Perhaps of more interest here are the changes during shorter periods of time, as river bends
form, grow, and cutoff. Most meandering rivers will show significant change of sinuosity
through time. Figure 5.5 shows the range of sinuosity between 1765 and 1930 for 24
reaches of the lower Mississippi River. Obviously the Mississippi River is composed of very
different reaches that behaved differently during the period of record. These are the types of
changes to be expected with time along any active river, but the changes also can be
avulsive and of a catastrophic nature with abandonment of one channel, and formation of
another. This frequently occurs on deltas, and alluvial plains, and it is exemplified by great
lateral shifts of the Indus River (Holmes 1968). Indeed, avulsion of the Mississippi River
down the Atachafalaya River channel is only prevented by major flood-control structures.
If as a result of climatic fluctuations and human activities, the sediment load, flood peaks,
and water discharge of a river are altered, a river response can be expected. However, the
type of response will depend upon the nature of the river. For example, sinuous rivers could
become straight, and braided rivers could become sinuous, or the changes could be very
minor, depending upon the sensitivity of the river (Schumm and Beathard (1976).
Figure 5.5. Variability of sinuosity between 1765 and 1915 for 24 Mississippi River reaches.
Numbers identify the reaches of Figures 5.6 and 5.7 (after Schumm et al. 1994).
The greatest modern river changes have been the result of human activity. The Mississippi
River was straightened and shortened 229 kilometers (142 miles) between 1933 and 1942.
The goal was to reduce flood peaks and to improve navigation. This great river experiment
yielded both beneficial and undesirable results. Floods were reduced, but channel stability
decreased (Winkley 1977, 1994).
5.3.2 Differences Between Reaches
Perhaps of greater interest to river engineers is the variability along a single river. One could
assume that large alluvial rivers should have a relatively uniform morphology because the
controlling factors of water discharge and sediment load should not vary greatly. However,
other factors intervene to cause considerable variability. For example, a glance at even a
coarse-scale map of the pre-cutoff Mississippi River reveals great variability. In fact, 24
distinct reaches were identified between Cairo, Illinois and Old River, a distance of 768 valley
kilometers (477 miles) (Figure 5.6). The reaches were identified by changes of valley slope,
5.8
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