River Response to Baselevel Change: Implications for Sequence Stratigraphy
Author(s): S. A. Schumm
Reviewed work(s):
Source: The Journal of Geology, Vol. 101, No. 2, 100th Anniversary Symposium: Evolution of
the Earth's Surface (Mar., 1993), pp. 279-294
Published by: The University of Chicago Press
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River Response to Baselevel Change:
Implications for Sequence Stratigraphy1
S. A. Schumm
Department of Earth Resources, Colorado State University, Fort Collins, Colorado 80523
ABSTRACT
Baselevel is the imaginary horizontal level or surface to which sub-aerial erosion proceeds. It is sea level. Controversy
surrounds the effect of baselevel change on river behavior, the rejuvenation of landscapes, and the delivery of sediment
to the shelf-slope depositional system. The effect of baselevel change depends upon many factors, such as rate of
change, amount of change, direction of change, river character, and dynamics and erodibility of the sediment source
area. In most cases the effects of baselevel change will be moderate, and they can be accommodated by changes of
channel pattern, width, depth, and roughness. Therefore, the delivery of large amounts of sediment to a shoreline or
continental shelf probably reflects not only baselevel lowering, but significant uplift of the sediment-source area and
perhaps climate change.
Introduction
In the 19th century, it was believed that the concept of global eustatic changes of baselevel provided a great unifying generalization that permitted worldwide correlation of erosion surfaces
(Chorley 1963). Subsequent recognition that continental stability was a prerequisite put an end to
these attempts because major transgressions and
regressions could be attributed to continental
warping (Chorley 1963, p. 959). Recently, geologists have assumed that eustatic baselevel changes
can be used as a basis for global correlation of
stratigraphic units (Vail et al. 1977; Posamentier
et al. 1988), and therefore, baselevel concepts may
provide a link between geomorphology, sedimentology, stratigraphy, and economic geology
that could be particularly beneficial to all concerned. Nevertheless, serious problems face the sequence stratigrapher, because the depositional record can be greatly complicated or even controlled
by climate change and tectonics (Sloss 1991; Miall
1986; Galloway 1989a, 1989b). In addition, much
depends upon river response to baselevel change,
because the complex fluvial system can respond in
varying degrees and by different methods.
There are two very different perspectives on the
effects of baselevel change. Geomorphologists have
concentrated on landform response, whereas stratigraphers have concentrated on the depositional
record. In order to obtain a fuller understanding
of the complex phenomenon of baselevel change,
several aspects of the baselevel problem will be
considered in this paper: (1) the definition of baselevel; (2) the effect of baselevel change on rivers;
(3) the effect of baselevel change on the continental
shelf; and (4) discrimination among the causes of
increased sediment delivery to the coast (baselevel
change, climate change, tectonics). It is clear that
with the available data solutions can only be partially achieved. It is hoped, however, that even
speculation concerning these topics may be of
value.
The Concept of Baselevel
Ninety years ago Davis (1902) published an influential paper in The Journal of Geology on baselevel, stream grade, and landscape planation in
which he attempted to clarify Powell's rather ambiguous description of baselevel. Although Powell
(1875) clearly stated that sea level is the "grand"
or general baselevel to which a landscape is lowered, he confused the issue by defining local and
temporary baselevels and by describing the base-
1 Manuscript received June 5, 1992; accepted December 1,
1992.
[The Journalof Geology, 1993, volume 101, p. 279-294] © 1993 by The University of Chicago. All rights reserved. 0022-1376/93/10102-004$1.00
279
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S. A. SCHUMM
level surface as an inclined plain. The Powell
definition of baselevel contained three ideas (Davis
1902): (1) baselevel is the lower limit of subaerial
erosion, which is sea level; (2) there are local and
temporary baselevels reflecting rock resistance and
structure; and (3) there is a baselevel that slopes
toward sea level and is determined by the gradient
of major streams. Unfortunately Powell confused
cause (baselevel) with effect (the baselevel surface),
and this has led to misunderstandings of the concept. Davis (1902, see Crickmay 1974, p. 170) illustrated this confusion by showing that baselevel had
been defined in the pre-1902 literature as: (1) sealevel at the coast; (2) a level not much above that
of the sea; (3) an imaginary surface sloping with
mature or old streams; (4) the lowest slope to
which rivers can reduce a land surface; (5) a slow
reach in a stream; (6) a condition in which rivers
cannot corrade or in which they are balanced between erosion and deposition; (7) a certain stage in
the history of rivers when vertical cutting ceases
and their slope approximates a parabolic curve; (8)
a plain of degradation; (9) an ultimate planation;
and (10) an imaginary mathematical plane.
Following this review Davis (1902; King and
Schumm 1980, p. 8) concluded that the concept of
baselevel should be limited to the first meaning,
"an imaginary level surface," and to define it simply as the level base with respect to which normal
sub-aerial erosion proceeds; to employ the term
"grade" for the balanced condition of a mature or
old river; and to name the geographical surface that
is developed near or very near to the close of a
cycle, a "peneplain," or "plain of gradation."
Davis did not solve the semantic problem (Malott 1928; Johnson 1929; Chorley and Beckinsale
1968; Thornbury 1969) A recent and even more
confusing and incorrect usage is illustrated by the
following definition: "baselevel, above which a
particle cannot come to rest and below which deposition and burial are possible" (Sloss 1962). Sedimentary particles do come to rest, and they are
stored above baselevel during the erosional evolution of a landmass. Perhaps Wheeler (1964a, 1964b)
has modified the definition to the greatest extent
by suggesting that, with constant sea level, baselevel fluctuates by rising and falling above and below the land surface as sediment supply and energy
conditions change. "At any given moment the
earth's lithic surface is divisible into innumerable
areas, each of which is characterized by one or
and erosion.
other of two processes-deposition
The boundary between any two or these areas is
at baselevel" (Wheeler 1964a, p. 603). Wheeler is
obviously not considering baselevel but rather the
fluctuation of stream profiles as a result of climatic
and tectonic change. He confuses baselevel with
stream profiles, and he states that baselevel controls nothing, although it clearly controls the lower
end of the stream profile. The AGI Glossary of Geology (Bates and Jackson 1987) still includes two
definitions which confuse cause (baselevel) and effect (erosion surface). A further problem is that, as
Thornbury (1969, p. 104) points out, we apparently
can't even agree as to its spelling (base level, baselevel, baselevel). I prefer the latter as it eliminates
the hyphen when the word is used as an adjective.
