Quaternary Science Reviews 19 (2000) 1763—1783
A review of catastrophic drainage of moraine-dammed lakes
in British Columbia
John J. Clague *, Stephen G. Evans
Department of Earth Sciences and Institute for Quaternary Research, Simon Fraser University, 8888 University Drive, Burnaby, BC Canada V5A 1S6
Geological Survey of Canada, 101 — 605 Robson St., Vancouver, BC Canada V6B 5J3
Terrain Sciences Division, Geological Survey of Canada, 601 Booth Street, Ottawa, Ont., Canada K1A 0E8
Abstract
Moraine-dammed lakes are common in the high mountains of British Columbia. Most of these lakes formed when valley and cirque
glaciers retreated from advanced positions achieved during the Little Ice Age. Many moraine dams in British Columbia are
susceptible to failure because they are steep-sided, have relatively low width-to-height ratios, comprise loose, poorly sorted sediment,
and may contain ice cores or interstitial ice. In addition, the lakes commonly are bordered by steep slopes that are prone to snow and
ice avalanches and rockfalls. Moraine dams generally fail by overtopping and incision. The triggering event may be a heavy rainstorm,
or an avalanche or rockfall that generates waves that overtop the dam. The dam can also be overtopped by an influx of water caused
by sudden drainage of an upstream ice-dammed lake (jökulhlaup). Melting of moraine ice cores and piping are other possible failure
mechanisms. Failures of moraine dams in British Columbia produce destructive floods orders of magnitude larger than normal
streamflows. Most outburst floods are characterized by an exponential increase in discharge, followed by an abrupt drop to
background levels when the water supply is exhausted. Peak discharges are controlled by dam characteristics, the volume of water in
the reservoir, failure mechanisms, and downstream topography and sediment availability. For the same potential energy at the dam
site, floods from moraine-dammed lakes have higher peak discharges than floods from glacier-dammed lakes. The floodwaters may
mobilize large amounts of sediment as they travel down steep valleys, producing highly mobile debris flows. Such flows have larger
discharges and greater destructive impact than the floods from which they form. Moraine dam failures in British Columbia and
elsewhere are most frequent following extended periods of cool climate when large lateral and end moraines are built. A period of
protracted warming is required to trap lakes behind moraines and create conditions that lead to dam failure. This sequence of events
occurred only a few times during the Holocene Epoch, most notably during the last several centuries. Glaciers built large moraines
during the Little Ice Age, mainly during the 1700s and 1800s, and lakes formed behind these moraines when climate warmed in the
1900s. Twentieth-century climate warming is also responsible for recent moraine dam failures in mountains throughout the world.
Warming from the late 1800s until about 1940 and again from 1965 to today destabilized moraine dams with interstitial or core ice.
The warming also forced glaciers to retreat, prompting ice avalanches, landslides, and jökulhlaups that have destroyed some moraine
dams. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction
Many lakes impounded by moraines are a threat to
people and property in mountain regions throughout the
world. The dams may fail suddenly, producing destructive floods with peak discharges far in excess of normal
flows. Notable examples include four large moraine dam
failures in the Cordillera Blanca of Peru between 1938
and 1950 (Heim, 1948; Lliboutry et al., 1977). The most
devastating of the four events was the 1941 emptying of
* Corresponding author. Fax: #001-604-291-4198.
E-mail address: jclague@sfu.ca (J.J. Clague).
moraine-dammed Lake Cohup which destroyed onethird of the city of Huaraz, killing more than 6000 people.
Recent large outbursts from moraine-dammed lakes have
also been reported in other parts of the Andes (Reynolds,
1992; Hauser, 1993), the Himalayas (Gansser, 1983;
Vuichard and Zimmerman, 1987; Ding and Liu, 1992;
Reynolds, 1995, 1998; Watanabe and Rothacher, 1996;
Cenderelli and Wohl, 1997), the mountains of central
Asia (Niyazov and Degovets, 1975; Yesenov and Degovets, 1979; Popov, 1990), and North America (Evans,
1987; O’Connor and Costa, 1993; Clague and Evans,
1994; Evans and Clague, 1994; O’Connor et al., 1994)
(Fig. 1; Table 1). The draining of Laguna del Cerro Largo
in Chile in 1989 involved the release of 229;10 m of
0277-3791/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved.
PII: S 0 2 7 7 - 3 7 9 1 ( 0 0 ) 0 0 0 9 0 - 1
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Fig. 1. Locations of well documented moraine dam failures (see also
Table 1).
water (Hauser, 1993), the largest moraine dam failure
documented in the literature.
Moraine-dammed lakes are common in the Cordillera
of western Canada, and many of them have drained
suddenly in recent years, producing floods and debris
flows (Fig. 2). The phenomenon, however, is not well
understood in Canada, partly because moraine dam failures have occurred in remote mountain valleys where they
have gone largely unnoticed. Recently, development associated with forestry and mining, hydro-electric power generation, and recreational activities has extended into these
remote valleys, and there is now more of a need to understand potential hazards associated with outburst floods.
In this paper, we review and discuss moraine dam
failures in British Columbia. Our main objectives are to
document how moraine dams form and fail, and to provide insights on failure processes and hazards.
Fig. 2. Known, historical moraine dam failures in British Columbia
(see Table 2).
2. Setting
Lateral and end moraines are common in glacier forelands outside present ice margins in most high mountain
ranges, including those in western Canada (Fig. 3). The
moraines were constructed during a recent period of cool
Table 1
Examples of moraine dam failures
Site
Location
Date of failure
Madatschferner
Galrittferner
Cohup
White Branch
Tempanos
Jancaruish
Austria
Austria
Peru
Oregon, USA
Argentina
Peru
August 7, 1890
December 13, 1941
July 1942
1942-1953
1950
6-10
Artesoncoda
Broken Top
Squaw Creek
Safuna Alta
Peru
Oregon, USA
Oregon, USA
Peru
July 16-17, 1951
October 7, 1966
September 7, 1970
1970
0.19
0.33
4.9
Moraine no. 13
Mingbo Valley
Soviet Union
Nepal
August 3, 1977
September 3, 1977
0.08
4.9
Dig Tsho
Nepal
August 4, 1985
8
Adapted from Costa and Schuster (1988, Table 5).
Volume stored in lake.
Outburst
volume(m;10)
Failure mechanism
Ice avalanche
Excess meltwater
Collapse of undercut
glacier
Ice avalanche
Ice avalanche
Earthquakeinduced piping
Melt of frozen soil
Melt of ice core?
