CHAPTER
12
Glacier-dammed ice-marginal lakes of Alaska
David F.G. Wolfe, Jeffrey S. Kargel, and Gregory J. Leonard
ABSTRACT
The climate across Alaska is changing, as are the
melting and other dynamics of glaciers and their
lengths, widths, thicknesses, and masses. One may
reasonably expect that terminus and surface
ablation of Alaskan glaciers would impact the
occurrence and sizes of glacier-dammed lakes
(GDLs) impounded by many of these glaciers. An
individual lake and its damming glacier may have
unique dynamics, exhibiting chaotic variations not
directly attributable to climate change. Statistical
analysis of a large GDL population, however, indicates that changes may be linked directly to shifting
climatic conditions, thus providing an empirical
basis for quantitative numerical models of future
behavior. In this chapter we show that the rapidly
changing distribution of glacier-dammed lakes is
neither simple nor homogeneous, but that lake
changes may be amenable to modeling. These lakes,
their impounding glaciers, and their changes
through time can be detected in satellite imagery.
Presented here is research enabled by the GLIMS
initiative, and the Terra/ASTER and Landsat programs. Archived imagery facilitated review and
change analysis of 538 previously mapped GDLs
across Alaska, U.S.A. All glacier perimeters within
the study area were also reviewed for additional
lakes forming since the prior survey in 1971.
Change analyses of the combined lake population
over time found nonuniform distributions of lake
losses and gains from 1971 to 2000 across Alaska.
Detected changes appear less related to elevation,
latitude, or maritime influence than to the complexity, origin and terminus types of damming glaciers,
the aspects of their ice dams, possibly the topographic gradient below and near the lake, and possibly the rate of recent temperature increases. The
Copper River Basin (CRB), for example, has
recorded low rates of atmospheric warming relative
to adjoining areas over the past 50 years, and it has
retained and developed the greatest proportion of
GDLs. A concurrent detailed remote-sensing and
field observation study of the CRB highlighted the
dynamics of individual GDLs and their damming
glaciers. Whereas there is no single typical or representative lake, Iceberg Lake, in the far eastern Chugach Mountains, provides an example showing how
a climatic shift to warmer conditions may result in
diminishment and even disappearance of these
lakes. Iceberg Lake was comparatively stable for
at least 1,500 years, but responded to >100 years
of thinning of its damming glacier with the initiation of episodic glacier lake outburst flood (GLOF)
drainages every year or two, starting in 1999. The
thinning of the damming glacier, in turn, is partly a
response to a moraine-dammed lake that has
formed since the 1970s at the terminus of the trunk
(Tana) glacier, catalyzing disarticulation of that
terminus and causing a furtherance of thinning of
264
Glacier-dammed ice-marginal lakes of Alaska
Iceberg Lake’s damming glacier. The entire system
including the Bagley Icefield, outlet glaciers, multiple GDLs (Iceberg Lake among them), and
moraine-dammed lakes, is dynamically complex;
some parts appear to be responding to climate
shifts, and other parts may be displaying intrinsic
unsteadiness of flow (including a surge/waste cycle
of Bering Glacier). In recent decades, at the same
time as known lakes have been diminishing or disappearing at lower elevations, GDLs have tended to
form and persist at higher elevations.
12.1
INTRODUCTION
Thorarinsson (1939) and Mottershead and Collin
(1976) hypothesized that fluctuations of glaciers
may be monitored by documenting changes over
time in ice-marginal lakes. The advent of remotesensing technologies may make this approach
superfluous; however, these same technologies
enable broad-scale review of such lakes, individually or as a population, to corroborate glaciological
and climate data as well as to monitor glacier lake
hazards. While much is known about dam failure
and lake release mechanisms, little research has
investigated the characteristics of lake-damming
glaciers or developed empirical data on development locations of glacier-dammed lakes (GDLs)
on a large-scale systematic basis.
In this chapter, we combine research results of a
2008 M.Sc. thesis (Wolfe 2008) on GDL changes
over time with interrelated concurrent research by
Kargel and colleagues funded through the NASA
International Polar Year (IPY) program. Our collaborative research applied satellite imagery over a
historic USGS GDL map sheet (Post and Mayo
1971) area (Fig. 12.1) to comprehensively update
lake locations, and add attributes and analyze
changes. Here, we first review Wolfe’s (2008) comparison of the historic and recent lake populations
and their damming glaciers, and the attribute data
developed for each population. Then we briefly
consider the unique dynamics of example glacier
lakes. One of these lakes drained regularly for
nearly a century, and another formed and drained,
once and finally, in less than 40 years. The NASAfunded research combined lake population change
data with ground-based and remote-sensing analysis of GDLs. One of these persistent lakes, Iceberg
Lake (a colloquial previously published name),
bridged the prehistoric-to-satellite time period with
no sign of complete lake drainage (Loso et al. 2004,
2006). Then, in 1999, just months after the launch of
ETMþ (Enhanced Thematic Mapper Plus) aboard
Landsat 7, and just prior to the launch of ASTER
(Advanced Spaceborne Thermal Emission and
reflection Radiometer) aboard Terra, Iceberg Lake
started episodic complete drainages. We review and
further investigate Iceberg Lake dynamics during
the Landsat 7/ASTER era as a case study to help
illustrate some complexities related to GDL formation, persistence, episodic drainage, and disappearance.
Because GDLs are, or have the potential to be,
recurring geohazards to life and property, reports
have called for regular documentation and monitoring of GDLs across Alaska both before (Stone
1955, 1963a, b, Kuentzel 1970) and after (Young
1980, Rice 1987) the Post and Mayo (1971) GDLmapping effort. Few monitoring efforts and no
comprehensive post-1971 mapping updates of
GDLs across Alaska were conducted, however,
until 2008. Monitoring is a necessary step toward
having a validated, predictive theoretical capability
to project future behavior and assess likely future
hazards.
Investigation of lake formation locations and
processes and characterization of lake damming
may facilitate development of empirical models of
anticipated changes in the near future and validation of numerical models of more distant future
glacier/lake evolution. These investigations also
provide insights into prehistoric periods of rapidly
changing glacierization, GDL formation, and GDL
drainage, including that of the huge Pleistocene/
Early Holocene Glacial Lake Ahtna of Alaska’s
Copper River Basin (CRB). Incorporation of these
data into GIS and numerical models and artificial
intelligence methods of landscape classification
may also facilitate lake detection, monitoring, and
glacierization change detection in remote areas
throughout the cryosphere. Incorporation of time
series data covering recent decades may also help
determine how glaciers have previously responded
to dramatic climate shifts in previous millennia.
Beginning in 1999 with the start of observations
by Landsat 7’s ETMþ, and in 2000 with ASTER,
15 m resolution imagery of glaciated basins of
Alaska was repeatedly acquired, and then archived
with the Land Processes Distributed Active Archive
Center (LPDAAC). Archived ASTER imagery
from 2000 to 2007 was reviewed across the 1971
GDL Map Sheet 2 (‘‘baseline map’’ hereafter)
extent (Post and Mayo 1971; Fig. 12.1). Landsat
7 ETMþ imagery was used here to supplement
Regional context 265
Figure 12.1. Glacier-dammed lakes of Alaska population study area, defined by Post and Mayo (1971, Map Sheet
2, black outline), and surrounding U.S state of Alaska, as well as Canada’s Yukon Territory and province of British
Columbia. Outline of study area: lake change varied in three distinct patterns progressing in bands from the
southeast (coastal mountains of the St. Elias and Chugach Ranges, just north of 60 N bisected by 140 W), through
the continental Wrangell (south of 62 N near 143 W) and coastal Kenai (60 N ,148 W) Mountains to the north
(63 N from 144 W to 148 W) and west (60 N to north of 63 N, 150 W to 154 W) Alaska Range areas. The
Chugach and St. Elias Mountains lost about half their historic lakes but gained substantial numbers of new lakes.
The Wrangell and Kenai Mountains lost a third of their historic lakes, with the Wrangells gaining more than twice the
number of lakes it lost. The northern Alaska Range lost about two thirds of their historic lakes, and gained few
(except along the Denali Fault), while the southern half of the western ‘‘Alaska Range’’ lost only about half its
historic lakes.
areas of limited ASTER coverage, as is done with
similar glacier-related mapping research (Raup et
al. 2000, Bamber 2006). Alaska glacier perimeters
and GDLs had been captured by 2006 across >90%
of the study area, enabling a comprehensive review
and analysis of previously inventoried lakes, and
documentation of new lakes that formed following
the 1971 survey. The analysis, in theory, also permits the evaluation of climatic conditions in higher
elevation areas unmonitored by long-term or firstorder weather-recording stations (Fig. 12.2).
12.2
REGIONAL CONTEXT
12.2.1 Geographic setting
The study area (Fig. 12.1) contains seven of the ten
tallest peaks in North America and encompasses
glaciated parts of central Alaska, U.S.A., and bordering parts of southwestern Yukon Territory and
northwestern British Columbia (BC), Canada, as
delineated by Map Sheet 2 in Post and Mayo
(1971: outline, Fig. 12.1). Covering six degrees of
266
Glacier-dammed ice-marginal lakes of Alaska
Figure 12.2. (a) Climatic temperature anomaly time series for Alaska and within the study area. The blue-shaded
region shows the span of years between the two lake inventories discussed in this chapter. (b) Temperature changes
over the 58 years from 1949 to 2007. The rate of temperature increase varied by location and by season across the
state, and, within the study area, it was greatest to the north and west and least in the southern areas. For the period
from 1949 to 2007 (data from 19 first-order weather stations, of which only 2 were above 500 m elevation), mean
spring temperatures increased an average of 2.2 C, mean summer temperatures increased an average of 1.3 C,
mean autumn temperatures increased an average of 0.5 C, and mean winter temperatures increased an average of
3.5 C. Overall, mean annual temperatures increased 1.9 C across the state.
latitude (58.5 to 64.7 N) and 23 of longitude
(135.4 to 158.5 W), the area includes 67.3 10 3
km 2 of glaciers (about 75% of the total glaciated
area of Alaska and bordering Canadian mountains;
Arendt et al. 2002) or more (Molnia, 2007). Major
rivers draining the area include the Alsek, Copper,
Kenai, Kuskokwim, Matanuska, Nenana, Susitna,
Tanana, White, and Yukon. The study area covers
the entire Alaska and St. Elias Ranges, and the
Chugach, Kenai, Talkeetna, and Wrangell Moun-
Regional context 267
tains, as well as portions of the Coastal Range and
the Chigmit Mountains. The entire set of glacierized
mountains, taken as a whole, form vast arcs, overall
concave south, embracing the Gulf of Alaska (Fig.
12.1; see also Fig. 13.1). The geological context of
the region is described in Section 13.2.1. The arcuate geometry and pervasive tectonic fabric of the
ranges in this region control the prevailing precipitation and temperature patterns and the glacier and
stream drainage orienations (see Chapter 13,
especially Figs. 13.2 and 13.4) and thus also influence the orientations of ice dams as documented
below. Due to the tectonic fabric, the larger glaciers
and their stream drainages, especially those initially
taking a northern route, commonly trace tortuous
paths to the sea. Glaciers in the Alaska and St. Elias
Ranges can originate from above 5,000 m, while
some glaciers descending from the southern flanks
of the Chigmit, Chugach, Coastal, Kenai, and St.
Elias Mountains terminate in northern Gulf of
Alaska (North Pacific Ocean) tidewaters.
