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Draft Fmal Completion Report [0 UDWR for Contract #93-1070. Amendment 3 12 Borland, 1951), so that most sediment is transported by suspension. While bed scour in sand-bedded rivers occurs during floods by the suspension of sediment, the sediment in motion moves at a much slower rate than the water flow. Increased sediment concentrations reduce water velocities further decreasing the rate of transport. Thus, while large volumes of sediment may be in transport, the net change in sediment storage within a reach may be small (Leopold and others, 1964). It should also be noted that, even for rivers transporting large quantities of sediment, the amount of sediment transported is small compared to the amount of sediment stored on the bed. banks, and in the floodplain. Discharge measurement records from the discontinued USGS gaging station near Ouray (station number 09307000) were re-analyzed by Schmidt (1994). His analysis for the gage cross section showed an annual scour and fill c)Cle of about 3 m during the passage of the spring flood (Fig. 9). Scour occurred on the ascending limb of the hydrograph, and filling occurred during the descending limb and subsequent low flows. As described below, this pattern of scour and fill was similar to. the pattern that was observed at some cross sections within the detailed study reach, both in terms of depth of scour and timing of scour and fill. Long-tenn Channel Response The response of rivers to disturbance is of concern to geomorphologists, ecologists, and engineers. A "disturbance" to a river may be either natural, such as the passage of a very large flood, or human induced, such as the closure of a dam. Regime theory considers a river to be an equilibrium expression of the long-term average of the hydrology of a basin (Y u and Wolman. 1987), but on a year-to-year scale, natural rivers are highly variable. Yu and Wolman (1987) developed a model for the dynamic adjustment of alluvial river width, They modeled channel width as a function of present discharge and past high flow events; the most recent events were given greater weight in the model, and the geomorhpic importance of past events decreased with time. Y u and Wolman's (1987) simulation model predicted that average channel width increased with high flow events, and then progressively decreased (i.e., "recovered"), until a flow event of great magnitude occurred. This model shows that the expected channel form of natural rivers varies over time, and is not static; increases in channel width occur only when certain threshold discharges are exceeded. In addition. channel narrowing continues until the peak discharges are sufficient to mllintllin or increase channel width. Prior to dam closure, the higher magnitude, but highly variable, flood peaks of the Green River formed a channel that was wider than that the current river (Andrews, 1986; Lyons and others. 1992). The simulation model