Excess runoff?
Ice avalanche
Reference
Eisbacher and Clague (1984)
Eisbacher and Clague (1984)
Lliboutry et al. (1977)
Hopson (1960)
Rabassa et al. (1979)
Lliboutry et al. (1977)
Lliboutry et al. (1977)
Nolf (1966)
Laenen et al. (1992)
Lliboutry et al. (1977)
Yesenov and Degovets (1979)
Buchroithner et al. (1982)
Fushimi et al. (1985)
Galay (1985); Vuichard and
Zimmerman (1987)
J.J. Clague, S.G. Evans / Quaternary Science Reviews 19 (2000) 1763—1783
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Fig. 3. Moraine-dammed Nostetuko Lake in the southern Coast Mountains of British Columbia as it appeared in July 1977 (view south; photograph
by J.M. Ryder). The large moraine impounding the lake was constructed during the Little Ice Age. Compare with Fig. 9, a photograph taken after the
outburst flood of July 19, 1983.
Fig. 4. Age distribution of Little Ice Age moraines in the Canadian
Rocky Mountains (modified from Luckman, 2000, Fig. 4).
climate that ended in the late nineteenth century (the
“Little Ice Age” (Matthes, 1939; Grove, 1988), which is
the last part of the Neoglacial period (Porter and Denton, 1967)). Researchers working in British Columbia and
Alberta have shown that these moraines date mainly to
the eighteenth and nineteenth centuries (e.g., Ryder and
Thomson, 1986; Luckman, 2000; Fig. 4). Little Ice Age
advances also occurred in other parts of the world during
this period (Grove, 1988).
In many areas, glaciers began to retreat from their
Little Ice Age moraines at the end of the nineteenth
century (Fig. 5). Retreat was rapid and substantial during
the warmer decades of the twentieth century, especially in
the 1920s and 1930s, and from about 1980 until today.
Lakes formed in closed basins between Little Ice Age
moraines and retreating glaciers (Fig. 5). In some instan-
Fig. 5. Schematic diagram showing the relation between glacier thinning and retreat, and the formation and destruction of moraine-dammed
lakes (modified from Evans and Clague, 1993, Fig. 8). Avalanching of
glacier ice (A) into the lake generates waves that overtop the moraine
dam (B), initiating breaching.
ces, lakes drained slowly soon after they formed as the
moraine dams were slowly eroded. In other cases, however, overflow channels on the moraines became naturally armored and the lakes persisted.
Most moraine-dammed lakes occur in cirques or the
upper reaches of steep-walled valleys behind end moraines (Fig. 6). Other, less common settings include trunk
valleys behind lateral moraines deposited by ice flowing
out of tributary valleys, and tributary valleys behind
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Fig. 6. Sketch showing principal locations of moraine-dammed lakes.
(1) Tributary valley, between moraine and glacier that deposited the
moraine; (2) tributary valley, in front of moraine deposited by trunkvalley glacier (moraine blocks mouth of tributary valley); (3) trunk
valley, adjacent to moraine of tributary-valley glacier (moraine blocks
trunk valley).
(Fig. 8). The sediment was transported both within and at
the surface of the glacier and released on a growing ridge
at the glacier snout. Some moraines consist largely of
coarse, blocky and bouldery material with a matrix of
sand and gravel. Blocks and boulders up to several
metres across may constitute up to 80% of the moraine.
These materials resemble rockslide deposits and, in fact,
are accumulations of supraglacially transported landslide debris. Other moraines consist of silty and sandy
diamicton and sandy gravel (Fig. 8). These deposits
contain 25—50% gravel-size material and are generally
finer than the blocky and bouldery deposits described
above. The sediment is crudely stratified; strata dip either
irregularly or away from the former ice-contact face
towards the distal edge of the moraine (Fig. 8). Some
moraines are ice-cored and may have only a thin cover
of sediment (Ostrem and Arnold, 1970). Others lack
ice cores but contain interstitial ice in pores within the
sediment.
4. Historical failures in British Columbia
lateral moraines deposited by ice flowing in trunk valleys.
Most, although not all, moraine-dammed lakes lie near
or above treeline and may be close to steep rock slopes
susceptible to landslides and snow and ice avalanches.
3. Characteristics of dams
Moraine dams may consist of a single moraine or two
or more moraines deposited during several Neoglacial
advances and separated by swales. Most moraines are
steep-sided (up to ca. 40°) and have width-to-height ratios of 0.1—0.2. Width-to-height ratios are generally
smaller than those of landslide dams and comparable to
those of constructed dams (Fig. 7; Costa and Schuster,
1988). Heights of a few tens of metres are common, and
some moraine dams are more than 100 m high.
Moraine dams comprise loose, poorly sorted, stratified
to massive sediment deposited directly from glacier ice
At least nine moraine dams failed in British Columbia
during the historical period — five in the Coast Mountains, one in the Saint Elias Mountains, and three in the
Columbia Mountains (Fig. 2; Table 2; Evans, 1987;
Ryder, 1991; Clague and Evans, 1992, 1994; Clague and
Mathews, 1992; Evans and Clague, 1993, 1994). Five of
the nine events are described briefly below to illustrate
the scope of the phenomenon. None of the failures was
directly witnessed.
4.1. Nostetuko Lake
Nostetuko Lake (ca. 1600 m asl), located at the head of
Nostetuko River 230 km north of Vancouver, is dammed
by a Little Ice Age end moraine that probably dates to
the 1800s (Fig. 3). The moraine was built by Cumberland
Glacier, which has since receded and now terminates on
a steep rock face above the lake.
Fig. 7. Cross-sections of Nostetuko and Queen Bess moraine dams, the Mayunmarca landslide dam, and the Oroville earth-fill dam (modified from
Costa and Schuster, 1988, Fig. 13). Inset defines moraine width (w) and height (h). Most moraine dams have width-to-height ratios similar to those of
constructed dams, but several times less than those of landslide dams.
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Fig. 8. Poorly sorted, bouldery sediments exposed in the breached, Little Ice Age moraine at Nostetuko Lake, British Columbia. Most moraine dams
in the Canadian Cordillera are composed of such materials. The height of the exposure is about 40 m.