12.2.2 Climate
Environmental characteristics vary substantially
across the study area, generally changing from
maritime to continental influences with increasing
latitude and increasing distance from the Gulf of
Alaska. Maritime-influenced coastal areas adjacent
to the Gulf of Alaska are classified (by data from
low-elevation first-order recording stations), as
temperate rainforests, with cool wet summers and
moderate winters (Wiegand 1990). These areas had
average annual temperatures above 0 C (Shulski
and Wendler 2007) with average annual precipitation during the 1971–2000 study period varying to
over 2,300 mm in areas such as Cordova, Alaska.
Continental influences affect inland areas, which
are classified as continental subarctic or boreal
climates, and are characterized by warm short summers and cold dry winters (Ricketts et al. 1999).
Inland areas had average annual temperatures
below 0 C with less than 400 mm average annual
precipitation during the study period (Shulski and
Wendler 2007). Continental climatic extremes are
greatest north of the northern Alaska Range and
Wrangell Mountains (Shulski and Wendler 2007).
Due to the orographic influence of the surrounding
mountains, the upper river valleys of the Kenai,
Matanuska, and Susitna drainages (north of the
Kenai, Chugach, and Talkeetna Mountains, respectively), and the lowland interior of the CRB also
have continental climatic extremes of temperature
and extremely low annual precipitation. Such
semiarid conditions persist into the mountain
slope areas where glacier tongues debouch from
the Chugach and Wrangell Mountains into the
CRB.
Low-elevation (<500 m asl) weather stations
across Alaska recorded a temperature regime shift
(almost a step increase) averaging þ1.9 C in 1976/
1977 (Fig. 12.2; Shulski and Wendler 2007). Within
the study area, the rates of recorded temperature
increases between 1949 and 2007 were heterogeneous. The recorded temperature increase averaged þ2.3 C over this period across the northern
and western parts of the study region (Stafford et al.
2000), while stations of the study area’s southeastern quarter, including the CRB, recorded increases
averaging only þ1.6 C (Shulski and Wendler 2007).
These values compare with a global mean warming
over the same period of 0.6 C (Hansen et al. 2012).
However, recent decades of Alaska’s warming have
not been anomalous relative to land areas across the
Earth’s Arctic and high northern temperate latitudes, which in general have exhibited far more
rapid warming than the Earth as a whole (IPCC
2007, NOAA 2011). Differing rates of mean annual
global radiation modeled for Alaska (MAGRA), as
summarized by Dissing and Wendler (1998, p. 172),
also appear to have changed in a similar pattern
across the study area. As demonstrated in the
review on the Chugach Mountains (Chapter 13 of
this book by Kargel et al.), many other areas of the
Arctic and high northern latitudes have undergone
similar regime shifts in temperature or glacier mass
balance in the 1970s and 1980s. Precipitation during
the 1949–2007 period has generally decreased
annually in continental climates and increased in
eastern coastal areas of the study area. Seasonally,
the timing, form, and duration of precipitation
events have changed in ways that may tend to fill
lake basins more quickly and more completely leading into the winter months.
12.2.3 Previous research
Stone (1955, 1963b) pioneered comprehensive GDL
mapping in the area, documenting 60 lakes along
primarily coastal range mountain glaciers in the
early 1950s. Other early researchers or explorers
documented individual or small numbers of GDLs,
including a few occurrences documented in native
oral traditional knowledge that extended flood
impacts to infrastructure back at least into the
268
Glacier-dammed ice-marginal lakes of Alaska
1880s. Following the Great Alaska Earthquake of
1964, two reviews evaluated changes in coastal
glaciers and the lakes they impounded. Post
(1968) reviewed 38 lakes, while Marcus (1968)
reviewed 46 lakes. Kuenzel (1970) was the first to
extend research into interior areas of Alaska, documenting 101 GDLs, while Young (1977) reviewed
the Canadian portion of study area, documenting
and evaluating the hazard potential of 200 GDL
basins to infrastructure.
The maximum fill extents of GDLs and updated
glacier perimeters were mapped by the USGS (Post
and Mayo 1971) from ground surveys and oblique
aerial photographs, acquired through the 1960s. A
combined 750 lakes were mapped across two map
sheets (Post and Mayo 1971). The baseline map
extent—outline (a), Fig. 12.1—discussed here
included all glaciated basins of Southcentral Alaska
mountains, as well as those of adjacent Canada,
whose drainages flowed into Alaska, west of Glacier
Bay. Lakes as small as 0.4 km 2 in surface area were
mapped. Glacier perimeters updated for and shown
on the baseline map represent the best comprehensive glacier extent map documentation published for the area through the time of the present
research. The 1960s era glacier perimeter updates
were never included on any subsequent maps.
Glacier perimeter-mapping efforts have since been
handicapped in the study area by pervasive surface
debris obscuring lower elevation glacier ice margins
and tongues. Most glacier perimeters shown on
topographic maps of Alaska, on which many current glacier change documentations are based,
therefore, have not been updated since their positions were documented in data acquired in the
1960s, 1950s, or even—as noted by Manley
(2008)—the 1940s. Furthermore, mid-20th century
Post and Mayo (1971) perimeter data had not yet
been included in global glacier-monitoring databases through 2005 (Bolch et al. 2010).
However, dramatic downwasting/thinning of
glaciers throughout central and coastal Alaska
has been documented over this period (Echelmeyer
et al. 1996, Adalgeirsdóttir et al. 1998, Arendt et al.
2002, 2006, Chen et al. 2006, VanLooy et al. 2006,
Larsen et al. 2007, O’Neel et al. 2008), particularly
at lower elevations (Rice 1987, Arendt et al. 2006,
Molnia 2007). This extensive loss of glacier mass,
particularly along the lower elevations of most
mapped GDLs of the time (Marcus 1968, Post
1968, Post and Mayo 1971) led area researchers
to believe there would be few GDLs to remap with
the present research. Because GDLs had not been
resurveyed and but few were monitored, there was
little knowledge of their changes or persistence.
Very large prehistoric GDLs have been documented in the study area by Ferrian and Schmoll
(1957), Karlstrom (1964), Clague and Rampton
(1982), Schmok and Clarke (1989), Froese (2001),
Wiedmer et al. (2010), and others. One of these,
Glacial Lake Ahtna, is described in the next chapter
(see Chapter 13 of this book by Kargel et al.).
12.3
METHODS
The term ‘‘glacier lakes’’ encompasses several
hydrologic features, as summarized and outlined
by Costa and Shuster (1988). Our work considers
only ice-marginal lakes impounded by actively flowing glacier ice (Fig. 12.3), following the lead of Post
and Mayo (1971). Lakes dammed by moraines or
stagnant remnant glacier ice are not included in the
lakes reviewed here because, in the case of the
former, despite being ice marginal they are not
glacier dammed and, in the case of the latter, while
ice dammed the ice is no longer actively flowing
from an accumulation zone to a terminus. Also
not considered here are supraglacial, englacial, or
subglacial lakes, which, while they could pose recurring hazards or add water volume to the GLOF
from a GDL release, were neither surveyed historically nor amenable to the visual form of assessment
done in either survey.
Primary lake population research was based on
comparisons of a historic analog map document
(see Map Sheet 2, Post and Mayo 1971), with satellite imagery archived with LPDAAC. The historic
map was scanned for this research, and then georeferenced to topographic maps of the U.S.A. and
Canada. Archived orthorectified ASTER L1B
imagery was downloaded in preprocessed Hierarchical Data Format Earth Observation System
(HDF-EOS) file format from the LPDAAC, which
was then reprocessed employing the cubic convolution resample method for visual interpretation.
Landsat imagery was downloaded from the USGS
GloVis site for Alaska and Natural Resources
Canada’s Geogratis site for Canadian parts of the
study area. The Post and Mayo (1971) baseline map
and image processing was conducted at Alaska
Pacific University, Anchorage. ASTER DEM processing was conducted at the University of Arizona,
Tucson.
Historically mapped lakes were relocated and
mapped from recent imagery within an ESRI
Methods 269
Figure 12.3. Lake types considered. All are ice marginal and dammed by the ice of an actively moving glacier. Lake
type Ais at the confluence of two intersecting glaciers. Lake type B is a sidewall pocket of the damming glacier. Lake
type C is backing into an unglaciated side valley. Lake type D is a glaciarized side valley where the glacier has
retreated or is otherwise not connected to the damming glacier. Lake type E is an unglaciated trunk valley that forms
a lake behind the damming tributary valley glacier.
ArcGIS platform. These GDLs were evaluated for
persistence over a minimum time span of 30 years
from 1971 to ca. 2000 (1999 to 2007). Recent (ca.
2000) imagery was reviewed to determine if there
was still a 1971 era lake in a given location, if it was
still dammed by ice, and whether the ice still
appeared to be active. The GDLs were characterized by either occurring only in the 1971 ‘‘historic
survey’’ (termed ‘‘absent lakes’’), or only in the
more recent 2008 survey (‘‘new lakes’’), or in both
surveys (‘‘persistent lakes’’). A declaration of
‘‘absent’ status of a historically mapped GDL was
based on multitemporal repeat imagery or the state
of the damming glacier shown on that imagery.
Recent lakes include both those identified as persisting from the earlier map, and ‘‘new’’ lakes iden-
tified by Wolfe (2008) on the imagery where none
had previously been mapped. All lakes were manually identified through the use of multitemporal
imagery, and enhancements of RGB color and
contrast elements, and then analyzed for a series
of exploratory locational and formation attributes.
Data were grouped by lake status (absent, persistent, or new) and then analyzed to identify changes
over time by comparing absent lake characteristics
with those of the persistent lake group and with the
group of new lakes. Lakes with surface area as
small as 0.4 km 2 , the minimum size mapped by Post
and Mayo (1971), were detected and mapped.
Recent imagery showed that the perimeters from
the population of low-elevation glaciers were consistently enveloped by barren, vegetation-free and
270
Glacier-dammed ice-marginal lakes of Alaska
Figure 12.4. Glacier ablation and lake changes: Melbern (A–C) and Castner (D–F) Glaciers. Terminus retreat
(A–C) formed and destroyed Lake Melbern in <40 years; thinning (D–F) left a plateau where a lake had been
mapped in 1971. Also note prominent trimlines (lower left of panels E–F), which align with the 1971 glacier
perimeter (panel D, right side) and denote mass wastage.
ice-free terrain extending laterally out to a prominent single-ridged lateral moraine (Fig. 12.4, panels
E–F). This lateral moraine coincided with the
glacier perimeter updated for the baseline map by
Post and Mayo (1971). Therefore, glaciers with
perimeters distinctly inset from this lateral moraine,
mapped as ice covered in 1971, were determined to
have retreated by downwasting.
Vertical elevation and calculated horizontal
attribute data (described below), for both the lake
basin and the damming glacier, were recorded for
the first time, for both historic and recent lakes, to
evaluate lake distribution changes and damming
glacier characteristics over time. Limited imagery,
or cloud-obscured imagery, dictated sample sizes
among attributes. The research is unique in that it
focuses on longevity and development of a population—all lakes of a given form: ice marginal and
active glacier dammed—across a contiguous region.
Other recent studies, such as Gardelle et al. (2011),
regarding Himalayan glacier lakes, reviewed a
noncontiguous sampling area of other specific lake
types (supraglacial or moraine dammed), which
exhibit different characteristics and behave differently than glacier-dammed ice-marginal lakes.
Attribute data collected were exploratory, defining
how and at what level significant differences were
detected. Because several factors were analyzed
Methods 271
together without a priori planned contrasts, the
level of significance for post hoc tests was adjusted
with the Bonferroni correction:
0:05
ðnumber of factors consideredÞ
(Field, 2005). Significance ( p) values of at least
0.005 were considered significant in the detection
of intergroup differences. The attributes described
in the subsections below highlight factors corroborating (1) glacier recession data for the region
mentioned earlier, (2) climate conditions from
lower elevations documented by Shulski and Wendler (2007), (3) glacioclimatic relationships, and (4)
Thorarinsson’s (1939) and Mottershead and Collin’s (1976) hypothesis that changes in monitored
GDLs could indicate fluctuations in the damming
glacier. All GDL and damming glacier data compiled for this study can be reviewed within Online
Supplements 12.1A, 12.1B, 12.1C, and 12.1D).
