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<br />eddies, pools, and runs. Bottom substrates are boulder, rubble, sand,
<br />and silt.
<br />
<br />METHODS AND MA TERIALS.-Continuous recordings of water
<br />temperature were made at Echo Park and Island Park. Temperature
<br />readings were also made at all stations with a pocket thermometer
<br />when fish samples were taken. Water quality analyses (dissolved
<br />oxygen, alkalinity, conductivity, pH, and turbidity) were made at
<br />least once each visit to a sampling station. Water-flow, water tempera-
<br />ture, and water-quality data were obtained from the U.S. Geological
<br />Survey records for Greendale and Jensen, Utah, gauging stations.
<br />From July, 1963, to October, 1966,667 collections containing 24,040
<br />fish were made. Most sampling was done from June through Septem-
<br />ber, but several weekend collecting trips were made in April, May,
<br />October, and November, Location of sampling sites was limited by
<br />vehicle and boat access to the river. Sites for intensive sampling were
<br />located at Lodore, Echo Park, Island Park, and Split Mountain. Supple-
<br />mental sampling stations were located at Little Hole, Bridgeport,
<br />Willow Creek, and Ouray (Figure 1). Fish populations were sampled
<br />with various types of collecting gear, including gill nets (223 collec-
<br />tions) , . seines of various mesh sizes (264 collections ) aad a boat-
<br />mounted 230-volt a-c electric shocker (121 collections). Tow nets, food
<br />strainers, and a modified scoop shovel with window-screen inserts were
<br />used to collect larval and small fish (59 collections).
<br />
<br />ECOLOGICAL IMPACT OF RESERVOIR OPERATION.-The
<br />Green River below Flaming Gorge Dam has changed greatly since the
<br />closure of the dam in late 1962. Chemical and physical characteristics
<br />of the river immediately below the dam are now dependent upon
<br />reservoir conditions at the level of the penstocks and upon the amount
<br />of discharge, Downstream from the dam, tributary streams join the
<br />river, which gradually attains pre-impoundment conditions. To assess
<br />changes caused by the dam, chemical and physical features of the river
<br />at increasing distances below the dam before and after impoundment
<br />were compared.
<br />Flow.-Before impoundment of the Green River by Flaming Gorge
<br />Dam, flow was the lowest during the winter months and increased
<br />gradually until peak run-off was reached in Mayor June. Following
<br />peak run-off, the flow receded during the summer months and was
<br />uniformly low during the fall months, Since completion of Flaming
<br />Gorge Dam, however, characteristic high spring flows and low winter
<br />flows have been replaced by a relatively stabilized seasonal flow pat-
<br />tern (Table 1). Monthly and daily flow rates have varied because
<br />
<br />30:)
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