It is unfortunate that Powell's baselevel concept
became so confused, but in order to define baselevel, as it will be used here, one need only turn to
the definition of the words base and level. Base is
the lowest part, and level is a horizontal line or
plane positioned along a horizontal axis. Therefore,
in the sense that the term was created, baselevel is
effectively sea level, although we know that rivers
erode slightly below it.
The Effect of Baselevel Change on the
Fluvial System
It seems very logical that baselevel lowering will
rejuvenate a drainage network and deliver large
amounts of sediment downstream. Support for
these conclusions comes from experimental studies (Holland and Pickup 1976; Schumm et al. 1987)
and from observations of the headward incision of
arroyos, gullies, and channelized streams (Schumm
et al. 1984). Fisk (1944) concluded that Pleistocene
sea-leveling lowering caused excavation of alluvium and bedrock incision far up the Mississippi
River valley, and on a smaller scale, even a single
meander cutoff can cause local steepening and upstream channel degradation (Winkley 1976). E. W.
Lane (1955), an eminent river engineer, concluded
that, when a river is affected by a baselevel rise or
fall or by a horizontal shift of the river mouth (figure 1), the river will degrade or aggrade to restore
its equilibrium profile. After all, the channel must
continue to carry its load of sediment with a given
water discharge and this requires a given gradient
that must be restored either by erosion (figure lb)
or by deposition (figure la,c).
In contrast to Lane, Leopold and Bull (1979) concluded that baselevel changes affect the vertical
position of a river only locally and to a minor extent. They argued that not only is stream gradient
important, but that channel pattern, roughness,
and shape (Rubey 1952) can also adjust, in order to
absorb the effect of baselevel change. They based
their conclusion partly upon a study of a cutoff in
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deposits .. . that the direct influence of the late-
a)
b)
c)
Figure 1. When baselevel changes or the position of the
mouth of a stream shifts laterally, the stream profile will
be established at a higher or lower elevation. (a) baselevel
rises from position 1 to 2, (b)baselevel falls from position
1 to 2, (c) the mouth of the stream shifts horizontally
from position 1 to 2 (after Lane 1955).
the canyon of the San Juan River, where baselevel
was lowered about 40 m, but the tiny streams that
drain the cutoff are apparently unable to incise into
the bedrock wall of the canyon so this may not be
a good example. Leopold et al. (1964, p. 260) cite
surveys that indicate a reservoir controls deposition in the river for only a short distance upstream
where the backwater curve intersects the original
streambed profile. The initial surveys showed that
the gradient of deposition above dams varied between 30 and 60% of the original slope of the channel, and even after an additional 22 yrs of sediment
accumulation, these slopes did not steepen significantly. It may be that the period of record is
too short, because, when this type of deposition
occurs, the channel is filled first, and then sediment is spread over the entire valley floor. It is not
simply a matter of filling the channel, therefore,
but also of regrading the valley, which will take
considerably more time. In contrast, channel incision occupies only a small part of the valley floor
and can occur rapidly.
Studies in the Mississippi valley by Durham
(1962) and Saucier (1981) contradict Fisk's (1944)
conclusions that the river incised into bedrock
throughout the lower Mississippi valley during
maximum glaciation and minimum sea level, because the valley was never swept clean of sediments. Saucier (1981) concluded from the "extent
and shape of the prism of Holocene backswamp
Wisconsin sea-level fall extended no farther upvalley than the latitude of Baton Rouge" (Autin et al.
1991, p. 552), 370 river kilometers above Head of
Passes. This is only a fraction of the distance to
the head of the Mississippi alluvial valley at Cairo,
Illinois, 1536 river kilometers upstream from Head
of Passes. Nevertheless, an effect extending 370
km upstream is neither local nor minor. Blum's
(1992) studies of the Colorado River of Texas lead
him to a similar conclusion. Furthermore, Pleistocene sea-level lowering caused only 23 m of incision by the Trinity and Sabine Rivers on the Texas
continental shelf (Thomas and Anderson 1989, p.
566). The subsequent sea-level rise caused deposition for a distance of only 150 km upstream
(Thomas and Anderson, 1993). Further investigation shows that there was incision to a depth of 40
m (Thomas and Anderson, 1993), but this is far less
than the 120 m of sea-level lowering (Fairbanks
1989).
The question regarding the impact of baselevel
change on the fluvial system does not have a ready
answer. If baselevel effect is minimal in some cases
but significant in others, then other variables must
play an important role. At least 10 variables appear
significant, and they can be grouped as follows: (1)
baselevel controls, including direction, magnitude,
rate, and duration; (2) geologic controls, including
lithology, structure, and nature of valley alluvium;
(3) geomorphic controls, including inclination of
exposed surfaces, valley morphology, and river
morphology and adjustability.
Baselevel Controls. The direction of baselevel
change obviously determines whether a river will
aggrade or degrade, but the magnitude of the
change appears to be most important. If baselevel
lowering is small, a channel can adjust to a change
of slope by changing its pattern, by increasing bed
roughness, or by changing shape, and in this manner accommodate a baselevel change. If the change
is large, river incision is likely, and if it is very
large, rejuvenation of the entire drainage network
may result.