Table 2
Moraine dam failures in western Canada
Site
Date of failure
Outburst
volume
(m;10)
Peak discharge Effect
(m/s)
Failure
mechanism
Tide Lake
1927—1930
Bridge Glacier
Klattasine Lake
Nostetuko Lake
1964—1970
1971—1973
July 19, 1983
1—2
1.7
6.5
(1000
'1000
ca 10 000
Flood
Flood/debris flow
Flood
North Macoun Lake
South Macoun Lake
Patience Mountain
Tats
July 1983
Before 1949
1951—1966
June 28, 1990
0.4
0.3
0.004
(1000
(1000
(1000
Flood
Flood/debris flow
Flood/debris flow
Flood/debris flow
Piping?
Ice avalanche?
Ice avalanche?
Queen Bess Lake
August 12, 1997
8
'1000
Flood
Ice avalanche
Flood
Ice avalanche
Reference
Clague and Mathews
(1992)
Ryder (1991)
Clague et al. (1985)
Blown and Church
(1985)
Evans (1987)
Evans (1987)
Evans (1987)
Clague and Evans
(1992)
Unpublished
Other partial breachings, accompanied by floods, may have occurred between 1935 and 1947.
On July 19, 1983, part of the toe of Cumberland
Glacier broke away and cascaded into the lake. A wave
generated by the impact of the ice avalanche moved
across the lake and overtopped the moraine dam. The
wave eroded the inner face of the moraine near the outlet
to a height of 4 m above the shoreline (Blown and
Church, 1985). The outflow channel on the moraine was
naturally armoured with a boulder lag, but the overtopping wave breached the armour and initiated catastrophic incision. Within four hours, the moraine had
been incised to a depth of almost 40 m and about
6.5;10 m of water had been released (Fig. 9; Blown
and Church, 1985). The resulting flood swept down Nostetuko and Homathko valleys to the sea (Fig. 10).
Over 1;10 m of sediment were eroded from the
moraine; most of this sediment was deposited as a large
fan on top of a former meadow directly downstream from
the dam (Fig. 9). Farther downstream, floodwaters extensively eroded unconsolidated deposits in Nostetuko valley, damaged large tracts of forest, and left piles of trees,
other woody debris, and coarse sediment on bars and
channel margins. Erosion was particularly severe at
bends along the river where colluvial fans extended out
onto the valley floor. Much of the sediment eroded from
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Fig. 10. Cross-sections showing the maximum depth of the 1983 Nostetuko outburst flood in Nostetuko River valley. Distances from Nostetuko Lake
are indicated at the upper left of the cross-sections.
channels and valley sides was transported short distances and then redeposited, contributing to local
aggradation of the valley floor. However, along steeper
reaches of the river, the valley floor was stripped clean
of sediment by the floodwaters (Fig. 11). Fine material
(sand, silt) was carried a long distance downstream
and deposited in overbank areas. Forest was flattened
where the flood and floating debris overflowed channel
banks.
The flood attenuated as it moved downvalley. The two
Water Survey of Canada gauging stations closest to
Nostetuko Lake were damaged and ceased functioning,
but two other gauging stations remained operational.
A hydrograph at a site 67 km downstream from dam
showed an increase in discharge from the background
level of 330 m s\ to over 900 m s\ in one hour
(Fig. 12). At a site a further 45 km downvalley, the increase in discharge was less and slower (Fig. 12). Never-
theless, the flood was still large enough at the mouth of
Homathko River, 115 km downstream from the dam, to
briefly inundate part of a logging camp. The average
velocity of the flood wave between the two operational
gauging stations was about 10 km h\.
In the 18 yr since the event, Nostetuko River has
incised its flood deposits, but it has not yet recovered its
pre-flood channel planform and morphology. This observation suggests that such rare flood events may affect
streams for decades.
4.2. Queen Bess Lake
Queen Bess Lake (1692 m asl) is located near the head
of a major tributary of Nostetuko River, 9 km northwest
of Nostetuko Lake. The lake is dammed by Little Ice Age
moraines built in the nineteenth and possibly eighteenth
centuries.
䉳
Fig. 9. Digital image of Nostetuko Lake and the breached, Little Ice Age moraine of Cumberland Glacier (view southeast). The snout of Cumberland
Glacier fell into the lake in July 1983 and generated a wave that overtopped and incised the moraine. The digital image consists of an orthophoto,
based on July 1994 aerial photographs, draped on a digital elevation model obtained from the same photos. Changes in surface elevation between July
1981 and July 1994 are shown in colour tones. Dramatic surface lowering is seen at the snout of Cumberland Glacier (the blue and purple area behind
Nostetuko Lake). This lowering is the result of the 1983 glacier avalanche and climate-related glacier retreat. Surface lowering at Nosteuko Lake
records the fall in lake level due to failure of the moraine dam. Note deposition of debris on the large fan in the foreground. Compare this figure with
Fig. 3, which is a pre-outburst photograph.
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Fig. 11. Lower Nostetuko River valley in August 1984, approximately one year after the 1983 outburst flood (view south, upriver). All sediments and
trees were stripped from the valley floor by the floodwaters, which were up to about 5 m deep here.
Queen Bess Lake drained suddenly on August 12—13,
1997, when its moraine dam was overtopped and incised
by a displacement wave triggered by a large ice avalanche
from Diadem glacier (informal name) (Fig. 13). The wave
overrode a 600 m length of the moraine crest to depths of
15—25 m (Fig. 14). The avalanche occurred during
a warm rainy period when large amounts of water were
discharging from the base of Diadem glacier. About
8;10 m of water escaped from the lake, producing
a flood that stripped vegetation from the valley floor
below the dam (Fig. 14).
The level of Queen Bess Lake dropped 8 m, and
a steep-walled ravine up to 15 m deep, 60—75 m wide, and
360 m long was carved into the end moraines at the site
of the former outflow channel (Fig. 15). Sediment eroded
from the ravine was carried downstream by floodwaters
and deposited on a fan 200 m from the moraine. Incision
of the moraine was limited by bedrock cropping out on
the floor of the ravine at its north end. Were it not for this
sill, outflow from the lake probably would have more
deeply incised the moraine, and a much larger volume of
water would have been released.
The flood wave surged 9 km down the west fork of
Nostetuko River to the main Nostetuko valley (Fig. 14).
It travelled another 12 km in Nostetuko valley to
Homathko River, and thence west down Homathko valley. Although the flood attenuated as it moved downvalley, it still was noticed by forestry personnel at the mouth
of Homathko River, 100 km from the source.
