In order to cope with alluvial rivers, an understanding of their complexity in space and
through time is necessary. Alluvial rivers differ in three ways:
1. Rivers differ among themselves depending on hydrology, sediment loads, and geologic
history (in other words, rivers differ among themselves).
2. Rivers change naturally through time and as a result of climate and hydrologic change.
3. Along any one river there can be considerable variability of channel morphology as a
result of geologic and geomorphic controls.
Information on these differences, especially the last two, will aid in predicting future river
behavior and their response to human activities.
An important consideration in predicting future river behavior and response is the sensitivity
of the channel. That is, how readily will it respond to change or how close is it to undergoing
a change without an external influence? For example, individual meanders frequently
develop progressively to an unstable form, and a chute or neck cutoff results, which leads to
local and short-term channel adjustments. The cutting off of numerous meanders along the
Mississippi River caused dramatic changes, as a result of steepening of the gradient, which
led to serious bank erosion and scour (Winkley 1977). However, Brice (1980) studied the
effect of cutoffs and channel alignment on numerous smaller streams, and he found that,
although some responded in a manner similar to the Mississippi, many did not. Those that
did not were characterized by stable banks and gentle slopes. For example, Stevens (1994)
found that the Citanduy River in Java has remained relatively stable after a total of 23 cutoffs
because it has very resistant clayey bed and banks. Therefore, some streams are sensitive
whereas others are not. Obviously, care must be taken before the behavior and response of
one stream can be extrapolated to another.
The differences among rivers is often reflected in their channel patterns. During an
experimental study of a channel in a laboratory flume, the channel changed from straight, to
sinuous and finally to braided, as flume slope (and thereby sediment load and stream power)
increased (Figure 5.3). The relatively straight River Nile and the relatively meandering
Mississippi River reflect geomorphic history as well as current geologic controls such as
active tectonics, tributary contributions and other factors that affect valley slope. For
example, the difference between the Nile and other large rivers may be due to the fact that
the Mediterranean Sea evaporated during early Tertiary time (Hs 1983, Said 1981). A
blockage at the Straights of Gibraltar stopped the inflow of Atlantic Ocean water into the
Mediterranean basin. This significant lowering of base level caused incision of streams
draining into the Mediterranean, and a deep canyon was cut which formed the Nile valley.
When sea level rose again, marine water entered this canyon, and marine sediments are
found as far up-river as Aswan. The Nile then filled this trough with fluvial sediments at a
relatively gentle slope that was needed to move water and sediment to the newly established
higher base level. In contrast to this unique history, the Mississippi River developed a
relatively steep valley slope as the result of the influx of outwash sediment from melting
continental glaciers. After retreat of the ice, the valley slope was steeper than was required
to transport the reduced sediment load, and the Mississippi developed a sinuous course in
order to reduce its gradient. The difference between these two great alluvial rivers,
therefore, has much to do with their geologic and geomorphic history.
5.6
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