The rate of change is also important, and some
experimental work has demonstrated that when
the lowering of baselevel is fast, a stream incises
vertically with little lateral migration, whereas
when the change is slow, considerable lateral migration takes place, permitting the channel to adjust its slope (Yoxall 1969; Wood et al. 1992b).
Also, with rapid incision, all discharge events,
even the largest flows, will be concentrated in a
narrow deep channel. This increases the energy of
the flow, and the affect of the baselevel lowering
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S. A. SCHUMM
is greatly enhanced. In alluvial valleys in the
southeastern United States much of the flow was
spread over the valley floor prior to channelization,
and this flow was, in effect, non-erosive. Following
initial rapid incision, floodwaters were concentrated in the enlarged channel, and incision moved
rapidly upstream (Schumm et al. 1984). Therefore,
rapid baselevel change appears to have a more significant upstream impact, than does a slow change.
Duration of baselevel changes is probably less
important than magnitude and rate, although it is
intimately related to both. If the duration is long,
the rate will be slow, and many adjustments of the
fluvial system can occur to negate the effects of the
long-term baselevel change. However, if baselevel
lowering is large, rapid and short, the impact will
be significant, and channel incision can still be occurring upstream, even after baselevel has returned
to its original position.
Geologic Controls. In many cases, bedrock or
structural controls will prevent or delay the effect
of a baselevel change from moving upstream. If so,
baselevel fluctuations will not affect the fluvial
system or be reflected in the stratigraphic record
until the local control is removed.
If the valley alluvium is cohesive, incision may
be propagated rapidly upstream. If there is a resistant layer (root mat, sod, compacted sediment, cohesive silt and clay over sand), a nickpoint can retain its identity and migrate for long distances
upstream. If the alluvium is sandy, the effect of the
baselevel change can be dissipated in the wide and
shallow channel that results when incision into
this type of sediment occurs.
Geomorphic Controls. The surface exposed by
baselevel lowering is very important in determining the impact of the lowering (Miall 1989). If baselevel is lowered abruptly, as along a scarp (figure
lb), the results will be very different from the situation when baselevel is lowered gradually, as when
a shoreline retreats across a gentle continental
slope (figure l c), or one equal to the gradient of the
river (Wood et al. 1992a).
The morphology of the valley in which the
stream flows is important in determining the ability of a river to adjust to baselevel change. If the
channel is in a wide valley or flowing across a
plain, it has the ability to shift laterally. A channel
confined within a narrow valley can only aggrade
or degrade.
Finally, and most important, is the morphology
and the sensitivity or adjustability of an alluvial
river. For example, depending upon its morphology, a river may be able to adjust its pattern and
shape in order to adjust to a baselevel change. If
the river is straight or sinuous and a baselevel fall
causes a steepening of the gradient, the river could
become more sinuous and adjust to changes of
slope without major incision. However, a braided
river could only incise.
Although all of the 10 controls are important,
not all require a detailed discussion. For example,
regarding geologic controls, either there are bedrock controls or there are not, and regarding baselevel controls, either baselevel rises or falls. Channel confinement by valley morphology or cohesive
alluvium will be discussed first because it has a
bearing on other controls.
Nature of Alluvium and Channel Confinement
The cohesiveness of the sediment forming the bed
and banks of a channel seems very important in
determining how far upstream the effect of a baselevel lowering can propagate. Experimental studies
in low-cohesion sediments have shown that nickpoints will not migrate indefinitely
upstream
(Brush and Wolman 1960) because: (1) as the nickpoint reach lengthens, its slope is reduced until
it is nearly equal to the slope of the stream; the
nickpoint cannot be identified when the slope of
the nickpoint reach is approximately 20% of the
average slope of the river; (2) as slope reduction
takes place, stream competence declines to the
point that bedload movement ceases; (3) considerable bank erosion takes place in cohesionless sediment so that widening of the channel occurs and
stream competence declines.
Unlike channels formed in low-cohesion sediment, channels in cohesive sediment incised along
the entire length of a flume (Begin et al. 1980,
1981). In Begin's experiments, water that spread
widely across the flume became concentrated in
the incised channel. This concentrated the energy
of the flow in the narrow deep channel, which, in
turn, enhanced incision and headward erosion. Begin's results are similar to those that occurred
when baselevel was lowered in a 9 by 15 m rainfallerosion facility. Because of the cohesive nature of
the sediment and a relatively large lowering of
baselevel, incision was propagated through the
drainage network to the smallest tributaries. As
a result, the drainage pattern was extended, and
sediment yields increased significantly (Schumm
et al. 1987, p. 34-44).
When incision occurs in valleys with fine sediments, water is concentrated in the incised channel, and erosion can extend for very long distances
upstream. For example, when baselevel was lowered 3 m by excavation, a headcut formed in the
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1100
1200
1300
1400
1500
1600
1700
1800
Time (years)
Figure 2. Changing length of lower Yellow River between 1200 and 1855 A.D. (from Ren and Xie 1990).
channel of the East Prairie River of Alberta, Canada, which flows through clayey and cohesive lacustrine sediments (Parker and Andres 1976). The
headcut migrated upstream 2134 m between 1966
and 1968 and an additional 8232 m between 1968
and 1972. The arroyos of the southwestern United
States, and channelized streams in the southeastern United States, also provide examples of longdistance channel incision in relatively cohesive
valley fills (Schumm et al. 1984). An excellent example of the effect of confining a channel is provided by an extraordinary record of Yellow River
deposition (1194-1855 A.D.) downstream of Yuntiguan, which is now 200 km from the sea (Ren and
Xie 1990). Before 1600 A.D. seaward extension of
the Yellow River delta was very slow, at a rate of
about 40 m/yr. Sediment was spread widely across
the north China alluvial plain as the river mouth
shifted frequently (figure 2). During the latter part
of the 16th century, an official of the Ming Dynasty (Pan Jixun) caused the river to be confined
by levees. This caused increased sediment delivery
to the delta, which grew at a rate of 1400 m/yr.