The floodwaters extensively eroded Quaternary deposits in the west fork of Nostetuko valley, damaged
tracts of forest, and left wood debris and coarse sediment
on bars and channel margins (Fig. 15). Margins of many
colluvial fans impinging on the valley floor were eroded
back tens of metres, leaving steep unstable scarps
(Fig. 15). Most of the sediment eroded from channels and
valley sides was transported short distances and then
redeposited, contributing to aggradation of the valley
floor. Steeper, rock-controlled reaches of the river
were stripped clean of sediment by the floodwaters.
Boulders up to 10 m across were transported in traction
through these reaches and deposited on more gently
sloping sections of the valley floor. Fine sediments
were deposited upstream of channel constrictions where
water was hydraulically ponded during the flood. In the
two years since the outburst, Nostetuko River has incised
its flood deposits, but it has not yet recovered its preflood channel morphology and is unlikely to do so for
decades.
4.3. Klattasine Lake
Klattasine Lake (1640 m asl) is situated at the head of
the west fork of Klattasine Creek, 35 km west of
Nostetuko Lake. The moraine impounding Klattasine
Lake dates to the nineteenth century. It failed sometime
between June 1971 and September 1973, causing the
sudden release of 1.7;10 m of water from the lake
(Clague et al., 1985). Escaping waters trenched the
moraine down to a bedrock sill, which now forms the
stable outlet of the remnant lake (Fig. 16). The cause of
the failure is unknown.
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tasine Creek (Fig. 18). This debris dammed Homathko
River, but the blockage was breached four days later and
a secondary pulse of water and sediment moved downstream. In the nearly 30 yr since the event both
Homathko River and Klattasine Creek have dissected
the debris fan.
4.4. Tide Lake
Fig. 12. (A) Hydrographs of the 1983 outburst flood from Nostetuko
Lake (modified from British Columbia Hydro and Power Authority,
1983, Fig. 4). The lower hydrograph is from a site 67 km downstream
from the dam and shows an increase in discharge from 330 m s\ to
over 900 m s\ in one hour; the upper hydrograph, from a site a further
45 km downvalley, shows a somewhat attenuated flood wave. (B)
Sketch showing attenuation of a hypothetical outburst flood as it
travels downvalley from a breached moraine dam (vertical and horizontal scales are dimensionless). (1) Site near the moraine; (2) mid-valley
site; (3) site distant from moraine. The total volume of the flood is the
same at each of the three sites, but flood height decreases and flood
period increases downvalley. Base flows also increase downvalley due
to normal inputs from tributary streams.
The Klattasine event is instructive because it was a
debris flow rather than a flood. The escaping waters
eroded sediment from the moraine dam and, more importantly, mobilized large quantities of sediment along
the channel and margins of the steep valley below the
lake. In this manner, the flood rapidly evolved into a debris flow that travelled 8 km to Homathko River valley
(Fig. 17). Destruction of forest in the path of the debris
flow indicates a fairly rapid movement, although debris
did not rise significantly at bends in the valley. Scour of
the valley floor by the debris flow locally destabilized
adjacent valley slopes, causing secondary slope failures
(Fig. 17).
The debris flow deposited a sheet of cobbly and bouldery sediment up to 20 m thick at the mouth of Klat-
Tide Lake (ca. 680—790 m asl), in the northern Coast
Mountains of British Columbia, differs from Nostetuko,
Queen Bess, Klattasine, and most other moraine-dammed lakes in the Canadian Cordillera in that it was
impounded at the toe of a glacier flowing into a trunk
valley (Case 3 in Fig. 6). The lake formed repeatedly
during the Neoglacial period when Frank Mackie Glacier advanced across Bowser valley and blocked Bowser
River (Clague and Mathews, 1992; Clague and
Mathewes, 1996).
At its maximum, Tide Lake was 9 km long, 200 m deep
at the ice dam, and had a volume of 1100;10 m.
Although probably stable for long periods, the lake occasionally drained catastrophically beneath or at the margin of Frank Mackie Glacier, producing jökulhlaups
with peak discharges of 5000—10,000 m s\ (Clague and
Evans, 1994).
Frank Mackie Glacier built a suite of lateral and end
moraines during the Little Ice Age. As this morainal
complex grew in size, it created a growing barrier to
drainage beneath the glacier. In the late 1800s or early
1900s, Frank Mackie Glacier retreated from the end
moraine and no longer directly blocked the Bowser River
drainage. Tide Lake, however, continued to be dammed
by the moraine. The moraine dam persisted until sometime between 1927 and 1930 (Hanson, 1932), when it was
breached by overflow from Tide Lake, perhaps during
a period of rapid snowmelt or heavy rainfall. Only a remnant of the moraine dam remains today. Tide Lake can
only re-form if Frank Mackie Glacier were to readvance
far enough to block Bowser River.
4.5. Tats Lake
Tats Lake (1500 m asl) is a small (1 ha), morainedammed lake located in the Saint Elias Mountains of
northwestern British Columbia. The lake is bordered on
three sides by a single, sharp-crested moraine built during the Little Ice Age. Along its eastern side, the lake is in
contact with the steep snout of a cirque glacier. On June
28, 1990, about 4000 m of water escaped from Tats
Lake, lowering its level about 0.4 m (Clague and Evans,
1992). The escaping waters eroded a steep-walled trench
up to 10 m deep and 25 m wide over a distance of 500 m
from near the crest of the moraine to its toe (Fig. 19);
5000—15,000 m of sediment were removed from the
moraine dam.
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Fig. 13. Queen Bess Lake and its breached Little Ice Age moraine in 1998, one year after failure (view west—southwest). An ice avalanche from Diadem
glacier (arrow) triggered a large displacement wave that overtopped and breached the end moraine (o is breached outlet).
The outburst produced a slurry of sediment and water
that moved downvalley either as a sediment-laden water
flood or a debris flow, locally entraining additional sediment from the valley floor. The first 2.7 km of the flow
path below the moraine has an average gradient of 11°,
whereas the final 1.3 km to Tats Creek is much steeper
(20° on average; 40° maximum). It is not known if a flood
or a debris flood traversed the relatively low-gradient
section of the valley below the moraine, but it is clear that
a debris flow passed down the steep, dominantly rocky
reach above Tats Creek. A considerable amount of sediment may have been incorporated into the flow as it
moved down this final steep reach.
The debris flow stopped as it entered Tats Creek. It left
a fan-shaped mass of debris about 100 m wide and 100 m
long, which temporarily stemmed the flow of the stream.
