CHAPTER 5
RIVER MORPHOLOGY AND RIVER RESPONSE
5.1 INTRODUCTION
Rivers and river systems have served man in many ways. Rivers are passage ways for
navigation and are essential to agriculture, particularly in the arid and semiarid parts of the
world. To a large degree, the flooding by rivers and the deposition of sediment on the river
valleys have been a means of revitalizing the river valleys to keep them productive. Rivers
have provided a means of traveling inland and developing trade. This has played a significant
role in the development of all countries wherever rivers of significant size exist.
Rivers have different alignments and geometry. There are meandering rivers, braided rivers,
and rivers that are essentially straight. In general, braided rivers are relatively steep and
meandering rivers have more gentle slopes. Meandering rivers that are not subject to rapid
movement, are reasonably predictable in behavior; however, meandering rivers are generally
unstable with eroding banks which may result in destruction of productive land, bridges, bridge
approaches, control works, buildings, and urban properties during floods. Bank protection
works are often necessary to stabilize certain reaches of many rivers and to improve them for
other aspects of flood control and navigation.
5.2 FLUVIAL CYCLES AND PROCESSES
Fundamental characteristics and processes governing the formation of river systems are
discussed in this section. A very general classification of rivers which considers their age is
introduced. The morphology of floodplains, deltas, and alluvial fans is described as well as the
processes of headcutting and nickpoint migration. The concept of geomorphic threshold
completes this section on fundamentals leading to a discussion of variability and change in
large alluvial rivers. Specific aspects of stream form and a simple geomorphic classification of
streams are presented later in this chapter.
5.2.1 Youthful, Mature, and Old Streams
One of the early methods to classify rivers was by relative age as youthful, mature, and old
(Davis 1899, see King and Schumm 1980). As a general concept this has validity with steep
irregular young streams becoming mature with a narrow valley, a floodplain, and a graded
condition; that is, the slope and energy of the streams are just sufficient to transport the
materials delivered to it. As time passes, the valley widens and a fully meandering channel of
low gradient develops. Unfortunately, for this classification, the lower Mississippi River would
be designated as old, when, in fact it is one of the most youthful rivers. It developed on the
alluvium deposited by drainage from the continental ice sheet perhaps 10,000 years ago.
More recently, rivers have been classified based upon type of sediment load (Schumm 1977),
and pattern (Brice 1982). Nevertheless, the concept of landform and channel evolution through
time is a valuable one. Hydraulic Engineering Circular (HEC) No. 20 (Lagasse et al. 2001)
5.1
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