During the 17th century, the levees deteriorated
during a period of war, and the river spread its sediments widely across the plain. The result was a
reduction of delta growth to about 200 m/yr. Following this period of disruption, an official of the
Qing Dynasty (Jin Fu), reconstructed the levee system, and the delta grew at a rate of 1700 m/yr
(figure 2). This rate decreased to an average of 600
m/yr until 1855. The maintenance of the river between substantial levees fixed the river position,
and as a result its bed aggraded to about 10 m above
the level of the North China Plain, as the delta
prograded. In 1855, a major flood breached the
levee at Tongwaxiang, which caused a 7 to 10 m
lowering of water surface elevation. This lowering
of local baselevel caused incision in the leveed
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channel, which was estimated to have extended
100 km upstream (Qian 1990).
These observations show how different the effects of baselevel change can be upon a confined
(leveed) and an unconfined channel. In the first
case, vertical adjustment is the only option. In the
second case, the river can aggrade or degrade, but
lateral shift is the most likely result. When a river
is unconfined, it will distribute its sediment load
widely over a plain. Avulsion will be frequent, but
the upstream effect will be much less than when
the channel is confined.
River Pattern Adjustment
Lane's (1955) argument that a stream will restore
its gradient at a higher or lower level following a
baselevel change (figure 1) is conceptually correct
because if the river is initially at grade, then to
move its sediment load and water discharge
through the channel, the original gradient of the
stream must be re-established. However, sediment
loads may be greater after channel incision and less
after aggradation. Another fallacy in Lane's argument is the assumption that the maximum downvalley slope, the valley slope, and channel gradient
are identical. This is not the case, and the twodimensional perspective provided by figure 1 leads
to erroneous conclusions. For example, a sinuous
river has a gradient less than that of the valley
floor.
Sinuosity (P) is the ratio between channel length
(Lc) and valley length (Lv), and it is also the ratio
between valley slope (Sv) and channel gradient (Sc)
as follows:
Le
Lv
Sv
Sc
(1)
Therefore, a straight channel has a sinuosity of
1.0, and the gradient of the channel and the slope
of the valley floor are the same. It has been demonstrated that rivers can respond to major changes
of water and sediment load primarily by pattern
changes (Schumm 1968), and that much of the pattern variability of large alluvial rivers such as the
Mississippi, Indus, and Nile reflect the variability
of the valley slope.
The lower Mississippi River below Cairo, Illinois can be used as an example. When the watersurface elevation, which is a close approximation
of bed elevation, is plotted against valley distance
a longitudinal profile of the floodplain or valley
floor is obtained (figure 3). This profile is irregular,
as a result of neotectonics and tributary influences
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S. A. SCHUMM
Figure 3. Profile of Mississippi
River valley between Cairo, IL and
Old River, LA. This is a profile of
the floodplain or valley floor. The
effect of river sinuosity is removed
by plotting water-surface elevation
against valley distance.
300
250
200
150
100
50
0
50
100
150
200
250
300
350
400
450
500
VALLEYMILES FROM CAIRO
(Schumm 1986). When sinuosity for each of the 24
reaches is obtained from four surveys covering a
period of 150 yrs (figure 4), it is apparent that the
steeper reaches of the valley profile have the highest sinuosity and are the most subject to variations
of sinuosity, as meanders form and cutoff. Figures
3 and 4 indicate that increased valley slope will
cause river pattern to change from low to higher
sinuosity, which, in turn, aids in the maintenance
of channel gradient on a varying valley slope (equation 1). However, if the valley slope change is too
great, braiding will result (Schumm et al. 1987, p.
172-173).
Figures 5 and 6 attempt to illustrate this concept
geometrically. Figure 5 shows the impact of the
lowering of baselevel in a valley with a stream of
sinuosity (P) of 1.5. The line A-C represents the
channel profile and the line A-B represents the profile of the valley floor. Points B and C are at the
river mouth, and points F and G are at the same
Figure 4. Sinuosity of Mississippi River for each reach
of figure 3 for four surveys.
location in the valley. The channel distance is onethird longer than the valley distance, and the difference in channel and valley slope reflects the sinuosity of the stream. The length of the channel is
1.5 times the length of the valley and, therefore,
the stream gradient is one-third less than the valley slope (equation 1). If in the worst case, a vertical fall of baselevel from B to D and C to E is assumed, channel incision and lateral erosion will
steepen the valley floor. If the channel is not confined laterally, it can adjust to the increased valley
slope (F-D) by increasing sinuosity to 2.0, and the
channel profile is extended to H. In this case, incision ceases at point F in the valley and at point G
in the channel because the increase of sinuosity
from 1.5 to 2.0 from G to H maintains a constant
channel gradient over the reach of increased valley
slope (F-D). The one-third increase of channel
length (sinuosity) between G and H compensates
for a one-third steepening of the valley floor from
F to D.
According to Lane's assumptions, the effect of
this baselevel fall would be propagated upstream
to point A, where an amount of erosion equal to
B-D would occur. However, because the stream
can adjust, the steepening of the valley floor will
not result in a change of stream gradient. Rather
the channel lengthens, and the effect of baselevel
lowering is propagated only a relatively short distance upstream. The distance will undoubtedly depend on local conditions and the original slope of
the valley floor, but this exercise supports Saucier's
(1991) contention that Pleistocene sea level change
in the lower Mississippi valley was effective only
as far as Baton Rouge. The probability that a large
river can adjust in this fashion is made more likely
by the fact that the baselevel changes in nature
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Figure 5. Effect of a baselevel fall
(B-D) on channel length and pattern.
See text for discussion.
will take place relatively slowly and not abruptly,
unlike during the experimental studies. The river,
therefore, has more time to adjust by changing sinuosity.