Soon, however, the stream overflowed the debris dam
and established a new course through forest along the
south side of the valley.
The cause of the Tats Lake outburst is uncertain, but
the two most likely possibilities are (1) a sudden influx of
water into the lake owing to drainage of a pond within or
beneath the glacier to the east, or (2) avalanche or calving
of ice from the glacier snout. The former explanation is
hypothetical because no evidence was found for the presence of a pond within or beneath the glacier. A few blocks
of ice were present in the lake in the summer of 1991
when we visited the site, but it is not clear if these could
have produced waves large enough to overtop and incise
the outlet.
The Tats Lake outburst, although small, is notable for
two reasons: (1) the moraine was not completely
breached during the event; and (2) a relatively small
release of water initiated a significant debris flood or
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Fig. 14. Map of the path of the 1997 Queen Bess outburst flood. Inset map shows the Queen Bess moraine dam, the upper limit of the displacement
wave, and the terminus of Diadem glacier at various times between 1949 and 1998.
debris flow that travelled over 4 km from the source.
The escaping waters did not completely cut down
through the moraine, which is unusual in British
Columbia, although there are many examples of this
behavior in other regions (e.g., Cascade Range in
Oregon; Laenen et al., 1992; Cordillera Blanca in
Peru; Lliboutry et al., 1977). Commonly, when water
begins to erode a moraine dam, outflow increases exponentially until either the lake is empty or further downcutting becomes impossible due to exposure of a bedrock
sill. At Tats Lake, on the other hand, the overflow
channel became naturally armoured as downcutting
proceeded.
5. Failure mechanisms
The cause of only two of the nine, documented, moraine dam failures in the Canadian Cordillera is known
with certainty. The Nostetuko and Queen Bess moraine
dams failed because they were overtopped by waves
generated by ice avalanches. The overflowing waters
eroded the crest and outer flank of the moraines, initiating catastrophic incision. Incision probably propagated
from the steepest part of the moraines back towards the
crest. Avalanche-induced wave overtopping is probably
responsible for some other moraine dam failures in British Columbia (Table 2), and this may be the most
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Fig. 15. Flood-devastated west fork of Nostetuko valley about 5 km below Queen Bess Lake (view south). Photograph taken in 1998, one year after
the August 1997 flood.
Fig. 16. Breached moraine at Klattasine Lake. View north through the breach from the shore of Klattasine Lake; person (circled) near outlet provides
scale. Photograph taken in 1987 by Isabelle McMartin.
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1775
Fig. 17. Diagrammatic sketch showing stages in the evolution of the Klattasine Creek debris flow (modified from Evans and Clague, 1994, Fig. 15). (1)
The moraine damming Klattasine Lake begins to fail; (2) escaping waters mobilize large quantities of sediment, initiating a debris flow; (3) the debris
flow rapidly moves downvalley and entrains additional sediment; (4) the front of the debris flow reaches Homathko River and temporarily blocks it;
secondary landslides occur in Klattasine valley. Stippled area " moving debris; black area " wake of debris flow.
Fig. 18. Fan at the mouth of Klattasine Creek. Bouldery debris deposited by the Klattasine Creek debris flow temporarily blocked Homathko River.
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Fig. 19. Tats Lake (T), source of the June 1990 outburst. Note the trench (arrow) eroded into the outer flank of the moraine damming the lake.
Fig. 20. Mechanisms of moraine dam failures.
common cause of failure in other mountain ranges
(Lliboutry et al., 1977; Ding and Liu, 1992). The very low
cohesion of morainal sediments accounts for the ease
with which they are incised by overflow, unless the surface is armoured by coarse material.
Ice avalanching is a common trigger for moraine dam
failures because many glaciers have retreated up steep
rock slopes since they abandoned their Little Ice Age
moraines (Figs. 3 and 9). Toes of glaciers clinging to steep
slopes are heavily crevassed and wet, and thus prone to
failure.
The Queen Bess and Nostetuko events are end members of a group of moraine dam failures attributable to
wave overtopping triggered by landslides and ice avalanches. One end member, represented by the Queen Bess
event, includes failures in which large displacements
waves are the primary source of the flood waters. Much
of the water that escaped from Queen Bess Lake was
propelled over the moraine by one or more waves. At the
other end of the spectrum of overtopping failures is the
1983 Nostetuko Lake event, described above, which involved a much smaller wave, produced by a smaller ice
avalanche. In this case, the wave initiated catastrophic
incision of the moraine dam but did not itself displace
much water from the lake. Nearly all of the water that
escaped from Nostetuko Lake did so through a breach in
the moraine that rapidly grew as the outburst progressed.
Other trigger mechanisms, which are either external or
internal to the moraines, are also important (Fig. 20;
Eisbacher and Clague, 1984; Costa and Schuster, 1988).
Overtopping and breaching of moraine dams can result
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1777
from unusual meteorological conditions — increased
streamflow during periods of rapid glacier retreat, heavy
rainfall, or rapid snowmelt. Such conditions must be
exceptional, however, as most moraines have been stable
since they formed more than 100 yr ago, a period during
which high overflows must have occurred.
Piping and settlement may gradually weaken a dam to
the point that a less exceptional event initiates failure.
Moraines consisting of silty and sandy diamicton and
sandy gravel may be susceptible to piping. Particle-size
analysis of a sample from the Nostetuko moraine, for
example, shows that the sediment internal erosion factor
(D15/D85) (Sherard, 1979) ranges from about 3 to 10
(Blown and Church, 1985). A value of 5 or more indicates
a potential for piping. Groundwater derived from the
lake may entrain silt and sand and carry them out of the
moraine. Piping may be facilitated where strata dip down
the outer flank of the moraine, focussing groundwater
flow.
Moraine dams containing ice cores or interstitial ice
are also vulnerable to failure (Reynolds, 1998). The ice
may melt if climate warms, causing the moraines to
subside and eventually fail. In addition, thaw flows and
slumps may further weaken the moraines. Climate warming from the late 1800s until about 1940 and from about
1965 to the present (Hansen and Lebedeff, 1987; Folland
et al., 1990) may have melted ice within some moraines
and contributed to their failure. Collapse of the dam
during an earthquake is another possible cause of failure.
An earthquake may also trigger an ice avalanche or
rockfall that generates waves that overtop the moraine.