12.3.3 Glacier stream order (complexity)
Glacier order was determined at both the ice dam
and the damming glacier terminus. Glacier order is
defined by the number of converging equal-valued
ice streams forming the damming glacier, as a
modification of the Strahler (1952) stream order
method.
The complexity of any ice stream segment was
determined by starting at the glacier origin, and
labeling each unbranched tributary (minimum 1
km long) as first order. Where any two equal-order
ice streams come together, an ice stream of the next
higher order is created. Median glacier order was
calculated for the glaciers damming each individual
lake within a population sample of lake types
(absent, persistent, new). Medians were compared
for significant differences in the magnitude of ice
stream order using the two-tailed Mann–Whitney
test.
12.3.4 Glacier surface gradient
12.3.1 Horizontal attributes
Horizontal attributes were calculated for lake position relative to glacier length. Lake location along
the length of the glacier was expressed in percent
upstream from the terminus and was also recorded
in kilometers from the terminus to the midpoint of
the ice dam.
ð12:2Þ
12.3.2 Mean glacier altitude (MGA)
MGA was calculated following data collection to
establish a constant elevation point between surveys. Using glacier origin altitude and glacier
terminus altitude (GTA; Davidovich and Ananicheva 1996) values derived from recent imagery
overlaid with U.S. NED 100 m contours, MGA
was calculated with the toe-to-headwall altitude
ratio (THAR) method following Meierding (1982)
and Torsnes et al. (1993), using a factor of 0.5,
where:
MGA ¼ ½ðglacier headwall or origin altitude
GTAÞ 0:5:
Glacier surface gradient was calculated following
the works of Reynolds (2000, 2008) and characterizes the slope of the area located between the
ice dam and the glacier terminus for each damming
glacier. Average glacier slope below the basin was
calculated in degrees as:
GDL altitude terminus altitude ðmÞ
arctan
:
distance from GDL to terminus ðmÞ
ð12:1Þ
As a gauge for interglacier comparison, MGA, as
used here, depicts relative rather than absolute
change, and is not intended to approximate the
ELA. MGA was used to compensate for the overall
glacier elevation increase associated with moving
inland, higher in latitude, and farther from maritime influences.
12.3.5 Damming glacier origin and
terminus types, and minimum–
maximum altitudes
Damming glacier origin and terminus types were
identified on recent imagery and each was grouped
by formation type:
. Origin types were grouped as either ‘‘cirque’’ or
‘‘ice field’’. Determination of glacier origin type
was based on reviews of imagery, overlaid with
100 m contour lines to indicate topographic variation. Cirque origins were interpreted to emanate
from bowl-shaped topography, where there was
little likelihood of noteworthy accumulations
from above the headwall, as the headwall was
usually too steep to retain snow/ice, and its rim
was a highpoint ridgeline. Lake-damming glaciers
originating from a relatively level contiguous ice
mass, with glacier outlets flowing into more than
one drainage, were termed ‘‘icefield’’ origin.
272
Glacier-dammed ice-marginal lakes of Alaska
. Glacier terminus types were defined as either land
or water-terminating (calving). Calving glaciers
terminated in either moraine-dammed lakes
(MDLs) or tidewaters (estuary-terminating
glaciers were excluded from this analysis).
. Maximum glacier altitude of the damming glacier
was recorded from the base of the headwall in
cirques, or at the crest interpolated between opposing aspect contours for icefields (ice divides).
. Minimum altitude (the lowest point) of the damming glacier was depicted in recent imagery. GTA
was interpreted as the farthest downvalley tongue
of active ice for land-terminating glaciers, and was
averaged across the width of the face for calving
glaciers.
12.3.6 Aspects of ice dams and
damming glaciers
Aspects of ice dams (Fig. 12.7, panels a–c, f–g) and
flow direction of lake-damming glaciers at both
origin and terminus were recorded for each basin.
Ice dam aspect refers to the direction the calving
face is oriented, and is typically measured as perpendicular to the orientation of the lateral margin
of the damming glacier (Fig. 12.7, panel h). Ice dam
aspect was recorded as the average direction the
dam faced: where a lake was dammed at the junction of two glaciers (Fig. 12.3, Lake Type A), for
example, the midline of the junction wedge was
identified as the ice dam aspect. This midline typic-
ally aligned with the medial moraine downglacier of
the junction.
Aspects were recorded as compass bearings.
Compass bearings were grouped into sixteen 22 wide azimuth classes—similar to the work of
Manley (2008) in a portion of the study area—each
separated by 0.5 from the next category. Aspect
estimates were grouped, and reclassified as the
median numerical value of the azimuth class within
which it fell (e.g., N ¼ 0, NNE ¼ 22.5, NE ¼ 45,
etc.). This method partially compensated for error
introduced in imagery acquisition (such as nadir
offset) and processing procedures.
12.4
RESULTS
The populations of both lake-damming glaciers
(henceforth, ‘‘damming glaciers’’; Fig. 12.5) and
ice-marginal glacier-dammed lakes (GDLs; Fig.
12.6) changed significantly and heterogeneously
across the study area between 1971 and 2000. Following a general synopsis, below, we first report
some notable changes in damming glaciers, and
then report notable changes in GDL attributes
which may be useful in developing models to predict lake development, growth, and demise. Raw
data are provided in Online Supplements 12.1A,
12.1B, 12.1C, and 12.1D.
A total of 214 damming glaciers were evaluated.
Nearly 30% of the damming glaciers of 1971 ceased
damming any lakes by 2000, while nearly 20% of
recent (2000) damming glaciers had not previously
Figure 12.5. Changes in lake-damming glacier population 1971–2000 across Southcentral Alaska, U.S.A. The
historic (ca. 1971) population of lake-damming glaciers (N ¼ 187, bar on left) was reduced by 54 glaciers, which
no longer dammed any ice-marginal lakes by ca. 2000. The recent (ca. 2000) lake-damming glacier population
(N ¼ 160) was supplemented with 31 glaciers, not previously shown impounding lakes, that had developed icemarginal lakes since 1971.
Results
273
differed significantly from those attributes shared
by glaciers that began impounding lakes since
1971. Lake occurrences are also shifting in relation
to precisely where along damming glaciers the lakes
are being impounded.
Through comparisons of the group of ‘‘absent’’
lakes with the group of ‘‘new’’ lakes, and the group
of 1971 damming glaciers with the group of 2000
damming glaciers, a set of attributes emerged that
show significant change over a short time period
coincident with atmospheric warming. Damming
glacier complexity, origin and terminus types, and
possibly aspect of the ice dam and glacier gradient
below the lake, appear to be important attributes
contributing to shifts in the locations of lake
formation. The formation of GDLs has shifted:
towards higher elevations, nearer the static mean
glacier altitude; to occur along longer more complex glaciers; and to points farther from damming
glacier termini, farther up the length of damming
glaciers. These changes are described below in more
detail.
Figure 12.6. Numbers of historic and recent glacierdammed lakes across central Alaska and adjacent
Canada. Of 538 lakes mapped historically by Post
and Mayo (1971), 263 (49%) are now absent and
275 (51%) persist; 71 of the persistent lakes (crosshatched area) were unfilled basins in all contemporary
satellite imagery, but still appear to have an ice dam. An
additional 141 new lakes were detected. Overall, the
number of glacier-dammed lakes decreased by 122
(23%) lakes (538 historic, 416 current) over 30 or more
years, but 34% of the current lakes are new.
been documented as damming any lakes (Fig. 12.5).
Lake-damming tidewater-calving glaciers (n ¼ 7)
were not abundant in the population, with only
one (which was in rapid unmitigated retreat) damming more than one lake in the 1971 survey.
Between the 1971 and 2008 GDL surveys, 263 of
the 1971-mapped lakes lost their ice dam or otherwise were no longer able to refill after draining.
Concurrently, however, another 141 ice-marginal
basins developed and began to impound water in
new areas along the damming glaciers (Fig. 12.6).
It is this loss of some lakes, concurrent with the
gain of new lakes, within the population and study
area, that enables statistical and geospatial analyses
of changes over time. There was a shift in the locations of ice-dammed lakes and of glaciers that dam
lakes. These changes were not random: specific
attributes shared by glaciers that used to dam lakes
12.4.1 Changes over time: Lake-damming
glaciers
A complete glacier perimeter survey of the study
area (Fig. 12.1) was conducted for the presence of
ice-marginal glacier-dammed lakes. In total, 214
lake-damming glaciers were evaluated for features
that may help explain the loss, development, or
persistence of ice-marginal lakes. Historically, 183
lake-damming glaciers were identified in an area
containing thousands of separate glaciers; 107
(58%) lost at least one lake due to loss of the ice
dam. Of these 107 glaciers, 54 (>29% of all historic
damming glaciers) no longer appeared to dam any
lakes. The recent survey (Wolfe, 2008) found 160
lake-damming glaciers, with 71 (44%) damming
new lakes; 31 of these (19% of all recent damming
glaciers; 44% of the glaciers damming new lakes)
were not previously known to dam lakes (Fig. 12.5).
Terminus retreat appeared to be the primary factor
in loss of the ice dam for 18% of the lake loss from
land-terminating glaciers, and 38% of the loss from
MDL-terminating glaciers. Thinning, or downwasting (Fig. 12.4E) appeared to be the primary factor
in loss of the ice dam for 82% of the lake loss from
land-terminating glaciers and 62% of the loss from
MDL-terminating glaciers.
Described in detail below are our evaluations of
features and properties that may explain the loss,
development, or persistence of ice-marginal lakes.
274
Glacier-dammed ice-marginal lakes of Alaska
Table 12.1. Summary of characteristics of lake-damming glaciers of Alaska and bordering Canada. The letter
preceding the ‘‘Minimum mean’’ and ‘‘Maximum mean’’ indicates the lake group from which the mean is
derived: a ¼ absent; p ¼ persistent; n ¼ new.
Attribute
Historical
mean
Recent
mean
Lake
group
Minimum
mean
Lake
group
Maximum
mean
838.9
1,006.5
a
783
n
1,118
534.9
418.5
a
(407)
n
(640)
27
36
a
21
n
41
Glacier length (km)
51.3
59.1
a
41
p
61
Glacier order at lake
2.9
2.8
n
3
a
3
Distance: terminus up to lake (km)
12.9
18.7
a
8
n
20
Glacier order at terminus
3.3
3.4
a
3
n
4
Glacier maximum elevation (m)
2,370.9
2,394.7
p
2,316
n
2,473
Mean glacier altitude (MGA) (m)
1,348.5
1,412.1
p
1,299
n
1,525
Number
(all lakes)
%
Number
(absent
lakes)
%
Number
(new lakes)
%
312
542
55
95
111
181
55
95
70
122
56
99
Lake elevation (m)
Elevation difference: MGA lake (m)
Glacier length below lake (%)
2 glacier gradient below ice dam
6 glacier gradient below ice dam
Cirque origin glaciers ¼ 127 (68.6%)
Icefield origin glaciers ¼ 58 (31.4%)
Historical
lakes
(%)
Absent lakes
dammed by . . .
%
Persistent lakes
dammed by . . .
%
68.3
31.7
193
52
79
21
146
105
58
42
These include damming glacier origin and terminus
types; damming glacier length; topographic gradient below the GDL; and glacier ice stream order, or
complexity (modifying Strahler, 1952), both above
the ice dam and for the length of the damming
glacier.
