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• <br />PHYSICAL AND CHEMICAL ENVIRONMENT <br />Many changes in the river environment have occurred since the con- <br />st?uction of Flaming Gorge Dam. A knowledge of these changes and of <br />those characteristics of the river which have not changed (such as <br />gradient) is necessary to interpret the findings related to distri- <br />bution and abundance of benthic invertebrates. <br />Gradient <br /> Differences in gradient divide the river into several ecologically <br /> distinct reaches (Table 2). Where the gradient is low the substrate is <br />=. _ °--composed of °fine sediments, and where high, rubble and gravel prevail. <br /> Little Mole (7.2 MBD) and Echo Park (64.4 MBD) represent areas of high <br /> gradient (9.9 and 15.9 ft./mi. respectively), Carr Ranch (42.7 MBD) is <br /> 14,n-an area-of-low --gradient -(2.5 ft./mi.), and Island Park (78.0 MBD) <br />represents an area of intermediate gradient (5.1 ft./mtt•.). <br />Discharge <br />Seasonal variations in flow have been greatly reduced by Flaming <br />Gorge Dam with its relatively stable power output. Mean monthly flows <br />fluctuated from 603 to 7306 cfs at Greendale before the dam was con- <br />structed and from 864 to 1947 cfs after (Table 3). In general, summer, <br />fall, and winter flows have increased and spring flow has decreased <br />since closure of the dam. Power demands on Flaming vorge Lam v1LCl1 <br />cause a diurnal fluctuation in flow. A frequent pattern of operation <br />at the dam is for maximum power production during daylight hours with a