As noted above, an outburst commonly continues until the moraine is completely breached or the lake is
emptied. Of course, any bedrock below the moraine, but
above the floor of the lake, will arrest the process, and, in
such cases, no further outbursts are possible. In some
cases, as at Tats Lake, an outburst may abort before the
moraine is completely breached due to armouring of the
overflow channel. The recognition of such partial breach
failures is significant because it raises the possibility that
multiple outbursts can occur from a single morainedammed lake.
the 1997 Queen Bess event). Ice-cored moraines may
thaw for decades or centuries until a critical threshold is
reached, whereupon failure may occur under normal
overflow conditions. The anticipated warming of climate
during the next century should accelerate this process.
A warmer climate may also increase the amount of water
that flows into moraine-dammed lakes, further destabilizing many dams. Finally, unusual external events,
such as a strong earthquake, may cause an otherwise
stable moraine dam to fail.
We propose a simple temporal model to explain the
occurrence of moraine dam failures following a formative
event such as the Little Ice Age (Fig. 21). A new population of moraine dams is created during an extended
period of cooler climate (i.e., the eighteenth and nineteenth centuries in the case of the Little Ice Age). Lakes
form behind these dams during the first retreat of glaciers
at the end of the cool period (the late 1800s or early
1900s, immediately after the Little Ice Age). The least
stable dams (e.g., those that are high and narrow, or that
have large amounts of water flowing over or through
them) fail soon after they form. If climate continues to
warm, as it did during much of the twentieth century,
dams that were formerly stable fail due to (1) overtopping
by waves triggered by ice avalanches and landslides, (2)
high overflow during storms; (3) high overflow caused by
sudden influxes of water from upstream glacial lakes, and
(3) melt of interstitial and massive ice. In the process, the
population of unstable moraine dams is reduced. If
glaciers continue to retreat, this population is not
regenerated, and the incidence of moraine dam failures
decreases.
Moraine dams in British Columbia persist longer than
landslide dams, except those consisting of coarse blocky
debris (Clague and Evans, 1994). The difference in longevity is due partly to differences in the sizes of upstream
drainage areas. Landslides generally dam streams with
large drainage areas, relative to the size of the dams,
and thus are overtopped by large flows. In contrast,
moraine dams have much smaller drainage areas, and
overflow discharges are relatively small under normal
conditions.
6. Longevity of dams
7. Outburst floods
Little Ice Age moraine dams have relatively low
width-to-height ratios and are composed of loose permeable sediment. These characteristics limit the stability
of the dams and suggest that they are likely to be shortlived. There are, of course, exceptions. Dams composed
of coarse blocky and rubbly debris may be stable irrespective of their morphology. Moraines that are low and
wide and have armoured overflow channels may persist
for centuries or millenia, although such dams can be
breached if overtopped by sufficiently large waves (e.g.,
7.1. Natural dam failure
A dam failure is a complex phenomenon that is controlled primarily by the form and material properties of
the dam and by the failure mechanism (Costa and Schuster, 1988). Most moraine dams have low width-to-height
ratios and consist of noncohesive granular materials,
thus the lakes impounded by them drain very rapidly.
Generally, however, an unusual event, such as an ice
avalanche, is required to initiate failure.
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Fig. 21. Hypothesized relation between climate (A) and the formation and failure of moraine dams (B). Moraines are built during a lengthy period of
cool climate such as the Little Ice Age. Lakes form behind the moraines when climate warms and glaciers retreat, as happened in the late 1800s and
early 1900s. Moraine dams begin to fail as climate continues to warm. The regional frequency of failures may increase to a peak decades later.
In contrast, width-to-height ratios of landslide
dams are typically high, thus when they are overtopped,
much more material must be eroded before a full breach
is developed than is the case for moraine dams. Peak
flood discharges of outburst floods from landslide-dammed lakes, however, are not necessarily smaller than
peak discharges of floods from moraine-dammed lakes
(Costa, 1985, 1988; Evans, 1986; Costa and Schuster,
1988).
The form, material characteristics, and failure mechanisms of moraine dams are fundamentally different from
those of glacier dams. Most glacier dams are much
larger than the lakes they impound, and floodwaters
must travel long distances beneath or over ice. Failure
of glacier dams generally involves erosion of the dam
by a meltwater stream flowing at the ice margin or
enlargement of a subglacial tunnel, sometimes with the
collapse of the tunnel roof (Walder and Costa, 1996).
Wholescale mechanical collapse of the dam and overflow
via a spillway are less common failure mechanisms (see
Clague (1987) and Mayo (1989) however, for an
example of a collapse failure). Enlargement of subglacial
tunnels by heat transfer and other processes is a slower
process than overtopping and incision of sediment dams,
thus glacier-dammed lakes that drain through tunnels
generally empty more slowly than do moraine-dammed
lakes.
7.2. Flood discharges
Study of moraine dam failures in British Columbia and
elsewhere indicates that flood size is controlled by many
factors, including the volume of water released from the
reservoir, lake hypsometry, the height, width, structure
and texture of the dam, the failure mechanism, and
downstream topography and sediment availability
(Costa and Schuster, 1988; Clague and Evans, 1994). In
general, flood size is proportional to the volume of water
released and, therefore, reservoir volume. Failures of narrow high dams commonly produce higher peak flood
discharges than failures of wide low dams. Dams containing large amounts of sand and gravel tend to fail more
rapidly, and produce floods with higher peak discharges,
than dams of coarser, rubbly and blocky material. Failure mechanism also influences flood size. For example,
large displacement waves (viz. Queen Bess Lake, 1997)
produce spiky, large-amplitude, low-duration floods,
whereas some incision failures generate floods with much
smaller peak discharges. Finally, the character of the
downstream flood path is also important. Floods in steep
narrow-floored valleys attenuate more slowly than floods
in more gentle broader valleys. Hydraulic ponding may
occur above constrictions in the flood path and thus
strongly influence the peak discharge and downstream
attenuation of the flood.
J.J. Clague, S.G. Evans / Quaternary Science Reviews 19 (2000) 1763—1783
1779
Fig. 22. Plot of potential energy of lake water versus peak discharge for selected failures of landslide, moraine, and glacier dams (modified from Costa
and Schuster (1988, Fig. 12); data from Costa (1985). Least-squares regression lines for the three types of dams and the envelope curve encompassing all
plotted data are also shown.
A variety of empirical equations have been proposed,
based on documented outburst floods from morainedammed lakes, to estimate peak discharges (Q ). The
most common variables in these equations are the volume of water released from the lake (V) and potential
energy of the impounded lake water (PE) — e.g.
