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These are large assumptions to make, but in many instances they are valid. However, at other times, changes must be anticipated and incor- porated into the logic train of the model. Whenever a substantial land use or channel change is anticipated, the biologist should be prepared to query the engineering/hydrology community concerning the resulting shape and character of the stream channel. In order to ask the. right questions, the biologist should have an elementary understanding of three basic concepts of fluvial geomor- phology-- equilibrium,.aggradation, and degradation. Aggradati on refers to a: persistent rise in the elevation of the streambed, whereas degradation is a persistent lowering.. A stream which is neither aggrading nor degrading is said to be in equilibrium. This equilibrium condition is not static. It fluctuates from year to year about some average condition as the stream experiences some annual pattern of scour and fill. As long as the net change in stream bed elevation over a number of years is near zero, the stream is said to exist in a state of "dynamic equilibrium." For a stream which is in a state of dynamic equilibrium and is expected to remain so, study results can be extended into time with relative certainty even though a particular cross section's shape is noticeably changed by scour or fill.. A corollary to the definition of "dynamic equilibriums" is that. a stable relationship exists between sediment yield and stream flows in the watershed. Therefore, if one cross. section is scoured, another is filled. The character of a specific study _ site may change, but if the river is truly in a state of equilibrium, its overall character or condition will remain constant and flow recommendations will be applicable into time. Aggradation and degradation are usually insidious, long term, processes reflecting a fundamental change in the relationship between sediment yield and stream discharge in the watershed. Over the long term, these processes may have more profound effect on a fishery resource than discharge itself. While it is difficult to quantify the magnitude of channel changes, a compilation of case histories assembled by Lane (1955) gives considerable 'insight into the types of changes to anticipate under different sediment yield/stream discharge conditions.. A reduction of discharge with no corresponding reduction in sediment load promotes aggradation. This type of impact is often associated with flood control reservoirs and out-of-stream diversions. The result of this type of perturbation is a reduction in both width and depth, and an increase in the width to depth ratio. Because the stream has less energy than required to transport sediment, the particle size of the substrate is typically reduced. These streams often experience encroachment by riparian vegetation which tends to stabilize the new channel configuration (Fraser, 1971).