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<br />over a long period, the river reach is considered to be in a state of dynamic <br />equilibrium. A river system in dynamic equilibrium will experience small, local and <br />relatively short-lived variations in the channel geometry that do not effect the <br />overall channel morphology. When the sediment transport balance of an alluvial <br />river reach is disrupted, long term sediment deposition or scour may induce wide- <br />spread changes in the channel morphology (e.g. width-to-depth ratio). Andrews <br />(1986), Butler (1988b), RCI (1991) and Lyons et a 1.( 1992) have all addressed the <br />concept of channel equilibrium. <br /> <br />The concepts of alluvial channel equilibrium, channel stability and <br />adjustment need to be defined in relation to appropriate time scales. Alluvial <br />channels have a bed and bank composed of the sediment that is being transported <br />by the river. According to Schumm (1977) stable alluvial channels are defined as <br />river channels experiencing no progressive adjustment during the last ten years. <br />The concept of an alluvial channel being in equilibrium as related by Schumm <br />(1977) and alluded to by RCI (1991) refers to a channel that has attained a <br />balance between its ability to transport sediment (sediment transport capacity) and <br />the sediment being supplied to it over the long term. An alluvial channel in <br />equilibrium is neither aggrading nor degrading although the channel itself may be <br />incised. RCI (1991) also makes the distinction between alluvial sand-bed channels <br />and alluvial cobble-bed streams. <br /> <br />River evolution can be viewed with different time scales. The geologist or <br />geomorphologist may be interested in changes related to paleoclimate variation or <br />tectonic uplift during a cyclic time span covering millions of years. These <br />processes may induce dynamic equilibrium changes in the channel gradient over a <br />very long time. On the other hand, the engineer often views the river in terms of <br />the downstream effects of a dam on the order of tens or hundreds of years. A <br />final, but important time scale is that of a steady-state, relatively short time span <br /> <br />22 <br />