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<br />changed and the width to depth ratio has been reduced (FLO 1996). The Little <br />Snake River sediment load is important to the Green River channel morphology <br />downstream of the Jensen Gage. <br /> <br />Effects of Flow Regulation on Channel Morphology <br />Stanford (1994) posed a question of whether lost flooded bottomlands and <br />nursery habitat in the Green River was caused by reduced sediment supply or lack <br />of extreme flow events that would move large quantities of sediment. The answer <br />is that both factors have contributed to the loss of flooded bottomlands. The <br />100-year period flood at Jensen was computed to be 51,200 cfs (1450 m3/s) <br />using pre-1963 peak flow data and 44,700 cfs (1266 m3/s) using post-1963 data, <br />indicating a decline in frequency of the higher peak flows. Similarly, 5-year return <br />period flows declined from 30,000 cfs to 23,000 cfs (850-650 m3/s). Data from <br />the Green River, Jensen gage indicates a 50% decrease in sediment load from pre- <br />to post-1963 (Andrews 1986). <br /> <br />Most large reservoirs in the West were constructed in the last 50 or 60 <br />years and alluvial river channels downstream of these reservoirs may require <br />several decades to adjust to the reduction in flow and sediment load. Williams and <br />Wolman (1984) related observed changes in alluvial rivers downstream of <br />reservoirs in the semiarid western United States and noted that channel <br />degradation below the reservoirs was completed within the first twenty years after <br />dam closure and that decreases in suspended load extended downstream for many <br />kilometers. Generally, an alluvial river's geomorphic response approaches a <br />condition of quasi-equilibrium relatively quickly after a disruption in the system, <br />depending on the location of significant sources of sand in the system. Andrews <br />(1986) stated that a century or more will be required for the Green River to adjust <br />to the effects of Flaming Gorge Reservoir. <br /> <br />30 <br />