12.4.1.1
Damming glacier origin data
Greater percentages of (1) absent lakes had been
dammed by cirque origin glaciers, and (2) persistent
lakes were more commonly dammed by glaciers
originating in icefields (Table 12.1) than expected
by chance—likelihood ratio (1 degree of freedom) ¼ 24.713, p < 0.001. Glacier origin types were
grouped into one of two classifications for analysis:
cirque, or icefield complex. Ice-field complexes
occurred primarily at lower latitudes, near maritime
influences. The medians of grouped cirque origin
elevations (2,700 m) and grouped icefield origin
elevations (1,800 m) differed significantly—
Kruskal–Wallis chi-squared test (1) ¼ 13.433,
p < 0:001. Lake longevity was not statistically
independent of origin type. Lakes were more likely
to persist if their damming glacier originated from an
icefield rather than a cirque. Cirque origin glaciers
historically dammed a total of 339 GDLs, while
icefield origin glaciers dammed 157 GDLs. Cirque
origin glaciers (63% of damming glaciers, impounding 65% of all GDLs) dammed 79% of absent lakes,
but only 58% of persistent lakes, despite higher
elevation origins. Icefield/complex origin glaciers
(34% of damming glaciers, damming 32% of GDLs)
dammed only 21% of absent lakes, but 42% of
persistent lakes.
Results
Damming glacier origins, combined across the
study area, predominantly had aspects facing north
or south.
12.4.1.2
Damming glacier termini data
Lakes were more likely to persist if the damming
glacier terminated in a lake rather than on land.
Termini calving into a moraine-dammed lake
(MDL) were, on average, >20 km from the GDL
ice dam, whereas land termini averaged <15 km
from the GDL ice dam. In addition, glaciers that
dammed lakes and were land terminated, were far
more likely to originate from a cirque rather than
an icefield, which, as noted above (Section
12.4.1.1.), increased the likelihood of ice dam loss.
Glacier terminus types were grouped into two
primary classifications for analysis: land or water
terminating. Of the 202 damming glaciers analyzed,
75% (151) terminated on land and 25% (51) terminated in water with a calving face. Damming
glaciers that terminated in (moraine-dammed) lakes
(n ¼ 44, or 22% of the sample analyzed) lost 28%
of their historically mapped GDLs while landterminating glaciers lost 54% of the GDLs they
had dammed historically. Of the seven tidewaterdamming glaciers, only one dammed more than a
single lake, presenting an insufficient sample size for
analysis.
Damming glacier termini had a slight tendency
toward north, south, and south-southwest aspects.
12.4.1.3
275
Damming glacier complexity
Glaciers damming new lakes were more complex
overall, and were an average 11% (4.7 km) longer
than those damming absent lakes (Table 12.2), and
7% (3.2 km) longer than those damming persistent
lakes (Tables 12.2, 12.3). Glacier order, or the complexity of a glacier system, as a function of converging ice streams, ranged in values from 1 to 6
for this population of glacier ice streams. At the ice
dam, glacier order did not differ notably between
new lakes and absent lakes. Lakes did appear to be
forming, however, along glaciers that are ultimately
more complex (higher order) at the terminus.
Glacier order at the termini varied by lake type,
being highest for new GDLs (median: 4.0) and
persistent GDLs (median: 3.3), and lowest for
absent GDLs (median: 3.0). Glaciers damming
new lakes, on average, were one full order of complexity above those of absent lakes at their termini
(Table 12.2).
12.4.1.4
Damming glacier gradient
Reynolds (2008) and Frey et al. (2008) found glacier
gradient to be a factor in the duration of supraglacial water impoundment. The relationship of
damming glacier gradient to water storage in icemarginal GDLs, however, has not previously been
documented. The present dataset hinted at this
relationship when unfilled glacier-dammed basins
were considered separate from filled basins (persis-
Table 12.2. Summary statistics for comparisons of absent (lakes mapped in 1971 that no longer form) and new
(found on year 2000 or newer imagery) glacier-dammed lake basins.
Attribute
Absent lakes
New lakes
Number
Median
Number
Median
U
p
Lake elevation (m)
254
780
136
1,000
11,771.5
<0.001
Elevation difference: MGA lake (m)
259
(650)
136
(400)
12,310.5
<0.001
Lake distance upglacier (% of glacier length)
239
18
131
40
8,568.5
<0.001
Glacier length (km)
240
38.5
131
43.2
13,370
0.02
Glacier order at lake
254
3.1
135
3.0
15,990
0.24
Distance: lake to terminus (km)
242
5.2
129
14.2
8,185.5
<0.001
Glacier order at terminus
254
3.3
135
4.0
14,755.5
0.02
U and p from Mann–Whitney (U) test; differences are significant where p 0:005. MGA ¼ mean glacier altitude. Elevation data
indicate relative rather than actual changes (see text); distance measures (km) are accurate on the horizontal plane, but do not account
for vertical change.
276
Glacier-dammed ice-marginal lakes of Alaska
Table 12.3. Summary statistics for comparisons of absent and persistent lakes (1971 glacier-dammed lake basins).
Attribute
Absent lakes
Persistent lakes
Number
Median
Number
Median
U
p
Lake elevation (m)
254
780
267
775
31,510
0.16
Elevation difference: MGA lake (m)
259
(650)
275
(470)
27,591.5
<0.001
Lake distance upglacier (% of glacier length)
239
18
250
30
19,583
<0.001
Glacier length (km)
240
38.5
250
40
26,401.5
0.02
Glacier order at lake
254
3.1
257
2.8
27,207.5
0.00
Distance: lake to terminus (km)
242
5.2
249
13
17,943.5
<0.001
Glacier order at terminus
254
3.3
261
3.3
31,745
0.38
U and p from Mann–Whitney (U) test; differences are significant where p 0:005. MGA ¼ mean glacier altitude. Elevation data
indicate relative rather than actual changes (see text); distance measures (km) are accurate on the horizontal plane, but do not account
for vertical change.
tent lakes with surface area). Of the 275 persistent
GDLs, 26% (71; 13% of all reevaluated lakes) were
unfilled basins that appeared to remain dammed by
active glacier ice. Damming glacier gradients tended
to be steeper below unfilled basins. Here, nearly
74% of drained, but persistently ice-dammed, lake
basins had gradients greater than 2 below the
dam, but just 38% of basins with impounded water
had gradients greater than 2 below the dam. These
figures suggest a greater likelihood of persistent
GDLs to drain more frequently, or to not refill,
where glacier gradients below the ice dam are
steeper than 2 . In the case of Iceberg Lake (case
study in Section 12.5) the gradient in the 1 km
below the ice dam to the trunk glacier confluence
has steepened by about half a degree of slope in the
last 50 years, during which the lake initiated drainages.
12.4.2 Changes over time: Glacierdammed lake population
Imagery acquired from 1999 to 2007 indicated 96%
(538) of the historically mapped GDLs had been
sufficiently imaged by 2006 to enable persistence
evaluation. Fifty-one percent (275) of evaluated
historic lakes persisted (Fig. 12.6) while 49% were
no longer capable of forming (‘‘absent’’ lakes).
Persistent lakes, compared with absent lakes,
tended to be farther upglacier (Table 12.3), more
likely dammed by icefield origin glaciers than by
cirque origin glaciers (Table 12.1), and more likely
dammed by calving terminus, or lacustrine, glaciers
than by land-terminating glaciers. The recent GDL
survey also found 141 newly formed lake basins
along glacier perimeters across the study area
(Fig. 12.6; also see Online Supplements 12.1A,
12.1B, 12.1C, and 12.2, which includes an oversized
plate showing all lake locations across the study
area). New lake location attributes, when compared
with those of absent lakes, tended to have more
extreme values or occurred within a narrower subset of the range of values than the differences found
between persistent and absent lakes. We elaborate
more on this below.
12.4.2.1
Lake locations along damming glacier
margins: absolute and proportional shifts
New and persistent lakes were farther upglacier
than absent lakes, both in terms of proportional
and absolute distances. Proportionally, new lakes
were 22%, and persistent lakes 12%, farther up the
damming glacier than absent lakes (Tables 12.2,
12.3). The median position of new lakes (40%),
measured as a percentage up the glacier length from
the termini, was significantly more than the median
position of absent lakes (18%; two-tailed Mann–
Whitney, U ¼ 8;568.5, p < 0:001). The median
position of persistent lakes was significantly greater,
as well, on average, than that of absent lakes (30 versus 18%, respectively; two-tailed Mann–Whitney, U ¼ 19,583.0, p < 0:001). In absolute terms,
the distance between a GDL and its recent damming glacier terminus differed significantly between
lake types (Kruskal–Wallis chi-squared test
Results
277
Table 12.4. Lake status since 1971, elevation, and damming glacier origin type by analysis region.
Prevailing
climate
Mountain
range region
abbreviation
Mean elevation by glacierized region
and lake status since 1971
(m)
Lakes dammed
by glacier origin
type (n lakes)
n
Absent
lakes
n
Persistent
lakes
n
New
lakes
Cirque
Icefield
KENI
22
325
47
604
12
496
3
68
CHUG
61
647
67
637
35
653
113
47
WARS
30
890
28
706
3
467
26
26
Maritime and
continental
STEL
59
772
58
843
41
1,279
105
53
Continental
WRST
7
1,221
29
1,309
16
1,694
37
2
TALK
0
n/a
0
n/a
8
1,834
8
0
EAKR
28
1,230
19
1,383
8
1,574
50
5
WARN
45
890
21
706
11
467
26
26
Maritime
Mean
783
831
1,118
KENI ¼ Kenai Mountains; CHUG ¼ Chugach Mountains; WARS ¼ Southwestern Alaska Range; STEL ¼ St. Elias Mountains;
WRST ¼ Wrangell Mountains; TALK ¼ Talkeetna Mountains; EAKR ¼ Eastern Alaska Range; WARN ¼ Northwestern Alaska
Range.
(1) ¼ 32.988, p < 0:001). New lakes were more than
9 km, and persistent lakes 8 km, farther from damming glacier termini, on average, than absent lakes.
The median distance between GDLs and glacier
termini was significantly greater for new lakes (median of 14.2 km) than for absent lakes (median of 5.2
km) (Table 12.2; two-tailed Mann–Whitney,
U ¼ 8,185.5, p < 0:001), and was also significantly
greater for persistent lakes (median of 13 km) than
for absent lakes (Table 12.3; two-tailed Mann–
Whitney, U ¼ 17,943.5, p < 0:001). All lake groups
were farther from the termini of land-terminating
glaciers than from the termini of water-terminating
glaciers, but the absent lakes group was closer to
water-terminating glaciers than either persistent or
new lake groups (Table 12.4).
12.4.2.2
Lake hypsography: above sea level (asl)
and below mean glacier altitude (MGA)
The elevation asl median of grouped new lakes
differed significantly from that of both the absent
(Table 12.2) and persistent lake groups. The differences in mean elevation asl between absent and
persistent lake groups, though, were insignificant
(Table 12.3). However, heterogeneous distribution
across the study area of these two groups (see oversize map, Online Supplement 12.2), with persistent
lakes concentrated in lower elevation coastal mountains, and absent lakes concentrated in higher elevation inland mountains (Table 12.4), obfuscated a
significant difference apparently contributing to
lake longevity. While the mean elevation of grouped
new lakes (1,118 m asl, n ¼ 133) was significantly
higher than both absent (mean elevation: 783 m asl,
n ¼ 250; two-tailed Mann–Whitney, U ¼ 11,556,
p < 0:001) and persistent (mean elevation: 831 m
asl, n ¼ 258; two-tailed Mann–Whitney, U ¼
12,028, p < 0:001) lake groups, these data tests
did not account for geographic distributional differences. Using the elevational midpoint of the glacier,
the MGA, to elucidate differences, however, indicated that both grouped new lakes (400 m below the
MGA) and grouped persistent lakes (450 m below)
were vertically closer to the damming glacier MGA
than absent lakes (670 m below) had been (Table
12.4). The vertical difference between the medians
of grouped new and absent lakes and the MGA was
significant
(two-tailed
Mann–Whitney,
U ¼ 12,310.5, p < 0:001; Table 12.2), as was the
vertical difference between the medians of grouped
persistent and absent lakes and the MGA (two-
278
Glacier-dammed ice-marginal lakes of Alaska
tailed Mann–Whitney, U ¼ 27,591.5, p < 0:001;
Table 12.3).
