Regulated Rivers - Hydro-Environmental Analysis: Freshwater Environments - page 63

Stage I. Sinuous, premodified
h < h c

h c = Critical bank height
= Direction of bank or
bed movement

h

Stage II. Channelized
h < h c

Stage IV. Degradation and widening
h > h c

Stage III. Degradation
h < h c

Te rrace

Floodplain

h

h

h

Slumped material

Stage V. Aggradation and widening
h > h c

Stage VI. Quasi equilibrium
h < h c

Te rrace Terrace

Bank
Bank-full

h

h

Slumped
material

Aggraded material Aggraded material

Stage II

Primary
knickpoint

Stage III

Stage IV

Stage V

Plunge
pool

Precursor
knickpoint

Stage VI

Secondary
knickpoint

Oversteepened reach

Aggradation zone

Aggraded material

FIGURE 3.21 Stages of channel evolution. (From Simon, A. and Hupp, C.R., Proceedings of the Forest
Federal Interagency Sedimentation Conference , Las Vegas, NV, U.S. Interagency Advisory Committee on
Water Data, Washington, DC, 1986; from Langendoen, E.J., Concepts: Conservational channel evolution and
pollutant transport system, Workshop presented at Mississippi State University, 2009.)

Destabilization may result from any change in the sediment load or low regime. For exam-
ple, increasing or decreasing the sediment loads to the system or the inlow hydrograph can
result in destabilization, such as from watershed modiications, as discussed in the following sec-
tion. Destabilization can also result from man-made channel modiications, or channelization.
Channelization may introduce steeper gradients and the steepened channel can cause channel deg-
radation, accelerated bank caving, and an increase in the downstream sediment regime (Schumm
1984; Brice 1982).
As illustrated by the channel evolution model, streams and rivers commonly respond to desta-
bilization by changing vertically by scour or ill (degradation or aggradation) or horizontally by
widening and migration. The Yalobusha River in north-central Mississippi illustrates the impacts
of channelization. European agricultural development in the Yalobusha basin in the nineteenth cen-
tury resulted in the restriction of the channel capacity due to the accumulation of silt and debris.
In the early twentieth century, a 12-mile ditch was excavated through the river's valley, after which