landscape may be relatively stable. Nevertheless, stability cannot be automatically assumed.
Rivers are, in fact, the most actively changing of all geomorphic forms.
Evidence from several sources demonstrates that river channels are continually undergoing
changes of position, shape, dimensions, and pattern. In Figure 1.2 a section of the Mississippi
River as it was in 1884 is compared with the same section as observed in 1968. In the lower
9.6 km (6 mi) of river, the surface area has been reduced approximately 50 percent during this
84-year period. Some of this change has been natural and some has been the consequence
of river development work.
Figure 1.2. Comparison of the 1884 and 1968 Mississippi River Channel near Commerce,
Missouri.
In alluvial river systems, it is the rule rather than the exception that banks will erode, sediments
will be deposited and floodplains, islands, and side channels will undergo modification with
time. Changes may be very slow or dramatically rapid. Fisk's (1944) report on the Mississippi
River and his maps showing river position through time are sufficient to convince everyone of
the innate instability of the Mississippi River. The Mississippi is our largest and most
impressive river, and because of its dimensions it has sometimes been considered unique.
This, of course, is not so. Hydraulic and geomorphic laws apply at all scales of comparable
landform evolution. The Mississippi may be thought of as a prototype of many rivers or as a
much larger than prototype model of many sandbed rivers.
Rivers change position and morphology (dimensions, shape, pattern) as a result of changes of
hydrology. Hydrology can change as a result of climatic changes over long periods of time, or
as a result of natural stochastic climatic fluctuations (droughts, floods), or by human
modification of the hydrologic regime. For example, the major climatic changes of recent
geological time (the last few million years of earth history) have triggered dramatic changes in
runoff and sediment loads with corresponding channel alteration. Equally significant during this
time were fluctuations of sea level. During the last continental glaciation, sea level was on the
order of 120 m (400 ft) lower than at present, and this reduction of baselevel caused major
incisions of river valleys near the coasts.
1.4
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