12.4.2.3
Aspect of ice dam populations
Ice dam aspect was assumed to be perpendicular to
ice flow orientation, which is, through averaging
a large population size, guided by the general linear
orientation of the mountain range. Mountain
ranges along the west side of the study area tending
to face northwest–southeast may have been statistically balanced by mountain ranges along the east
side of the study area, tending to face northeast–
southwest; between these west and east side ranges
were ranges that generally faced north–south. Combining all the data, the averaged aspects of damming glaciers tended to face north or south.
Ice dam aspects shifted between historic and
recent lake populations (Fig. 12.7). The aspects of
ice dams of now-absent lakes were strongly oriented
east or west (Fig. 12.7a), as might be expected from
a population of glaciers flowing north or south. The
aspects of persistent lake ice dams, however, ranged
primarily from the northwest to the northeast or
east (Fig. 12.7b). The aspects of new lake ice dams
faced an even narrower range of north to east
aspects than persistent lake ice dams (Fig. 12.7c).
It is unclear how much influence the disproportionate rate of climate change recorded across the
study area (Fig. 12.2) may have had on ice dam
aspect changes in the population over time, compared with the disproportionate shift in the lake
population toward differently oriented geography.
It is also not clear—and was not analyzed—which is
the important feature in lake loss and development:
the aspect of the ice dam, or the aspect of the basin
that the lake is backing into.
12.4.2.4
Ice dam and lake-type changes
through time
Of the different types of ice dam formations, it
appeared as though valley blocking (Type E,
Fig. 12.3) and tributary detaching (Type D, Fig.
12.3) have historically resulted in the largest lakes.
No new lakes formed and few persisted behind
valley-blocking glaciers; only one new lake formed
in recent times due to a tributary glacier detaching
from the trunk glacier. The majority of new lakes
appeared to form in sidewall pockets and unglaciated side valleys. The one source of potentially
developing large GDLs is along the margins of
nunataks and coastal mountain icefield perimeters,
on plateaus above steeply descending outlet
glaciers.
These findings indicate both a change in glaciers
that dam lakes and changes in locations along the
horizontal plane of damming glaciers and a change
in vertical elevations at which lake formation occurs
(Online Supplements 12.1A, 12.1B, and 12.1C). The
data also may suggest a lake formation sensitivity
to climate changes, such as atmospheric temperature increases; precipitation type, timing, or duration; and/or, in the case of aspect, perhaps, cloud
cover.
12.4.2.5
Changes in big lakes and some
notable anecdotes
Most lakes in both the Post and Mayo (1971) and
Wolfe (2008) surveys were relatively small, with
80% of lakes having surface areas <1 km 2 each.
In the former survey, only 15 lakes (3% of the
historic population) had a surface area >5 km 2 .
Net size changes and disappearance within this
group of 15 lakes alone accounted for over 60%
of total measured surface area loss between surveys.
In the recent survey, there are only six lakes (1% of
the recent population number) with >5 km 2 visible
surface area. All six of the larger lakes in the recent
population were persistent. Only one new lake had
an apparent surface area >1 km 2 in the 2008 survey. Clearly, the largest GDLs of modern times
across this study area are disappearing. In Section
12.5 (‘‘Case study’’) we examine in depth one of the
few persistent larger lakes, Iceberg Lake. Additionally, we look anecdotally at two other large lakes,
which no longer form, to demonstrate the complexities involved in describing, defining, and analyzing
large-scale trends from any one individual glacierdammed lake. All three of these large lakes were
backed into large low-gradient valleys in nearcoastal mountains.
Melbern Lake, in the Coastal Range of southeastern Alaska, had an ice dam facing east, and
lasted about 40 years with only one drainage event,
which cataclysmically destroyed the ice dam in 2001
(panels a–b, Fig. 12.4). Lake George, in the far
western Chugach Mountains, had an ice dam facing
south, and released cyclically for about 100 years,
according to reports dating back as far as Mendenhall (1899). Iceberg Lake, farther from the coast
and higher in elevation than the other two, persists
in the far eastern Chugach Mountains with a
north-facing ice dam. In existence for over 1,500
Results
279
Figure 12.7. Ice dam and glacier flow aspects. Note scale change in each graph. The change in GDL population—
loss of lakes in some areas and gain in others—had a cumulative outcome of an overall change in prevalent ice dam
aspects. Historically, lake dam aspects were well distributed (panel f ¼ data of panel a þ panel b). Recent lake dam
aspects, by contrast, tended to be restricted to northerly and easterly trending aspects (panel g ¼ data of panels b
and c combined)). Panel d combined with e shows the general trend of damming glaciers from origin (d) to terminus
(e), indicating lake-damming glaciers within the study area flow generally north or south off generally east to westtrending mountains. Each aspect category trajectory represents the central value (e.g., 180 for south) of a 22 range
of aspects (e.g., 169–191 ) encapsulated in that category designation. Numerical values in scales represent
numbers of ice dams/glaciers per aspect category.
years, it did not have its first complete drainage
until 1999.
Melbern Lake, as labeled in Clague and Evans
(1994)—it is given the lake ID ‘‘STEL BCCan114P06’’ in Wolfe (2008)—formed at about 250 m asl at
the coastal toe of the St. Elias Range of Southeast
Alaska, behind Konamoxt Glacier, beginning in the
1960s. At that time, Post and Mayo (1971) mapped
ice-marginal lakes on both sides of Melbern Glacier
at its junction with Konamoxt Glacier, a tributary
to Melbern that persisted to become a trunk valley
dam as Melbern retreated (Fig. 12.4, panel A).
These two lakes coalesced to become a large trunk
valley lake by the late 1970s, and grew steadily as
the Melbern terminus continued to disarticulate
into the 1990s (Clague and Evans 1994). By the
early 1990s, the lake had grown to 12 km 2 in surface
area and was considered to be one of the largest
280
Glacier-dammed ice-marginal lakes of Alaska
GDLs on Earth (Clague and Evans 1994). There is
no documentation that Melbern lake ever drained
entirely and then refilled. A review of 2001 Landsat 7
ETMþ imagery indicated it drained cataclysmically
in a single subglacial drainage event; its waters subsequently erupting in a lower moraine-dammed lake
on the downvalley side of Konamoxt Glacier in a
boil over 500 m in diameter. The damming Konamoxt Glacier went into a rapid retreat phase for the
next several years. Later ASTER imagery confirmed continued retreat of Konamoxt Glacier
(Fig. 12.4, panel C) out of the water to become land
terminating by 2007. The 2001 event is the first and
only known drainage. Melbern Lake existed for less
than 40 years.
Lake George, elevation 80 m asl, was documented by Stone (1955, 1963a) and Boggs and
Duffy (1996), among others (reviewed by Norris,
2007). The damming Knik Glacier originates from
about 2,700 m asl and is 35 km long, with the ice
dam at the terminus face buttressed by a bedrock
cliff. Lake George appears to have formed as the
tributary glaciers (George and Colony) detached
from the trunk at the close of the Little Ice Age
(LIA). Three lakes eventually merged into one as
meltwater collected behind the ice dam (Boggs and
Duffy, 1996). Native oral traditional knowledge, as
documented by Mendenhall (1899), tells of the first
catastrophic flood—wiping out three villages in the
floodplain—occurring prior to 1898, perhaps as
early as the 1870s. Covering a 67 km 2 area, with
a depth when full of over 50 m, the lake was the
largest GDL in North America in 1967. It released
over 2.3 10 9 m 3 at greater than 11,000 m 3 /s
(Boggs and Duffy, 1996). Following the initial
release, the lake refilled and released episodically.
Eventually, the maximum lake volume and flood
sizes decreased, and frequency of release increased,
until it became an annual or biannual occurrence.
Within three years of the Great Alaska Earthquake
of 1964 (magnitude 9.2), with an epicenter in the
western Chugach Mountains near the origin of
Knik Glacier, Lake George ceased refilling (Boggs
and Duffy 1996) after 100 years of drain and release
cycles (Stone 1963a, Post and Mayo 1971).
12.5
CASE STUDY: ICEBERG LAKE
12.5.1 Overview
Iceberg Lake (the colloquial name used by Loso et
al. 2004, 2006)—also known as Bunch (Pavlis and
Sisson 1995), Blanche (Stone 1963b), and Bering
GlacD6-01 (Wolfe, 2008)—is located at latitude
60 45 0 40 00 N, longitude 142 57 0 00 00 W, in the eastern
Chugach Mountains off the Gulf of Alaska (Figs.
12.8–12.14; Online Supplements 12.3 and 12.4). Iceberg Lake (IL) is about midway between Melbern
Lake and Lake George (Section 12.4.2.5) but at a
much higher elevation (933 m asl; Loso et al. 2006)
than those two (Melbern Lake, 250 m; Lake
George, 80 m), as well as over 100 m above the
mean elevation of all persisting lakes in the larger
study area, and 300 m higher than the mean of lakes
persisting across the Chugach Range (Table 12.4).
IL, listed as persistent since the 1971 survey, has an
ice dam facing north, and is located 15.9 km (46%
of the damming glacier length) from the ca. 2000
Tana Glacier terminus, 300 m below mean glacier
altitude (Wolfe 2008). It is thought to have formed
by detachment of a tributary glacier, whose remnants include Chisma West Glacier. Silt deposition
in the lake formed a nearly continuous little-disturbed varved sedimentological record for at least
1,500 years (Loso et al. 2006). Since 1999, however,
with initiation of the first ever (Loso et al. 2004,
2006) complete drainage (Fig. 12.9), IL has had
glacier lake outburst floods (GLOFs; also termed
jökulhlaups) in late summer to early fall, nearly
every year (Table 12.5, updated from Loso et al.
2006), leaving an iceberg-choked pond of about the
same size every time (Fig. 12.10). Our satellite
image time series (Fig. 12.11) illustrates those events
primarily with before-drainage and after-drainage
scenes.