Q "0.045» (Walder and O’Connor, 1997);
Q "0.0013PE (Costa and Schuster, 1988). Potential
energy is calculated as the product of dam height, reservoir volume, and the specific weight of water.
Plots of Q vs. » and Q vs. PE show considerable
scatter (Fig. 22), reflecting the diverse characteristics of
moraine dams, basin geometry, and flood paths, as well
as downstream changes in peak discharge and errors in
estimating discharge. Direct measurements of flood discharge are virtually impossible, thus a variety of indirect
methods are commonly used to estimate peak discharges,
including drawdown rates and measurements based on
hydraulic formulae or post-flood channel surveys, all of
which are subject to error. Many published estimates of
peak discharge come from hydrographs that are considerable distances below the dam. Such estimates are
generally smaller than the peaks at the breach because
floods attenuate as they move downstream (Fig. 12; Cenderelli and Wohl, 1997). Downvalley attenuation of outburst floods results from interactions of the flood wave
with the channel, including flow retention outside the
channel, frictional resistance to flow, acceleration and
deceleration, flow losses, backwater effects, deposition,
and channel constrictions and expansions (Manville
et al., 1999).
Notwithstanding these problems, the data in Fig. 22
show that peak discharges of floods from moraine-dammed lakes differ from those of other types of natural dam
failures. For the same potential energy, moraine dams
produce larger flood peaks than most glacier dams because, as mentioned earlier, the failure mechanisms of the
two types of dam are different.
Another technique for estimating peak discharges of
floods resulting from failures of morainal and other dams
is to formulate the problem as critical flow through an
enlarging outlet (Walder and O’Connor, 1997; Manville
et al., 1999). This approach establishes the relative importance of breach growth rates and lake volume and
shape on the peak discharge of a dam breach flood.
A dimensionless measure of peak discharge Q is defined
relative to the dimensional discharge Q :
QH"Q (gd)\,
where g is gravitational acceleration and d is the amount
of lake lowering or breach depth. QH is primarily control
led by the dimensionless parameter g, where
g"kH»H"k» (gd)\,
in which kH is the dimensionless rate of breach growth,
k is the rate of breach growth, »H is the dimensionless
volume of water discharged, and » is the volume of
water discharged. Dimensionless measures of breach erosion rate, breach geometry, and lake shape are derived
from the dimensional estimates of (1) the volume of the
lake and the amount of water discharged during the
outburst, (2) the hypsometry of the lake, (3) the breach
erosion rate, and (4) the breach geometry. Peak discharges obtained using this approach may be more accurate than those determined from empirical relationships
derived from historical dam failures. Dimensionless analysis, however, requires knowledge of the rate of breach
growth, which is often unknown and must be estimated.
This approach has not been widely applied to failures of
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moraine-dammed lakes, but it has considerable potential
and can be used to test peak discharge estimates obtained
by other methods.
8. Hazard assessment and mitigation
8.1. General considerations
Peak discharges of floods from moraine-dammed lakes
generally are many times larger than the peak flows of
rainfall and snowmelt floods in the same basin. The large
discharges and long travel distances of outburst floods
must be taken into account when developing valleys
below natural dams.
Estimates of peak discharges of outburst floods may be
obtained by dimensionless analysis or from empirical
relationships such as those shown in Fig. 22. Costa and
Schuster (1988) have argued for the use of conservative
peak-discharge estimates for planning purposes where
there is a potential loss of life or property damage. These
can be obtained from the envelope curve encompassing
data points for all natural dam failures for which there
are reasonable estimates of peak discharge (e.g.,
Q"0.063PE, Fig. 22).
A more detailed assessment of the magnitude and
possible impact of a particular outburst flood can be
made by taking into account the morphology and internal characteristics of the dam, the character of the flood
path below the dam, and the availability of sediment and
vegetation for entrainment by floodwaters. Discharges
may increase substantially through addition of easily
eroded sediment and woody debris. If sufficient sediment
is added, the flood may transform into a debris flow. This
occurred, for example, when Klattasine Lake drained in
the early 1970s. Historical examples indicate that, following failure of a moraine dam, a debris flow can only form
and be sustained on gradients steeper than 10—15°
(Fig. 23), and only if there is an abundant supply of
sediment downvalley of the dam (Clague and Evans,
1994; O’Connor et al., 1994). Moraine dams contain large
volumes of sediment, but additional material generally
must be picked up in the valley below the dam to generate a true debris flow. Bulking of flood flows with sediment and plant debris, by as much as a factor of four, has
important consequences for the assessment of outburst
flood hazards (Laenen et al., 1992; O’Connor et al., 1993),
but the phenomenon remains poorly understood.
Most historical floods from moraine-dammed lakes in
the Canadian Cordillera have occurred in remote mountain valleys. Recently, however, many of these valleys
have been opened to forestry, mining, and recreational
activities. Such development will continue in coming
years, increasing the likelihood that outburst floods and
related debris flows will damage human works.
8.2. Hazard assessment
An assessment of hazards associated with moraine
dams commonly begins with an airphoto search. At this
stage in the investigation, all moraine dams should be
considered potentially unstable, although subsequent
field study may indicate that many of the dams are
unlikely to fail.
Fig. 23. Profiles of the paths of historical floods (solid lines) and debris flows (dotted lines) caused by failures of moraine dams in British Columbia.
Debris flows are only generated and sustained on slopes steeper than 10—15°.
J.J. Clague, S.G. Evans / Quaternary Science Reviews 19 (2000) 1763—1783
The likelihood that a particular dam will fail can be
assessed, at least qualitatively, by studying (1) its
morphology and internal characteristics, (2) the size,
hypsometry, and hydrology of the reservoir, and (3) the
proximity of the reservoir to slopes that are susceptible to
landslides or avalanches.
One or a combination of the following factors may
indicate that a moraine dam has a high probability of
failure: (1) the width-to-height ratio of the dam is small;
(2) the dam lacks an armoured overflow channel; (3)
outflow occurs mainly by seepage through the dam; (4)
the reservoir surface is normally at or just below the crest
of the dam; (5) one or more highly crevassed glaciers cling
to steep slopes directly above the reservoir; and (6) slopes
above the reservoir are subject to rockfalls.