Figure 6 shows an abrupt rise in baselevel from
B to D for a valley profile A-B and a channel profile
A-C. Under Lane's assumptions, deposition would
backfill the valley causing deposition equal to B-D
at point A. This situation, identical to deposition
behind a dam, will produce deposition, but when
the valley aggrades to a slope (G-D) equal to the
original channel gradient (A-C), which is necessary
to transport water and the sediment load to the
new shoreline at point F, deposition should cease.
On the reduced valley slope (G-D), sinuosity will
decrease from 1.5 (A-H) to 1.0 (H-F). In this way,
the gradient of the channel is maintained, and deposition ceases at point G in the valley and point
H in the channel. In this case, the effect of an
abrupt rise in baselevel was not propagated upstream to point A because the effect was absorbed
by an adjustment of channel pattern from H to F.
Figures 5 and 6 show only two examples of baselevel change, where the sinuosity of the river before the change was 1.5. However, sinuosity could
range from 1.0 to about 2.5. If the sinuosity of the
river was 1.2 in figure 5, then the adjusted sinuosity would be about 1.6 instead of 2.0. If the sinuosity of the river was 2.0 instead of 1.5 in figure 6,
then the adjusted sinuosity would be about 1.3 and
instead of a straight channel, a sinuous channel
would cross the shelf.
Further evidence for the type of channel response shown in figures 5 and 6 is demonstrated
by the experimental studies of Jeff Ware (1992 oral
comm.). He lowered baselevel relatively slowly to
a maximum of 12 cm in a flume with a total length
of 18.4 m. This change would have doubled the
channel gradient. However, the effect of the baselevel lowering extended only 4 m upstream, and
the change in baselevel, as in figure 5, was accommodated by an increase of sinuosity from 1.2 to 1.5
in the lower 4 m in the flume. It is clear, however,
that pattern change did not totally compensate for
the change of baselevel. This channel widened and
roughness increased, thereby assuming part of the
adjustment to the baselevel change. If some of the
adjustment is accomplished by a depth decrease
(channel widens) and a roughness increase, then
some sinuosity would persist even in the example
of figure 6.
The Manning Equation for velocity of flow (V)
reveals that velocity is a function of hydraulic radius or average flow depth (R) gradient (S) and
channel roughness (n) as follows:
V
Figure 6. Effect of baselevel rise (B-D) on channel
length and pattern. See text for discussion.
1.49 R2/3 1/2
n
(2)
Ware's experiments showed that a sinuosity increase, which resulted in a slope decrease, was only
part of the adjustment, and both depth (R) and
roughness (n) adjusted to decrease velocity and
stream power. Therefore, the obvious pattern
changes (figures 4, 5, and 6) may only absorb part
of a valley slope or baselevel change, as suggested
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S. A. SCHUMM
by Leopold and Bull (1979). In addition, upstream
incision was producing more sediment, which required a steeper gradient.
Figure 7 carries the discussion farther by illustrating a change of baselevel from B to C, across
continental shelfs of different gradients (Miall
1989). In examples a and b, the slope of the continental shelf is gentler than that of the stream channel. In example c, it is identical, and in examples
d and e, it is steeper. In example a, the decrease in
slope is large. The channel cannot adjust to this by
a change of pattern or other channel characteristics
(equation 2), so aggradation takes place, and a
wedge of sediment is deposited. This increases the
gradient of the shelf, and it would decrease the gradient of the stream from D to C but a decrease of
sinuosity compensates, as in figure 6. The slope of
the channel between D and C is the slope required
by the stream to transport the sediment load to C
as in figure 6. In example b, the reduction of slope
at B is not great, and the channel can compensate
by straightening and reducing sinuosity without
deposition. In example c, the slope of the shelf is
identical to the slope of the stream, and therefore
the channel can extend itself across the surface
without aggradation or degradation. Although as it
does so, the new channel will probably form by
developing natural levees and scouring slightly
into the existing surface. In example d, the slope
of the shelf is steeper than that of the stream, but
the increase is not large, and the stream can accommodate this by an increase in sinuosity between B and C as in figure 5. However, in example
e, the slope of the shelf is much steeper, and the
channel cannot adjust only by a pattern change.
Incision takes place, and the channel probably widens and braids. This effect of this will be propagated upstream for some distance (C to D) until
the channel can accommodate the slope change by
a change of pattern as in figure 5 as well as by a
change of shape and roughness (equation 2).
The discussion so far has been with regard to
the response of sinuous streams to an increase or
decrease of slope (figures 5, 6, 7). A braided stream,
however, may not be able to adjust to a steeper
valley slope. Experimental studies of the effect
of baselevel change on braided streams by Germanowski (1990, written comm.) show that in
these wide channels, formed in low-cohesion sediments, a baselevel lowering causes great channel
instability. Nickpoints formed as baselevel was
lowered at the end of a flume by removal of 1.9 cm
thick boards, but they quickly lost their identity
by reclining. The amount of incision decreased progressively upstream until the rejuvenated reach
Figure 7. Effect of baselevel change across continental
shelves of different inclination. See text for discussion.
merged with the unaffected upstream reach (figure
8). After 8.5 hrs, incision had not extended beyond
9 m upstream (Run 3D). With time the channel
would have undoubtedly adjusted further, but this
experiment best illustrates the declining rate of incision upstream and the declining rate of sediment
delivery, as shear stress is progressively distributed
over a greater channel length. As discussed earlier,
the situation may be very different when a channel
is confined and its ability to shift laterally is restricted.