Iceberg Lake is dammed by a large, 30 km long,
fourth-order, east-facing, unnamed tributary glacier of Tana Glacier (Fig 12.8, 12.10A; Online Supplement 12.3, A–D); here we refer to this glacier
primarily as ‘‘the damming glacier’’, and sometimes
by the provisional term, Kieffer Glacier, in tribute
to the founder of GLIMS, Hugh H. Kieffer (Uhlmann et al. 2012). A continuous depositional record
of clastic varves (presumed to be annual couplets of
summer sand and winter silt) and Bouma-type turbidity flow sequences indicate that the lake existed
continuously for over 1,500 years until 1999 (Loso
et al. 2006, 2007). Those sedimentological studies
provide a unique perspective on lake–glacier–
climate dynamics over a millennial scale period of
time. Varve thicknesses appear to increase in the
post–Little Ice Age period and also appear to indicate a Medieval Warm Period (Loso et al. 2006,
2007). However, our further assessment of the complex lake dynamics, and the analysis of Diedrich
Case study: Iceberg Lake 281
Figure 12.8. (Top panel) Iceberg Lake vicinity (upper right) and location within the larger ice-marginal glacierdammed lake survey area in southern Alaska (inset). Proximity to the Gulf of Alaska (along base of figure) is shown,
as are the Bering and Tana outlet glaciers of the Bagley Icefield (mostly out of view to the right/east), and the status
of nearby ice-marginal glacier-dammed lakes. The red triangles represent new lakes, the blue diamonds persistent
lakes since the 1971 survey, and black pluses absent lakes. (Bottom panel) Location of Iceberg Lake within the
eastern Chugach Mountains. Kieffer Glacier is provisionally named. Map base: ASTER 321-RGB false-color
composite image mosaic (August 10, 2004 and August 8, 2003). Some of the new GDLs mapped in the top
panel can be seen along the northeast side of Tana Glacier. Figure can also be viewed in higher resolution as Online
Supplement 12.4.
and Loso (2012) as well, establishes that there cannot be a unique and simple, reliable deconvolution
of varve thickness with respect to the effects of
glacier dam height (related to lake area and the
concentration or dispersion of sediment over that
area), meltwater input, or sediment concentration
in the meltwater. All of these factors are related in
some way to climate variability, but they record
different components of the system: meltwater input
responds rapidly to annual changes in summer temperature but also responds slowly as glacier area
changes; meltwater turbidity and sediment load
282
Glacier-dammed ice-marginal lakes of Alaska
Figure 12.9. Iceberg Lake Landsat ETMþ time series captures a drainage event over a 47 h 48 min period in late
August 1999. (Panel A) The nearly full lake basin on August 27, 1999. (Panel B) A completely drained lake two days
later on August 29, 1999, again with stranded icebergs near the lake outlet at the southeast end (both images ETMþ
432-RGB false-color composites). Figure can also be viewed in higher resolution as Online Supplement 12.5.
Table 12.5. Chronology of Iceberg Lake, Alaska, lake drainage events from satellite
and field observations.
Drainage event
a
Year AD
Date a and comments
1
1999
August 27 (Landsat 7 ETMþ; Fig. 12.9)
2
2000
August 15
3
2002
August 15
4
2003
August 3
5
2004
August 26
6
2005
Before August 11
7
2006
September 5 (photos taken/provided by C. Larsen)
8
2007
Drained before August 2 (Landsat TM)
9
2008
Drained prior to August 20 (authors’ site visit)
10
2009
July 29–August 6 (authors’ site visit)
11
2010
August 13(?)
Date of jökulhlaup commencement, 1 day; 1999–2004 dates from Loso et al. (2006).
Case study: Iceberg Lake 283
Figure 12.10. Iceberg Lake, eastern Chugach Mountains. (Panel A) The lake almost completely drained on
August 8, 2003 (image base: ASTER 321-RGB false-color composite). The yellow box indicates the view area
shown in panel B. (Panel B) Oblique aerial image also showing a separate lake drainage event six years later in
August 2009. Note stranded icebergs congesting towards the lake outlet (bottom center-right) and several more
clusters towards the top of the lake. Stepped trimlines and terraces can also be seen along the east side (on the right
of the photo) of the lake, indicating historic lake levels (photo: J. Kargel). Figure can also be viewed in higher
resolution as Online Supplement 12.6.
likewise include fast and slow-responding components; and height of the glacier ice dam responds on
centennial scales as the damming glacier and trunk
glacier thin and thicken in response to climate
oscillations. Furthermore, both precipitation and
temperature, which themselves may change
asynchronously, are involved in each of these variables, and thus the varve thickness record may not
bear a simple and reliably decipherable link to temperature, or any other single climate parameter,
alone.
Several distinct strandlines at IL (Figs. 12.9B,
12.10B; Online Supplement 12.3, J–N) have been
documented, dating, from highest to lowest, as
pre 1825, 1825–1834, 1834–1867, 1867–1957, and
1957–1999 (Loso et al. 2004, Loso and Doak
2006). These strandlines are thought to represent
discrete lake levels (shorelines) during successively
lowering highstands as the height of the glacier
ice dam decreased. Sturm (1986), however, documented the development of a terrace that is
geomorphologically similar to Iceberg Lake’s
strandlines during a GLOF from another large
GDL in Alaska. By analogy, Iceberg Lake’s strandlines may indicate previous partial sudden lake
drainages, such as those indicated by Stone
(1963b) occurring in 1948 and 1951.
12.5.2 Satellite observations
The first known complete GLOF drainage from IL
was fortuitously captured before and after draining,
within a <48 h time span, by Landsat 7 ETMþ
imagery in August, 1999 (Fig. 12.9) as discovered
by the lead author during archive review for this
research. Imagery obtained since 1999 show the
lake either full (approximating the 1957–1999 IL
shoreline of Loso et al., 2004) or nearly empty
(Fig. 12.9; Online Supplements 12.3, 12.7A, and
12.7B; Table 12.5). These images—especially those
with a low lake level—also readily show vegetation
trimlines and terraces (Figs. 12.9B, 12.10B). Only a
couple of the trimlines and terraces are visible in
ASTER imagery; several more are apparent from
the ground. Such terraces are often a signature of
glacier-dammed lakes and their former basin water-
284
Glacier-dammed ice-marginal lakes of Alaska
fill footprints (likened to ‘‘bathtub rings’’), which
generally may be formed by a complex combination
of near-shore wave and ice reworking of near-shore
sediment, proximal sediment deposition, and, as
documented elsewhere by Sturm (1986), erosion
(saturated soil slumpage) during drainage. The classic strandlines are very common features of many
glacier lakes (both ice-dammed and proglacial
lakes), which commonly exhibit drawdown features
overprinting episodic drainage and refilling signatures in response to climatic and glaciological perturbations of the system. The former Lake George,
discussed above, has such strandlines (Stone, 1955,
1963a), as does Strandline Lake (Sturm, 1986) on
the west side of the study area. The details of
strandline origin may depend on the unique attributes and processes of each individual lake and
environment, such as sediment type and clast sizes,
the rapidity of drawdown, and so on. The causes
(geomorphological origins) of Iceberg Lake’s strandlines are unknown, except that they obviously are
associated with former discrete lake levels. In the
case of Iceberg Lake, some of the higher strandlines
are further emphasized by vegetation and lichen
(which Loso and Doak 2006 used to date the
strands) differences, thus suggesting some discrete
age differences, and by alignment with lateral drainage channels paralleling the glacier margin.
Every component of the integrated climate–
glacier–lake system around Iceberg Lake is highly
dynamic (Diedrich and Loso 2012). Fig. 12.11
shows a satellite time-series of Iceberg Lake, and
the complete time series is shown for a broader area
extending to the growing terminus lake of Tana
Glacier in Online Supplements 12.7A and 12.7B.
The broader context Iceberg Lake, showing most
of the drainage basin associated with Tana Glacier,
including Iceberg Lake’s damming glacier, is shown
in Fig. 12.8, whereas the subbasin that drains only
into Iceberg Lake is shown in Fig. 12.12. As we will
describe, Iceberg Lake exists in close dynamic association, both influenced by and influencing, the
wider glacier–lake system.
The supplemental time series also illustrates a
distinct thinning and retreat of the lateral and terminal extents of the Tana and IL-damming glaciers.
We have not measured the thinning, but it is apparent from the retreat of the margins of the glaciers.
Indeed, nearly every glacier covered by the time
series—lake terminating and land terminating,
large and small—has undergone notable retreat
and thinning between 1986 and 2011. Further, the
thinning or downwasting is clearly demarcated
across the larger study area, defined by Post and
Mayo (1971) and shown in Online Supplement 12.3,
panel D as a distinctive vegetation trimline extending 50 m or so above the present elevations of most
glaciers in the area. Fig. 12.4, panels e–f, shows a
case where an ice-marginal lake, mapped by Post
and Mayo (1971), is now a drained basin perched at
the top of the lateral moraine left by recent decades
of glacier thinning. This same type and extent of
recent thinning has strongly affected Kieffer and
Tana Glaciers, and thus has affected the hydrology
of Iceberg Lake.
The 2009 images in the Online Supplements
12.7A and 12.7B version of the time series form
an interesting trio: July 13, July 29, and September
15 images show a drawdown of Iceberg Lake
between the first two images and nearly complete
drainage by September 15. The second and third
authors visited the lake on August 6–8, 2009, at
which point the lake was nearly empty (Fig.
12.10B and Online Supplement 12.3). Careful
examination of the two July images in the satellite
time series (Online Supplement 12.7B) revealed that
as Iceberg Lake lowered, Tana Lake and its outflow
increased; this likely indicates Iceberg Lake drainage had begun at the time of the July 29 image.
Not shown in our time series but inspected for
this chapter, early Landsat imagery from 1972 to
1973 show Tana Glacier’s tongue, though already
thermokarstic, fully extended to its end moraine,
which is now abandoned by the glacier and has
become partially vegetated. The terminus of Tana
Glacier in the early 1970s (Landsat 1 MSS
L107001773189 from July 1973) was in approximately the same position as depicted in a 1914
topographic map (USGS 1914). While some thinning of Tana Glacier prior to the early 1970s is
evident in the early Landsat imagery from the existence of lateral moraines and trimlines above the
glacier level, resolvable ponds were absent on Tana
Glacier’s tongue throughout the 1970s. However,
development of ponds and lakes on Tana Glacier’s
tongue and along its terminus was well underway by
the mid-1980s and probably began around 1980;
lake growth has continued rapidly since then
(Online Supplement 12.7B).
12.5.3 Field observations
At the time of the second and third authors’ 2009
field visit to IL, drainage along the margin of the
damming glacier was extremely rapid, compared
with 2008 (when the first and second authors
Figure 12.11. Iceberg Lake 21-step satellite time series captures a sequence of summer to early autumn fill–drain cycles from 1986 to 2011. Images: Landsat TM
and ETMþ false-color composites (432-RGB), enhanced TM near natural composites (321-RGB), and ASTER false-color composites (321-RGB). Figure can
also be viewed in higher resolution as Online Supplement 12.7A and as slides on 12.7B.
Case study: Iceberg Lake 285
286
Glacier-dammed ice-marginal lakes of Alaska
Figure 12.12. Iceberg Lake drainage basin (yellow line), lake area (white-outlined blue DEM), and lake bathymetry derived from ASTER DEM. Figure can also be viewed in higher resolution as Online Supplement 12.8.
visited), indicating the GLOF was still active on
August 6, 2009, albeit nearly complete. Flow into
Iceberg Lake in August 2009 also was quite high (as
indicated at the inlet stream, where it swept an adult
grizzly bear rapidly downstream in full flotation as
the bear tried to cross) compared with 2008. During
the 2009 visit, many icebergs were observed
stranded on the still-saturated lakebed, extending
almost to the terminal moraine of Lumpy Glacier
(Fig. 12.10B and Online Supplement 12.3), but far
fewer existed—and only close to the residual lakeshore—during the July 2008 field visit. At the time
of the 2009 visit, besides the icebergs, the stillsaturated silt and clay across the exposed lakebed
indicated rapid drainage just prior to our arrival,
consistent with satellite-imaging evidence. We also
made observations of the water inlets/sources and
outlet of Iceberg Lake (Online Supplement 12.3).
The nearly fully drained extent of Iceberg Lake
stood in stark contrast to other periods when the
lake basin was filled, shown in the satellite time
series and also captured in an air photo in 1951
by Kirk Stone (Stone 1963b; Fig. 12.13).
12.5.4 Satellite era hydrology
In this section we first examine the volume of
Iceberg Lake, with particular attention paid to
the volume of water required to refill the lake in a
one-year period. Next, we consider the possible
magnitudes of the known fluxes from precipitation
and from iceberg calving. We show that these
are insufficient sources to explain the satellitedocumented one-year refilling time, leading us to
speculate that unseen sources of water (the two
key possibilities being net negative surface balance
of the glaciers and/or subglacial water reservoirs
draining into Iceberg Lake) must make up the rest.
