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Channel Processes: River Stability Concepts

Within the scientific community, the terms channel stability, equilibrium, quasi-equilibrium, dynamic equilibrium and regime channels evoke various interpretations. It is necessary for those working with river systems to have a consistent working definition of what constitutes a stable river. A review of the literature provides insight into previous interpretation of terms, that all appear to be synonymous, or at the least, have a common thread of similarity. Davis (1902) defined a "graded " stream as the condition of "balance between erosion and deposition attained by mature rivers". Mackin (1948), as reported by Leopold et al. (1964), defined a graded stream as "one in which, over a period of years, slope is delicately adjusted to provide, with available discharge and with prevailing channel characteristics, just the velocity required for the transport of the load supplied from the drainage basin. The graded stream is a system in equilibrium; its diagnostic characteristic is… "that any change in any of the controlling factors will cause a displacement of the equilibrium in a direction that will tend to absorb the effect of the change." The controlling factors described by Leopold et al (1964) were width, depth, velocity, slope, discharge, size of sediment, concentration of sediment and roughness of the channel. If any one of these variables were changed it sets up a series of concurrent adjustments of the other variables to seek a new equilibrium. Strahler (1957) and Hack (1960), used the term "dynamic equilibrium" referring to an open system in a steady state in which there is a continuous inflow and output of materials, in which the form or character of the system remains unchanged. Equations showing river variables as a function of discharge were derived by Leopold and Maddock, (1953), and by Langbein, (1963). These hydraulic geometry relations described adjustable characteristics of open channel systems in terms of independent and dependent variables in quasi-equilibrium (not aggrading nor degrading). Streams described to be "in regime" are synonymous with "stable channels." Equations describing three-dimensional geometry of stable, mobile, gravel-bed rivers were presented by Hey and Thorne (1986). Additional equations and discussion on stable river morphology were presented in Hey (1997). Regime channels, as discussed by Hey (1997) allow for some erosion and deposition but no net change in dimension, pattern and profile for a period of years. Processes of stream channel scour and or deposition have to occur in a natural stable channel, but over time, if this leads to degradation or aggradation, respectively, then the stream would not be stable.

These concepts of stream channel stability, the "graded river," regime, quasi-equilibrium, and/or dynamic equilibrium together form a central theme in WARSSS. The central tendency, amidst the indeterminate nature of the complex variables that shape channels and provide for erosion and deposition, is that rivers progress toward their most probable form (Leopold, 1994). As previously described in Chapter I, a stable channel is one whose most probable form amongst rivers is the channel morphology that, over time, in the present climate, transports the water and sediment produced by its watershed in such a manner that the stream maintains its dimension, pattern and profile without aggrading nor degrading (Rosgen, 1996).

As stated previously, the geomorphic role of rivers is to transport the flows and the sediment debris of its watershed while maintaining its dimension, pattern and profile without aggrading or degrading. The balance in the sediment input/output is central to the equilibrium of the river channel. Stream channel instability, or dis-equilibrium, is often associated with an excess load of sediment and/or size of delivered sediment beyond the carrying capacity of the river; thus, increases in storage and aggradation can result. Dis-equilibrium can also occur due to stream bank instability causing an increase in width/depth ratio and change in slope and pattern. An increase in the bankfull channel width changes the distribution of energy such that, for the same flow, there is a marked decrease in both shear stress (eqII11b) and in unit stream power (eqII11a). The corresponding reductions in shear stress and stream power that result from a higher width/depth ratio decrease the depth, velocity and slope. These channel and hydraulic changes decrease sediment transport competence and capacity, leading to excess sediment deposition. The adjustment of the channel in the presence of the high width/depth ratio and excess sediment deposition results in an increase in sediment supply from channel enlargement (widening) and lateral adjustments resulting in accelerated streambank erosion. During major floods, channels with a high width/depth ratio can aggrade, causing loss of capacity, filling of pools and significant loss of aquatic habitat. The need to calculate competence and capacity of sediment transport is a requirement in maintaining natural stable channels.

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