It can be concluded that, depending upon the
circumstances of baselevel lowering, there can be
aggradation, degradation, or little change (Butcher
1989). Nevertheless, this is a very simplistic view
of the response of the fluvial system to change. For
example, figure 8 shows that, as incision took
place in the lower part of the flume, there was aggradation upstream in response to the sediment being fed into the channel. When baselevel was
raised in this experiment, deposition eventually
extended upstream to the 9 m limit of incision, but
degradation was occurring above this point, as a
result of the previous baselevel lowering. In fact,
in most incised channels, upstream degradation
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(a)
(b)
3
4
5
6
7
8
9
10
11
12
13
14
1
16
17
18 l19
Distance Down Channel (m)
Figure 8. Longitudinal profiles of braided streams
formed during baselevel lowering. Profiles indicate three
stages of adjustment during experiment 3. Effect of baselevel change ceased at meter 10 (9 m upstream) after 8.5
hrs (from Germanowski 1990 and written commun.).
causes downstream aggradation so that the stream
reaches are out of phase. Furthermore, studies of
incised channels reveal that the increased sediment loads generated by incision will lead to deposition in a channel (figure 9). For example, stream
capture rejuvenated a stream in Texas, which led
to rejuvenation of tributary streams, larger sediment loads, and downstream deposition (Shepherd
1979; Gardner 1983). This response has also been
noted during experimental rejuvenation of drainage networks (Schumm et al. 1987, p. 95). The end
result of this complex response of rejuvenated fluvial systems is highly variable sediment delivery
to the depositional basin, which is characteristic of
fluvial sedimentary deposits (Johnson et al. 1985;
Badgley and Tauxe 1990; McRae 1990; Schumm
1981).
Effects of Baselevel Change on the
Continental Shelf
Morphologic changes of the shelf-slope channels
are important, but the schematic two-dimensional
sketches of figure 7 provide only part of the story.
In figure 7a, where significant deposition is required to maintain the gradient of the channel, an
alluvial fan or fan-delta will form as the unconfined channel shifts widely across the shelf (Figure
10a). A broad convexity will form in cross section,
and the shelf topography will become irregular and
assume the morphology of a gently sloping bajada
surface that rises toward the major channels that
cross the shelf. In figure 7b,c, and d, the channel
has adjusted its pattern to accommodate the shelf
(e)
Figure 9. Evolution of an incising channel from initial
incision (a, b), and widening (a, b, c, d) to deposition (c,
d), and eventual stability (e). This evolution occurs at
one cross section through time, but it can be observed
along the channel from upstream (a) to downstream (e)
(after Schumm et al. 1984).
slope, and deposition will be minor. Nevertheless,
natural levees will form and channel avulsion will
be common as channels become inefficient and are
abandoned (figure 10b). The shelf will be a dynamic
surface modified by frequent avulsions. In figure
7e, the shelf is incised, and sediment will be delivered to the shelf edge through the incised channel
(figure 10c). Unlike examples a and b of figure 10,
the incised channel should remain fixed in one position until it is filled by deposition.
Small channels will also develop on the shelf, as
runoff from the sloping surface is concentrated and
dendritic drainage networks form (figure 10). These
channels will incise less than the main channel,
but because they are at a lower elevation than the
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S. A. SCHUMM
Figure 10. Plan views and cross sections of three different shelf situations: (a) slope of shelf is gentle (figure 7a)
and deposition produces a fan delta, which in the cross
section rises above the shelf surface and sets the stage
for incision or avulsion, (b) slope of shelf is similar to
valley slope (figure 7c) and the channel extends across
the slope building natural levees and being subject to
avulsion, (c) slope of shelf is steep (figure 7e) and incision
occurs leaving smaller shelf channels perched above the
incision.
main channel (figure 10a,b) they can readily capture it and shift its mouth laterally. This type of
channel behavior has been well documented for a
different environment at the foot of the Book Cliffs
(Rich 1935), the Henry Mountains in Utah (Hunt et
al. 1953), and the Beartooth Mountains of Montana
(Ritter 1972), but the general morphologic situation is similar.
Much of the preceding discussion relates to sealevel change over sloping surfaces of different inclinations (figures 7, 10). However, when sealevel
lowering brings baselevel to the edge of the continental shelf and down the continental slope, channels will incise and valleys will form. In fact, there
should be small, medium, and large valleys on the
shelf as follows: (1) large deeply incised valleys
(figure 10c) are the extension of major rivers draining large areas of the continent, (2) medium-sized
valleys are the extension of small rivers that drain
from smaller drainage basins, and (3) small valleys
that formed on the shelf by runoff draining from
only the shelf to form dendritic drainage patterns
(figure 10). For example, channels and valleys of
different dimensions have been described by VanWagoner et al. (1990) on the Gulf Coast.
A series of experimental studies concerning the
effect of shelf inclination, rate of baselevel change,
and steepness of the sediment source area (sediment-yield rates) on the response to baselevel
change were performed at Colorado state University (Koss 1992; Wood et al. 1992a, 1992b). Koss
used the 9.2 by 15.5 m rainfall-erosion facility
(figure 11) to determine how a lowering of baselevel across a shelf sloping at 1% and down a 22%
slope affected the stream system and the shelf and
slope morphology. His experimental design resembled figure 7b with a shelf inclination less than the
slope of the stream draining to the slope. During
12 hrs, Koss lowered the water level 7.2 cm, which
brought the shoreline across the shelf and 3.2 cm
below the shelf break (figure 11). This water level
was held constant for 4 hrs, and then the water
level was raised to its original position on the shelf.
Under these conditions, a fan delta developed at
the mouth of the stream that drained the sediment
source area (figure 11). During the first 3 cm of
baselevel fall, the shoreline remained on the shelf,
the delta prograded rapidly, and water from the
stream spread over the delta and shelf surface.
Once the shoreline crossed the edge of the shelf
and dropped below it, many small valleys formed,
and they extended upslope toward the available
water. The channel or valley that captured the
most water grew fastest, and as a result it captured
more of the water supply and eventually joined the
main channel at the delta head. When this occurred, fluvial sediments from the source area were
moved to the shelf edge and down the slope to form
a shelf-edge delta (figure 11). As this channel now
conveyed all of the runoff from the source area, the
other defeated channels ceased headward growth.