The approximate volume of water contained in
Iceberg Lake at the time of an August 2004 ASTER
image (Fig. 12.14B) was calculated using the
ASTER DEM for an August 2003 image (when
the lake was mostly drained, Fig. 12.14A) and the
shoreline from the August 2004 image. The volume
was determined by first generating a best-fit lake
surface elevation for the 2004 image, using contours
from the 2003 ASTER DEM. The volume below
this lake surface elevation was then calculated
within ESRI ArcGIS software (3D Analyst tools).
We calculated a maximum depth exceeding 70 m
(excluding the unknown residual depth of the small
lake in the August 2003 scene) with an average
depth of 55 m (not counting the residual lake),
comparing well with the ground-measured 60 m
of Loso (2009). With an area of about 4.4 km 2
(Loso et al. 2006; ground measurement) to
Case study: Iceberg Lake 287
Figure 12.13. Iceberg Lake, eastern Chugach Mountains. (Left panel) The lake filled in 1951. Oblique aerial image
captured by Kirk Stone (lake color modified by authors to enhance lake area and shoreline perimeter). (Right panel)
Mostly drained lake in August? 2008. Blue dotted outline indicates approximate predrained lake level. Note retreat of
Chisma-West Glacier between images (right-panel photo from J. Kargel). Figure can also be viewed in higher
resolution as Online Supplement 12.9.
Figure 12.14. Calculation of lake volume for Iceberg Lake. (Panel A) 10 m contours draped over ASTER 321RGB. Contours generated from the ASTER DEM product derived from the underlying August 2003 ASTER image
when the lake was mostly drained (there remained a small residual lake near the dranage outlet). (Panel B) The filled
lake one year later. The white line is a best-fit contour representing the lake level. Volume calculations were
generated using this lake level. Note that the two time steps also document another drain–fill sequence that
occurred over a single-year interval (August 8, 2003 to August 10, 2004). Figure can also be viewed in higher
resolution as Online Supplement 12.10.
288
Glacier-dammed ice-marginal lakes of Alaska
5.46 10 6 m 2 (our DEM calculations), the total
lake volume was about 299 10 6 m 3 . Assuming
nearly all of the lake volume drained in less than
48 hours (as was the case documented in the 1999
paired imagery set, Fig. 12.9), the mean drainage
rate during the GLOF exceeded 1,700 m 3 /s, with
peak discharge undoubtedly several times this
value. The lake volume estimate (propagated also
into the mean discharge estimate), includes the
usual artifacts and errors of ASTER DEMs and
might be of the order of 10% of the reported
volume.
The time series shows that the lake can refill to
the IL shoreline of Loso et al. (2004) in as little as
1 year, though sometimes it may take 2 years. The
drainage basin covers about 92.7 km 2 (interpretation of divides from ASTER DEM, including parts
of the damming glacier and small areas upglacier).
This may be compared with other estimates of
66 km 2 (Loso et al. 2004) and 74 km 2 (Loso
2009), where the difference from our estimate is
related to areas of the damming glacier that we
included. Our field photographs show that additional waters drain into the lake beside the ice
dam (Online Supplement 12.3, G, H, I, O).
Total water contribution to IL, averaged across
the basin, was calculated to be 299 10 6 m 3 /92.7
km 2 ¼ 3,200 mm yr1 (estimated with 10% uncertainty) during the August 2003–August 2004 filling
cycle. This compares with mean annual precipitation at nearby Gulkana Weather Station (a deep
interior, ‘‘continental’’ setting) of about 280 mm
yr1 , and about 2,340 mm yr1 at coastal (maritime) Cordova Tide Station. Iceberg Lake has a
climate that is intermediate between those two
stations. We estimate local precipitation at Iceberg
Lake to be about 1,000 mm yr1 . Thus, water flux
into Iceberg Lake due to annual precipitation runoff is probably less than 93 10 6 m 3 yr1 (i.e.,
30% of the total water influx into the lake during
that one-year filling cycle). Next we examine possible additional sources of water flux into Iceberg
Lake to resolve the mathematical deficit.
Calving flux may be directly estimated from the
speed of ice moving onto the calving front (about
43 m yr1 from 1986 to 1999, according to the
time series displacement of a medial moraine into
the calving front), the length of the calving front
(about 1,500 m), and the height of the calving front
(minimum 63 m, which does not include the
subaqueous part; measured from field photos).
Allowing for converging flowlines and the necessary
acceleration of ice toward the ice-calving front, and
Table 12.6. Components of water influx to Iceberg
Lake for a one-year filling time.
Component of water influx
Mean annual precipitation over the basin
Damming glacier calving flux
Additional flux (net annual melting þ
englacial/subglacial flow)
Total
Estimated
million
m 3 yr1
<93
5
>201
299
for the calving front to extend somewhat beneath
the waterline, and considering a density of the
glacier of about 850 kg m 3 , we find that calving
flux may be of the order of 5,300,000 m 3 yr1 (water
equivalent, estimated with 20% uncertainty)—
that is, only 2% of total water influx. Whereas
icebergs are impressive from the ground level, up
close, the ‘‘sanity check’’ for this calculated
number is the comparatively small area of icebergs
in every satellite image of the time series compared
visually with lakebed area throughout the time
series.
We may reasonably infer that most of the water
influx during the August 2003–August 2004 filling
cycle was due to net annual negative balances of the
glaciers and melting of high-altitude perennial
snowfields to make up the rest (Table 12.6). The
negative balance magnitude must have been around
200 10 6 m 3 yr1 or more. For 52% glacierization
of the basin (Loso et al. 2006), this amounts to a
balance of glaciers close to 4.2 m yr1 water
equivalent (approaching 5 m yr1 change of
glacier thickness after allowing for micro and
macroscale porosity such as air-filled bubbles and
crevasses). We note that the satellite time series
indicates very little accumulation area in the entire
damming glacier. In some years there has been little
accumulation area even in the Bagley Icefield,
which feeds the Tana Glacier, whose thinning
affects the dam height of Iceberg Lake. Thus, the
whole glacier system is being starved of precipitation, which explains the rapid thinning rates of the
entire system. Ultimately, this thinning translates to
lowering of the ice dam height, which commonly
initiates GLOF cycles from GDLs, as appeared to
be the case with Iceberg Lake. Even before the
GLOFs started, ice dam lowering has progressively
lowered the threshold for GLOFs and thus caused a
Case study: Iceberg Lake 289
general lowering of the lake’s highstands, as Loso et
al. (2004, 2006) documented.
We see thinning happening for every glacier
contained in the area of the time series. This
pervasive thinning extends to most areas of Alaska.
For example, Berthier et al. (2010) found thinning
rates between the 1950s and 2007 mainly in the
range of 1 to 2 m yr1 for glaciers west of the
Copper River. However, that is far less thinning
than would be needed to supply water to fill Iceberg
Lake. Larsen et al. (2007) have documented as
much as 640 m thinning of glaciers to the southeast
of Iceberg Lake since the late 1940s, with an average
area mass loss from thinning in excess of 12 km 3 /yr.
However, the most rapid thinning rates primarily
affect tidewater glaciers, though most land-terminating glaciers in the region also are thinning.
A few glaciers in the Berthier et al. (2010) study
similarly have substantially greater absolute values
of negative balances than typical; they also have
exceptional
lacustrine
or
tidewater-calving
dynamics (such as Columbia Glacier). This process
along with those huge thinning rates may seem
irrelevant to the IL system, but a 1973 Landsat 1
image of IL and the Tana Glacier terminus shows
that the present terminus moraine-dammed lake
developed almost exclusively from dominating
damming glacier contribution to the Tana Glacier
terminus. Calving flux into IL, as a source of refill
water, is a volumetrically minor part of IL’s total
water budget. Whereas Tana Glacier is calving and
may be causing great loss of ice volume for the
whole system, including propagation upglacier to
induce thinning of IL’s damming glacier, it would
have little to do with rapid upglacier water production and filling of IL.
The lake has not necessarily filled to the same
level nor emptied every year; but drainage has been
a nearly annual event since 1999 (Table 12.5). We
would deduce glacier surface elevation balances less
negative than the 5 m yr1 estimated here during
years when IL only partially refills. Finally, we do
not discount further possible contributions from
subglacial drainage into IL of other glacier lakes
(supraglacial, englacial, or subglacial), unseen and
unevaluated here.
In many parts of the world, GLOFs of the magnitude produced by Iceberg Lake would be catastrophic and tragic; it would garner newspaper
headlines for days or weeks after a tragedy. Here,
with no human habitation or development nearby,
GLOFs are rarely even noticed. We know that they
occur because satellite imagery and reports by
travelers and bush pilots show the lake there one
day and gone another, leaving nothing but stranded
icebergs and saturated silt to indicate the recent
drainage event.
12.5.5 Possible causes of Iceberg Lake’s
dynamical evolution
As Loso et al. (2004, 2006, 2007) and Diedrich and
Loso (2012) have documented and explained, and
as our observations support, the fundamental control of IL’s maximum levels and drainage behavior
is ice dam height, which, in this case, is a direct
function of the damming glacier’s thickness. The
series of strandlines is related to progressive lowering of damming height and an episodic response
and overall lowering of IL’s maximum fill level as
well as a lowering of the minimum level following
drainage events. As mentioned, current knowledge
shows complete drainages starting only in 1999,
although Stone (1963b) reported partial drainages
in 1948 and 1951.
Iceberg Lake’s basin is crossed by a large moraine
ridge; this is either a former lateral moraine left by
Kieffer Glacier, or an end moraine left by Chisma
Glacier as it detached from Kieffer Glacier and then
retreated further. For a long period of time, this
moraine marked the southward extent of Iceberg
Lake, but as Kieffer Glacier thinned and its margin
retreated, Iceberg Lake expanded to the south,
where the remnant lake is now positioned after
drainage events. The 1951 air photo by Stone
(1963b; Fig. 12.13) shows that this moraine was
submerged. As the damming glacier retreated, there
must have been a point where ice effectively
detached from Iceberg Lake and, for a time, the
lake would have been a moraine-dammed lake. This
condition might not have lasted long as the current
geometry, following expansion of IL south of the
moraine ridge, again involved damming by Kieffer
Glacier. It may have been this incising of the moraine ridge that triggered conditions whereby episodic complete drainage of IL could occur.
A common GDL release mechanism involves
hydrostatic pressure lifting or floating the ice dam
once the lake fills to 90% of the dam height, but
photographs have documented the full lake nearly
overtopping this ice dam. The moraine dam relieved
this pressure against the basal ice, allowing Iceberg
Lake to fill until it overtopped the ice dam, forming
lateral channel incisions, until the moraine dam was
incised at some point. How or when the incision
occurred has yet to be addressed.
290
Glacier-dammed ice-marginal lakes of Alaska
Thinning of the damming glacier, however,
brought about long-term episodic drawdown of
IL leading up to the recent period of episodic
drainage and partial refilling due to the lake now
being in contact with the ice. But what is driving or
causing the damming glacier to thin? The entire
system—extending from the Bagley Icefield to outlet glaciers such as Tana and Bering, and Iceberg
Lake’s damming glacier, and multiple GDLs (IL
among them) and moraine-dammed lakes—has
incredibly complex dynamics. The following variables and mechanisms may be involved:
1. Atmospheric warming—leading to increased
glacier surface ablation and an overall negative
mass balance of the system, driving glacier thinning and lowering ice dam height.
2. Changing precipitation patterns—while coastal
areas are receiving increased annual precipitation, continental areas are seeing a decrease.
The seasonality of snow and rain precipitation
is also changing.
