WSP05833

• File Number: 8062
• Description: Federal Water Rights
• State: CO
• Basin: Statewide
• Date: 10/24/1983
• Author: S A Schumm
• Title: River Meanderings - Proceedings of the Conference of Rivers - 1983
• Water Supply Pro - Doc Type: Report/Study

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RIVER MEANDERING

One of the basic problems with time is that an individual does not
live long enough to appreciate or to understand the present sufficiently
to use it for prediction. Many geomorphic processes operate too slowly
to provide an adequate record for prediction. The record ;s too short,
and frequently this means that the geomorphologtst concentrates on
rapid and dramatic landscape changes (landslides. badland erosion, high
energy rivers). As a result. a bias toward small I relati~ely-rapidly-
changing features may be introduced into the literature. for example.
most studies of meander cutoffs document significant alterations of a
channel at. above, and below the cutoff. but Brice (27) shows that only
a fraction (about 20%) of the rivers that he studied did, indeed,
adjust significantly to meander cutoffs. Clearly. for the great range
of ri ver types genera 1 i za t ions a re dangerous. .

Another way of appreciating this problem is to consider a shifting
life span (8). A human with a life span of one day would likely con-
clude that rivers are static. If it was 100 years the perception
would be that rivers change position and morphology. If ~t was 10,000
years the person would note changes, as a result of climatic and
tectonic instAbility and the eVOlutionary development of rivers and
drainage systems.

An example of the importance of time is provided by a meander of
the Cimarron River near Perkins, Oklahoma (Fig. 1). From the point of
view of the highway engineer who built the bridge at this site in 1953
there was no evidence of potential change, although there was erosion
of the south bank upstream of the bridge. which was rip-rapped in 1957.
In 1959 the erosional attack shifted to the north bank (28). The high-
way engineer had no record of change or any suspicion of potential
change at the bridge site, but obviously it was in jeopardy from
meander shift (Fig. 1), and a geomorphic study would have revealed
thIs fact.

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Area or size of a watershed is closely related to many
character\stics such as stream gradient, cnannel dimensions, etc.
Furthermore. as scale changes (size) the explanation may change as the
size and comple~ity of the feature increases or decreases. For example.
although local variations of stream gradient can be explained by
variation of bed-material size. the gradient of a long river segment
is better explained by water discharge (lB). Consider also the
apparent controls on sediment yields for drainage basins in areas of
increasing size. For a group of small drainage basins in a local area
of similar climate and geolOgy, the control of sediment yield ~ill
likely be the average slope of the basin or relief, but for a larger
area the control will be geologic (10) and for regional or contInental
studies the control can be climatic (14). Also, as ba~1n size increases
sediment yield per unit area decreases (22).

Scale is very important in prediction. The longer the time span
involved and the larger the area the less accurate will be predictions
that are based upon present conditions or on short records. Therefore,
extrapolation of modern records is hazardous. and geomorphic predictions
for periods in excess of perhaps 1000 years should be based upon worst-
case conditions of climate change. tectonic activity and base-level
change, which involves an understandi~g of geologic history.

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RIVER MORPHOLOGY AND BEHAVIOR

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location:

Even the smaller components of a landscape such as first-order
streams have a considerable range of potential energy from mouth to
drainage divide. The morphology varies and the material and energy
flow varies, Sediment de11ver~ ratIos (ratIo of sediment export to
sediment production in a basin) are usually below 1.0. indicating
sediment storage and a decrease of sediment yield per unit area down.
stream. Therefore. at anyone time components of the system may be out
of phase. In other words some portions of a river may be degrading
while others are aggrading.

In high energy landscapes not all components need to function 1n
phase. Some tributaries or river reaches may b~ stable, whereas others
are aggrading or degrading. depending on local circumstances or the
rate at which they respond to changes In the main channel (22, 25),
This complexIty is the result of energy differences wIthin the basin
and the random occurrence of hydrologic and meteorological events
throughout the system. Therefore. conclusions about future conditions
wIll depend on the part of the system studied, For example, rejuvena-
tion of a drainage basin by a base-level change will cause a wave of
accelerated erosion to advance upstream. Depending on the size of the
basin, the lag time for features near the drainage divide may be very
long, and events in one part of the basIn may be very different from
those occurring elsewhere for hundreds or even thousands of years.

The Cimarron River bridge site (Fig. 1) is an example of both a
time and location problem, but an even better, although e~trerne,
example is a small steep badlands area northeast of lusk. Wyoming
(Fig. 2). The small drainage basins can be subdivided into three zones
as follows (1): 20ne 1 is the headwater high-sediment production area,
Zone 2 is an intermediate zone of sediment storage and transport and
Zone 3 is a braided-stream transport zone. Seasonal changes during one
year produce dramatic changes in the three zones as follows:

Zone 1: An idealized sequence of cross sections in lower zone 1
Is shown on Figure 2a. The numbers, 1 through 4, Identify the altitude
of the valley floor and channel at dIfferent tImes durin9 the year. In
late winter and early spring the bedrock floor of the valley is exposed
(1), but the adjacent hillslopes are covered wIth siltstone fragments
produced by freeze-thaw (l). SprIng rains mobilize this sedIment and
cause valley filling (2a and 2b). PrecIpitation on bare slopes and
upstream bedrock during late spring and summer yields flashy runoff
with a low sediment load, which causes incision and removal of much of
the stored alluvium (3). Subsequent runoff events cause further inci-
sion back to the bedrock valley floor (4) leaving a valley-side
terrace (3).

Zone 2; At stage 1 in Zone 2 (Fig. 2b) a valley fill is present
(l). Early spring runoff incises this alluvium (2), and produces a
terrace or multiple terraces. As sediment is introduced to Zone 2.
the result of major incIsIon of the Zone I valley fill, aggradation
raises the valley floor (3). When Zone 1 sediment production decreases,
as the channels reach bedrock, the Zone 2 alluvIal fill is eroded
episodically (4a, 4b, 4c), Zone 2 Is clearly out ~f phase wIth Zone 1,

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