The major cross-shelf channel did not always
form at the center line of the delta; rather it followed the track of the maximum water flow. In
one experiment it bisected the delta (A), and in the
other it followed the margin of the delta (B). As the
water level rose, the small shelf valleys were filled
with reworked shelf sediments, whereas the crossshelf valleys were filled with sandy source-area
sediment.
These experiments suggest that coarse sediments will be found in the cross-shelf valley and
finer reworked shelf sediments will fill the small
shelf-edge valleys. Therefore, the experiments help
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ently had little or no influence on source-area
stream behavior. As the delta formed there was deposition in the lower valley above the delta, but
the effect was felt only in the lower part of the
drainage system. This experiment further supports
the conclusion that a baselevel change alone may
not greatly influence the upstream fluvial system.
Implications
Figure 11. Map showing results of baselevel lowering
experiments. When water level was lowered slowly
across the shelf from position 1 to position 2, a shelf
delta (A) formed. When the water level fell below the
shelf edge, numerous channels incised into the shelf to
form small valleys (v). When one channel captured the
flow, a cross-shelf valley formed that, during one experiment, bisected the shelf delta (A). However, during another experiment, the cross-shelf valley formed on the
margin of the delta (B). Both valleys delivered sediment
from the source area and incised into the delta and shelf,
and slope deltas, (A', B1) formed. Slope of the drainage
basin was 5%, of the shelf 1%, and of the slope 22%
(from Koss 1992).
to explain the three different types of channels or
valleys identified on the shelf in the rock record
(VanWagoner et al. 1990, figure 26). The master
stream forms a large incised valley filled primarily
with fluvial sediment. Smaller valleys that are incised to shallower depths may reflect distributaries
or smaller streams crossing the shelf. Finally the
smallest valleys, that formed wholly on the shelf
and were never able to tap a supply of mainland
sediment, are filled with silts and clays. In addition, the seaward extension of a valley may not
follow the shortest route across the shelf.
Koss also determined that, although the fluctuating water level should have produced significant
changes in the channels of the source area, it appar-
for Sequence Stratigraphy
The sequence stratigraphic models proposed by
Vail and his colleagues (Vail et al. 1977; Posamentier et al. 1988) and their significance for hydrocarbon exploration appears to depend upon a global
mechanism, and they have chosen eustatic sealevel change. However, a controversy has developed concerning this assumption, and Miall (1991)
and Galloway (1989a, 1989b) believe that tectonism and changes of sediment delivery rates also
control the character of the shelf-slope deposits.
Sloss (1988, 1991) also argues for the significance
of tectonics and concludes that the eustatic explanation is too simple. Galloway (1991a) states that
sediment delivery, subsidence rate, and eustatic
sealevel change can be of varying significance in
sequence stratigraphy. Sediment delivery can be affected by tectonics, isostatic adjustment, climate,
and of course sea-level change.
The controversy concerning the effect of a baselevel change on river response is not surprising,
considering the complexity of the fluvial and shelfslope systems. A large number of variables influence the impact of baselevel change. As demonstrated above, baselevel changes can have major,
intermediate, or minimal effects depending on bedrock and structural controls, the nature of the affected river, the amount and rate of baselevel
change, the inclination of the newly exposed shelf
surface, and perhaps the hydrologic character of
the river. The impact on the shelf and slope will
depend on their gradient and on the type and quantity of sediment delivered from the sediment
source area, which depends upon erodibility of the
available materials, climate and relief. Therefore,
on a global basis, the effect of a sea-level change
will be highly variable.
Although a variety of effects are possible, the
propagation of the effects of baselevel change along
a large alluvial river probably will be moderate,
moving upstream, but not for long distances. Total
rejuvenation of the drainage system is not expected, although the effect will be greatest where
baselevel change is great, incision is rapid, and the
rivers are confined (figure 2).
All of the above casts doubt upon the conclusion
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of Aubrey (1989) that incision progressed inland
320 km from an early Cretaceous shoreline. Sealevel fall was about 50 m, which is consistent with
45 m of incision near the coast, but incision was
30 m about 320 km inland. This baselevel decline
would have increased channel gradients over this
distance by only 0.00016, which could be accommodated readily by channel adjustment (figures 5,
6, and 7). A much greater baselevel change would
be required to cause 30 m of incision so far from
the coast. Because subsidence is cited as the explanation for the subsequent filling of these incised
valleys, Weimer's (1983) conclusion that uplift of
much of the region was the cause of the inland
valley incision seems more reasonable.
As long-term accelerated sediment delivery to
the shelf-slope would be unlikely with a simple
baselevel lowering, it is probable that large accumulations of sediment reflect not only baselevel
change, but perhaps uplift of the sediment source
area, which, in turn, could lead to orographic climate change. Perhaps Sloss (1991) was correct in
suggesting that a pulse or expansion of the earth
causes the sequence-stratigraphic relationships by
changing both baselevel and relief of the sedimentsource area. It is likely that orogeny, epeirogeny,
and eustasy act together (Johnson 1971), perhaps
in response to changing rates of subduction (Gurnis 1992), interplate stress (Cloetingh 1986), or
changes of heat loss or flow (Worsley et al. 1984).
A major problem when dealing with complex
systems is convergence, which occurs when different processes or causes produce similar results
(Schumm 1991, p. 58-61). For example, increased
sediment deposition on the shelf and slope could
be caused by baselevel change, climate change, or
tectonics. In fact, the explanation could be multiple with all three factors interacting to produce the
stratigraphic results. Nevertheless, it may be possible to discriminate among the effects of baselevel,
climate change, and tectonic influences. For example, a climate change or uplift that produced a
higher sediment load relative to discharge would
lead to downfilling of a valley. The increased sediment load would lead to deposition where the valley profile lessened, and the valley would fill progressively in a downstream direction (figure 12a).