3. Detachment of some tributary glaciers feeding
Tana Glacier and IL’s damming glacier—thus
cutting off some of the supply of ice and contributing to thinning of what remains.
4. Influence of glacier lakes as a result of adding a
calving flux and frontal melting flux—thus also
contributing to downwasting.
5. Bouyancy-driven reduction in basal shear stress
and large-scale disarticulation of the Tana Glacier
tongue—as the proglacial moraine-dammed lake
has grown since the 1970s.
6. Intrinsic unsteadiness of glacier flow—including
the surge/waste cycles of Bering Glacier and indications of surge-like behavior of Tana Glacier
(Arendt et al. 2008). This behavior may be
related to formation and subglacial drainage of
supraglacial and ice-marginal lakes. It may also
be promoted by high-elevation thickening (due
to increased snow precipitation) in concert with
low-elevation thinning (due to warming temperatures), by increased longitudinal gradient
caused by the thickening/thinning pattern, and
by possible increased sensitivity of basal slip
conditions to turning the basal meltwater supply
on and off due to lake drainages.
Ultimately, each of these variables may be influenced by atmospheric warming, though surge–
waste activity has a long-documented history of
occurring with glaciers in long-term steady state
across the larger study area. All of these variables
are interrelated and involve meltwater in some way,
and ultimately each is individually capable of influencing IL’s ice dam height and the lake’s drainage
behavior. However, each associated process occurs
over different (albeit, poorly defined) timescales.
The surge–waste cycle of the Bering Glacier profoundly influences the Bagley Icefield (Burgess et al.
2012), which is a primary source of the Tana
Glacier. Even though Bering Glacier drains the
Bagley Icefield down the opposite side of the range
from the Tana Glacier and Iceberg Lake, there is a
propagation of effects into the Tana Glacier
(Arendt et al. 2008), due to influences on the location of the drainage divide (Burgess et al. 2012), and
so there could be influences as far as Iceberg Lake.
However, the surge–waste timescale for Bering
Glacier is about 15 years (i.e., far less than the
century length response time of Tana Glacier), indicating a complex relationship. The Tana Glacier,
however, has exhibited a thickening/thinning pattern in concert with the recent Bering Glacier surge
(Arendt et al. 2008), and this would affect Iceberg
Lake more directly, since the timescale for dynamical responses is much shortened compared
with how Tana would respond to Bering Glacier’s
dynamics. The first five processes listed above have
much longer timescales and so they could very well
be controlling the century-long thinning of IL’s
damming glacier, and hence, the drainage behavior
of the lake.
From all available data, our best surmise is that
Tana Glacier’s tongue and the whole system
reached a Little Ice Age maximum around the turn
of the 20th century; the tongue then was stably
positioned but started thinning, and that condition
prevailed until the 1970s. Other glaciers in Alaska
also generally were near their maximum elongations
around 1900, and then had lost mass, many with
supraglacial and glacier-dammed lake formation,
by the 1970s. From the 1970s, satellite imaging of
the Iceberg Lake/Tana Glacier area is fairly comprehensive and clearly shows transitions (1) from a
thermokarstic but relatively ‘‘dry’’ tongue of Tana
Glacier in the 1970s, (2) to a pond-riddled surface
in the 1980s, (3) to larger supraglacial and icemarginal lakes in the 1990s, (4) to rapidly coalescing
lakes in the early 2000s, and (5) to a disarticulated
shattered state starting about the time of our first
visit in 2008. Meanwhile, Iceberg Lake was full
and remained fairly stable from earliest Landsat
imaging in the early 1970s, with minor lake level
fluctuations, until 1999, as recognized by Loso and
his colleagues. The varve evidence accumulated by
Discussion and conclusions 291
those researchers indicates that the comparative
stability of Iceberg Lake had prevailed for the preceding 1,500 years, at least, until the drain/fill cycles
started in 1999.
Mass loss from the Little Ice Age maximum to
the 1970s was primarily a response to post-LIA
warming, because anthropogenic warming had
barely begun by the 1970s. However, the rapid
degradation of the whole system from the Bagley
Icefield to the Tana Glacier, Kieffer Glacier, and
Iceberg Lake in the late 20th and early 21st century
probably is a response mainly to anthropogenic
warming, which we think probably includes the
1975–1978 increase of temperatures in Alaska
(Fig. 12.2). This renewed phase of glacier thinning,
lake formation, and retreat on Tana Glacier and the
tributary Kieffer Glacier has produced conditions
under which Iceberg Lake has started its present
regime of repeated drain/fill cycles and has most
likely set the stage for the lake’s eventual permanent
disappearance.
12.6
DISCUSSION AND CONCLUSIONS
Ice-marginal glacier-dammed lakes are unique
complex dynamic manifestations of hydrologic
cycles associated with active glaciers and the array
of processes and changes that their damming
glaciers exhibit. Over recent millennia and within
recent historic times, both GDLs and the glaciers
that dam them have changed dramatically. It is
expected that lakes invariably will form on and near
the ablation zones of glaciers, and that any icedammed lakes are fundamentally unstable and subject to growth or episodic drainage or disappearance, triggered by stochastic or periodic dynamical
fluctuations of glaciers that would exist whether the
glaciers were overall in decline, advance, or steady
state. Alaskan glaciers are retreating and thinning,
as has been documented in many publications, and
the loss rates have been increasing in recent decades. However, the rate of change has differed by
location: different regions (or mountain ranges) in
Alaska are behaving differently across the study
area; but also within a glacier system, results differ
by origin type, terminus type, aspect, and perhaps
gradient. In this research, significant changes found
heterogeneously distributed horizontally within a
glacier system, as well as vertically, longitudinally,
and latitudinally across the study area, appeared to
be similar to differing rates of temperature increase,
precipitation change, and modeled solar input
across the study area.
Here, following the hypothesis first put forth by
Thorarinsson (1939) but utilizing modern methods,
we have identified changes in GDLs and lakedamming glaciers that occurred between the 1971
survey by Post and Mayo and the ASTER/ETMþ
era, a period marked by a nearly stepwise increase
in temperature (Fig. 12.2A) and other climatic
factors.
The changing dynamics of some larger icemarginal GDLs give an indication of the array of
variables associated with lake development, longevity, drain and refill cycles, and demise. These
large lakes also provide insights into how melting
and thinning/retreating of glaciers can cause disappearances of, or initiate episodic drainage behavior
from long-time persistent lakes. The case of Iceberg
Lake illustrates some of the complex controls on
the formation, stability, and drainage of GDLs.
The lake formed in a tributary valley vacated by
the Chisma Glacier, as it detached and retreated
from the trunk valley–damming glacier. The lake
remained in a relatively stable state of hydrologic
equilibrium so long as the damming glacier was
thick, flowing, and tending to maintain closure of
drainage conduits or maintaining contact with the
moraine dike. As the damming glacier thinned since
the LIA, Iceberg Lake was drawn down in stages, as
Lake George had been. In recent decades, the damming glacier has thinned dramatically, at a greater
rate, and retreated from the moraine dam, enabling
the lake to establish basal ice contact. The lake
therefore has emptied, almost completely, on a
nearly annual basis. The immediate control on
Iceberg Lake’s volume is thus the dam height and
the vigor of flow of the damming glacier. Ultimately, control of the damming glacier and, by
extension, of Iceberg Lake is the overall mass balance of the entire system extending up to the Bagley
Icefield. As climatic changes have increased surface
melting, and melting due to glacier–lake interactions (especially on Tana Glacier’s tongue), this
has caused decreased ice volume in the system,
decreasing dam height, and reduced Iceberg Lake
volume. Thus, atmospheric warming and increased
melting have contributed to destabilization of the
ice dam, leading to frequent drain and refill cycles of
Iceberg Lake.
While there have been few lakes tracked over
long time periods—such as Lake George due to
GLOF impacts on downstream infrastructure,
and Iceberg Lake due to well-preserved presumably
292
Glacier-dammed ice-marginal lakes of Alaska
annual sediment layers—there has been little
research into where a lake might form. There has
also been a dearth of research comparing the
attributes of lake-damming glaciers. Updating a
glacier dammed lake population map in this study
has yielded an opportunity to obtain and analyze
the attributes of both damming glaciers and the
locations of the lakes for two time periods separated
by distinct atmospheric warming (2.35 C on the
west side of the study area; 1.77 C on the east side;
Fig. 12.2B).
As a result, we have put together a few characteristics of lake-damming glaciers and GDLs and
documented their changes during a warming
period:
. The recent glacier-dammed lake survey found 160
lake-damming glaciers, with 71 (44%) damming
new lakes; 31 of these (19% of all recent damming
glaciers; 44% of the glaciers damming new lakes)
were not previously known to dam lakes.
. Lakes are forming beside longer (8 km) more
complex (up from third to fourth order) glaciers.
. Greater percentages of (1) absent lakes had been
dammed by cirque origin glaciers, and (2) persistent lakes were more commonly dammed by
glaciers originating in icefields than expected by
chance.
. Thinning, or downwasting, appeared to be the
primary factor in loss of the ice dam.
. Lakes were more likely to persist if the damming
glacier terminated in a lake rather than on land.
. Glaciers damming new lakes were more complex
overall, and were an average 11% (4.7 km) longer
than those damming absent lakes, and 7% (3.2
km) longer than those damming persistent lakes
. The mean elevation of grouped new lakes was
significantly higher than both absent and persistent lake groups. Both grouped new lakes and
grouped persistent lakes were vertically closer
to damming glacier mean glacier altitude than
absent lakes had been.
. Lakes tend to persist longer and form anew farther
up the length of damming glaciers (population:
6 km farther from the terminus; new lakes 20%
farther up their damming glaciers than absent
lakes were up theirs), and 120 m vertically closer
to the mean glacier altitude.
As an indication of the dynamic complexities
involved, lakes were more likely to persist if they
were dammed by a lake-terminating glacier than if
they were dammed by a land-terminating glacier,
related, we presume, to greater ice dam loss through
thinning than by terminus retreat. The population
of lake-damming glaciers appeared to be melting in
place—terminus stagnation—and have far less terminus retreat than other studies have indicated of
the overall glacier population.
Furthermore, we developed empirical data on
some characteristics of glacier-dammed lake development locations. A few of the variables most
clearly identifiable that could readily be incorporated into GDL detection and monitoring models
include
. Mean lake elevation in maritime zone mountains
was 500–750 m asl.
. Mean lake elevation in continental mountains was
900–1,700 m asl.
. Lakes were typically located 400–640 m (vertically) below mean glacier altitude.
. Lakes were typically located 8–20 km upglacier
from the terminus (or 20–40% up the length of the
damming glacier).
. Lakes were uniformly near a stream order of 2.9.
. Ice dams primarily face aspects between northnorthwest and east in the recent population.
As the GLIMS database continues to expand, many
of the background data required to develop these
statistics for other areas are becoming available.
12.7
ACKNOWLEDGMENTS
This chapter is dedicated to the memory of Austin
Post, a remote-sensing and glaciology pioneer. Dr.
Michael G. Loso and Dr. Roman Dial, Alaska
Pacific University (APU), Anchorage, assisted with
D.F.G.W.’s research. Portions of this research were
funded by a NASA IPY grant to J.S.K., and by
APU and AWRA grants to D.F.G.W. Assistance
with clarification on the 1971 lake research from the
late Austin Post is gratefully acknowledged, as is
the provision of high-resolution DOQQ aerial imagery of Chugach National Forest glaciers by the
USFS, and the cooperation and suggestions provided by NOAA, USGS, and USNPS personnel.
ASTER data courtesy of NASA/GSFC/METI/
Japan Space Systems, the U.S./Japan ASTER
Science Team, and the GLIMS project.
References 293
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