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<br />. <br /> <br />. <br /> <br />. <br /> <br />SPECIAL STUDIES (Continued) <br /> <br />o <br />c.o <br />w <br />U~ <br /> <br />snallow aquifers and eventut~~ into Piceance and Yellow Creeks (Weeks <br />and otners, 1974, Figure 18). <br /> <br />B. Flaming Gorge Reservoir <br /> <br />Since 1966, tne U.S. Geological Survey bas been carrying out a <br />series of studies on tbe water quality of Flaming Gorge Reservoir as <br />part of a continuing program to assess the water quality of the Colorado <br />River Basin. C71he results of the latest of these studies are given in <br />BoIke (197B). <br /> <br />This study found tbat circulation of water in Flaming Gorge Reser- <br />voir was due cbiefly to insolation, inflow-outflow relationships, and <br />wind. Thermal stratification of the reservoir occurs from mid-spring to <br />late autumn. Tbis has generally resulted in a well-mixed isothermal and <br />oxygenated epilimnion; a moderately mixed metalimnion marked by a <br />thermal transition and decreased dissolved-oxygen concentrations; and an <br />unmixed isothermal and oxygen-depleted hypolimnion. Oxygen depletion <br />generally first occurs in the upper part of the reservoir near the <br />confluence of the Blacks Fork and Green River arms. Since tbe <br />dissolved-oxygen depletion usually develops simultaneously in both arms, <br />it was concluded that this is a function of reservoir stratification. <br /> <br />The decomposition of organic matter, which is deposited in the <br />bottom sediment in this area of the reservoir at the former confluence <br />of the Blacks Fork and Green River arms, a metalimnetic oxygen minimum <br />was noted. This waS most apparent in July. Since neither chemical nor <br />seston analyses could explain this anomaly, it is attributed to the flow <br />characteristics of the reservoir. <br /> <br />In September 1975, excessive algal production was observed in an <br />area extending from approximately 7 miles (11 km) below the confluence <br />of the Blacks Fork and Green River arms to 9 miles (14 km) up the Blacks <br />Fork arm and 12 miles (19 km) up the Green River arm. Tbis algal bloom <br />turned tbe reservoir a "pea green" color and extended from bank to bank. <br />Vertical sampling indicated that the bloom extended to a depth of from <br />26 feet (8 m) to approximately 49 feet (15 m). The 49-foot (15 m) li~tj <br />corresponds to the depth of the euphotic zone. (See BoIke, 1976.) <br />During October 1975, no extensive algal production was observed in the <br />reservoir. <br /> <br />Excessive algal production in the reservoir could result in a large <br />oxygen demand when the algae dies and bacterial decomposition occurs. <br />This could significantly reduce the dissolved-oxygen content of the <br />reservoir making areas unsuitable for fish and other aquatic organisms. <br />The development of near-anaerobic conditions also could result in the <br />mobilization of nutrients and potentially toxic trace metals from the <br />reservoir sediment. Large-scale blooms have seldom occurred in the past <br /> <br />116 <br />