In contrast, valley filling as a result of sea-level rise
would cause backfilling (figure 12b). The deposits
should differ in several ways, although it may not
be possible to detect in outcrop. The downfilling
deposit should wedge out both up and downstream, whereas the backfilling deposit should
thicken downstream.
The downfilling deposit
should fine downstream, as the coarsest sediments
a
b
Figure 12. Comparison of (a) downfilling and (b) backfilling in a valley.
would be deposited upstream, and it should
coarsen upward, as the coarser sediment eventually moves downstream on a steeper valley slope
(figure 12a). This type of valley-fill deposit should
be similar to a thin deltaic deposit. There should
also be a marked change in the character (mineralogy, grain size) and volume of the sediment delivered, as erosion progresses in the upper part of the
drainage basin. In contrast, the backfilling deposit
would fine upward as the coarser sediments are
progressively deposited farther upstream. Also, the
character and quantity of sediment should not
change because most of the sediment source area
would not be affected by a rise of baselevel.
It might also be possible to distinguish among
baselevel, climate, and tectonic influences by the
change of sediment delivery with time (figure 13).
Based upon experimental studies (Schumm et al.
1987) and field observations (Gellis et al. 1991),
rapid baselevel lowering produces a major shortterm pulse of sediment (figure 13a). As incision
progresses through the drainage network, less sediment is produced and much of it is stored in the
newly developed incised channel (figure 9). Germanowski's (1990) and Gardner's (1983) experiments, which showed that the rate of erosion following baselevel lowering declined exponentially
as the initial nickpoint lengthened (figure 8), support this conclusion. A similar result could be anticipated from Ware's experiment when baselevel
lowering was compensated by an increase of sinuosity, a decrease of depth, and increased roughness.
When the baselevel change is slow, the impact will
be less and it will be delayed (figure 13a).
The effect of uplift in the sediment source area
should be pronounced and longer-lasting than the
effects of a baselevel change, and there will be an
exponential increase of sediment production as relief increases (Schumm 1963). There will be a lag
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291
span involved in the sketches of figure 13. The rising limb of each curve will depend upon the rates
of baselevel, tectonic, and climate change. Abrupt
changes of baselevel in channelized streams of
Mississippi set in motion a series of channel adjustments that were essentially
completed in
about 50 yrs (Schumm et al. 1984), whereas some
of the large arroyos in the southwestern United
States require at least 100 plus yrs to produce a
curve similar to figure 13a (Gellis et al. 1990;
Schumm 1991).
a) BASELEVEL
b) TECTONICS
Conclusions
TIME
Figure 13. Effects of (a) baselevel, (b) tectonic, and (c)
climate change on sediment delivery to the coast. See
text for discussion.
before the effect reaches the coast, but with time,
the decline of sediment delivery will be less than
for the baselevel change, and the higher rate of sediment delivery would persist for a long time (figure
13b). The magnitude of the response would, of
course, depend upon the quantity of sediment
stored in the uplifted landscape and the resistance
of the rocks affected. Weak rocks will continue to
produce large amounts of sediment, whereas resistant rocks will produce less after the weathered
mantle is removed (figure 13b).
A climate change (figure 13c) will have divergent effects (Langbein and Schumm 1958). A
change from a humid to semiarid or to a markedly
seasonal climate would produce the greatest increase of sediment, whereas a change from humid
to subhumid or to superhumid might have little
effect (Langbein and Schumm 1958). A change
from humid to arid would greatly decrease sediment delivery. Climate changes will be rapid
in comparison to most baselevel and tectonic
changes, and therefore, the response will be rapid
(figure 13c).
Unfortunately, little can be said about the time
It appears that for long sustained sediment delivery
to the coast, uplift is the most likely explanation,
because the upstream effect of baselevel change
will be moderate. This conclusion is supported by
the fact that the large rivers with large sediment
loads usually originate in orogenic belts (Dickinson 1988; Audley-Charles and Curray 1977). It is
further supported by Galloway's (1989a) conclusion that "Volumes of sediment excavated by valley incision of coastal plains exposed at relative
low stands are inconsequential in the context of
sequence stratigraphy volumes. Basins cannot be
filled by self-cannibalization." An important point
is that incised valleys on the continental shelf
range in width from "less than several miles to
many tens of miles" (VanWagoner et al. 1990).
This means that lateral erosion has widened the
valleys and the channels are not confined. The Yellow River example (figure 2) confirms that when
channels are not confined, the impact of baselevel
change will be limited in an upstream direction.
It is unfortunate that so many factors influence
the effect of baselevel change on the fluvial and
shelf-slope systems. Under certain circumstances,
the effect of baselevel change will be local and minor; under other conditions, it will be significant.
In most cases, however, large alluvial rivers will
be able to adjust to altered slopes by adjustments
of sinuosity, channel dimensions, and roughness
(equations 1 and 2). Therefore, except under extreme conditions, changes of baselevel will not rejuvenate the entire drainage network. A combination of baselevel lowering, which would rejuvenate
the lower part of the fluvial system, and uplift,
which would rejuvenate the upper part, would deliver large quantities of sediment to the coast and
produce the depositional sequences of great interest. Figure 14 is an attempt to summarize these
conclusions visually. For most alluvial rivers, the
effect of a baselevel change can be very significant
at the coast (km 0), whereas the effect of increased
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S. A. SCHUMM
relief depends upon its location. If near the coast
it will be dominant, but even at a great distance
(1000 km) it will have an impact at the coast, the
magnitude of which depends upon the quantity of
sediment delivered. In short, baselevel changes
could affect a valley for perhaps 300 km, as in the
Mississippi valley, whereas increased relief and, of
course, a climate change can affect the entire fluvial system.
ACKNOWLEDGMENTS
Distance
(Km)
Figure 14. Relative effect of changes of relief and baselevel (based upon an idea of Henry Posamentier). Distance is upstream from baselevel at 0 km. See text for
discussion.
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