Once the likelihood of dam failure is known, the downvalley effects of an outburst flood can be assessed. Peak
flood discharges can be estimated using the empirical
equations mentioned earlier, although such estimates
assume complete emptying of the lake. Discharge attenuation and bulking must be taken into account when
predicting the height of the flood wave at various points
downstream from the dam (Laenen et al., 1992). The
possibility that the flood could transform into a debris
flow must also be considered. As pointed out by
O’Connor and Costa (1993), these complexities make it
difficult to formulate reliable deterministic models for
assessing hazards from moraine-dammed lakes. A hazard
assessment may have to be based on empirical data from
case studies (e.g., Costa, 1988).
8.3. Hazard mitigation
No efforts have been made in western Canada to
prevent or reduce the impact of outburst floods from
moraine-dammed lakes because, until recently, valleys at
risk from such floods were unsettled and undeveloped. In
contrast, in other parts of the world where people and
property are at much greater risk from outburst floods,
countermeasures have been taken (Lliboutry et al., 1977;
Yesenov and Degovets, 1979; Eisbacher, 1982; Eisbacher
and Clague, 1984). Control measures that have been
applied to moraine-dammed lakes include excavation of
trenches and tunnels to lower or empty reservoirs, construction of paved revetments and low earthen dams to
stabilize outlets and increase freeboard, the use of siphons to drain water from lakes, and construction of
downvalley retention basins to trap floodwater and debris (Lliboutry et al., 1977; Reynolds, 1992, 1993, 1995,
1998; Grabs and Hanisch, 1993; Reynolds et al., 1998).
The following two examples, taken from Lliboutry et al.
(1977), illustrate common types of control measures and
potential dangers of constructing such works.
Laguna Jancarurish was impounded by a large, Little
Ice Age end moraine at the head of an inhabited valley in
the northern Cordillera Blanca of Peru. The lake drained
1781
by seepage through the moraine, but the freeboard between the lake surface and the lowest point on the moraine was only 10 m. Numerous small ice avalanches fell
into the lake from a nearby glacier in the 1940s, and
concern mounted that the moraine might be overtopped
and incised by a displacement wave from a large avalanche, triggering a catastrophic flood or debris flow
(termed “aluvión” locally). In 1949, the Comisión de
control de la Lagunas de la Cordillera Blanca (CCLCB)
began to lower the lake by digging a trench across the
moraine. The digging proceeded in steps — the trench was
lowered while a gate blocked outflow from the lake; the
gate was then opened and the lake lowered to the level of
the trench; and so forth. By 1950, the lake had been
lowered 15 m in this manner. In October 1950, the trench
was deepened an additional two metres and a stone
revetment was being constructed under the precarious
protection of the gate. On October 20, the toe a partly
submerged apron consisting of avalanched snow and ice
broke away at the opposite end of the lake and generated
large waves that overflowed the moraine, destroying the
gate. The escaping waters began to erode the distal flank
of the moraine, which was formed almost entirely of sand
and pebbles. The erosion advanced upslope towards the
crest of the moraine, and a waterfall formed that swept
the revetment away and caused the moraine to fail. The
level of the lake fell 21 m in a short period of time, and
6—10;10 m of water were released. The resulting
aluvión may have killed as many as 500 people in the
valley below the dam.
One of the most dangerous moraine-dammed lakes in
the Cordillera Blanca in the late 1960s was Laguna de
Safuna Alta. The lake began to form in 1950, and by
1969, about 5;10 m of water were impounded between a steep end moraine and the retreating glacier. The
surface of the lake in 1969 was 18 m below the lowest
point of the moraine. Water escaped by seepage through
the moraine. In 1968 and 1969, a tunnel was dug through
the moraine, 80 cm above the lake surface and lined with
sections of iron pipe encased in concrete. Plans were
made to completely drain the lake with powerful pumps,
but in May 1970 before the project could be implemented, a powerful earthquake caused the moraine to
move and settle. The tunnel cracked in numerous places
and settled at its upstream end below the level of the lake.
Water escaped through the tunnel and through natural
pipes within the moraine. Discharge grew exponentially
but fortunately ceased after the lake had fallen 38 m.
A new 150 m long tunnel was later dug, rendering
Laguna de Safuna Alta safe.
9. Conclusions
Most moraine-dammed lakes have formed during the
last 200 yr as glaciers retreated from their Little Ice Age
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maximum positions. The moraines are commonly steepsided and consist of coarse, poorly sorted, cohesionless
sediment. Most moraine dams fail when they are overtopped and incised by unusually large overflows. Anomalous overflows result from heavy rainfall or snowmelt,
from sudden influxes of water from upstream glacial
lakes, or from waves triggered by snow and ice avalanches or rockfalls.
Outburst floods from moraine-dammed lakes typically
are many times larger than snowmelt and rainfall floods
in the same basins. In many cases, discharges increase
rapidly to a peak and then drop off to background levels.
Peak discharges are controlled by many factors, the most
important of which are lake volume, dam height and
width, internal properties of the dam, failure mechanism,
and downstream topography and sediment availability.
Moraine dam failures generally produce larger floods
than do glacier dam failures, in part because enlargement
of tunnels within ice is a slower process than overtopping
and incision of sediment dams.
Floods triggered by moraine dam failures may transform into debris flows as they travel down steep valleys.
Historical data from British Columbia indicate that such
flows can only form and be sustained in channels steeper
than 10—15° and only where there is an abundant supply
of sediment in the valley below the dam. Although most
outburst floods attenuate as they move downvalley, entrainment of sediment and plant debris by floodwaters
may, in some instances, increase discharge. Such flow
bulking has important implications for hazard appraisal.
A hazard assessment of a moraine dam is made by
studying the dam, its reservoir, and the surrounding
terrain. A dam may be hazardous if it has an ice core or
contains interstitial ice, if it has a low width-to-height
ratio and lacks an armoured overflow channel, if the
reservoir surface is at or just below the crest of the dam,
or if the reservoir is close to slopes subject to ice avalanches or rockfalls. Climatic warming may cause some
moraine dams to fail, both by melting ice cores and by
increasing the amount of water flowing over the moraines.
Acknowledgements
This study draws on many data sources and personal
communications from a large number of researchers
— too numerous to list individually. We are particularly
indebted to John Costa, Jim O’Connor, and Joseph Walder, whose work on natural dam failure helped us focus
our thoughts. Tonia Williams drafted some of the figures.
Fig. 9 was produced by Tim Albert of Geosolutions
Consulting Inc., Nepean, Ontario. Jim O’Connor, Lionel
Jackson, Jr., and Willie Scott provided thoughtful critiques of a draft of the paper.
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