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<br />OOO~Oq <br /> <br />temperature can also affect timing of spawning, duration of <br />incubation, and timing of gonadal maturation (Fry 1971; Matthews <br />and Maness 1979; Armour 1991). Water quality of a stream is <br />influenced by changes in water temperature which affect <br />solubility of dissolved gases, deoxygenation rates and <br />synergistic toxicity (Theurer et al. 1984). <br /> <br />Water temperature is thought to be a critical macrohabitat <br />variable affecting the suitability of forage fish habitat in the <br />central Platte River (Fannin and Nelson 1986, USFWS and USBOR <br />1990). Low flows in combination with changes in physical, <br />chemical or biological factors can create poor water quality <br />conditions impacting fish populations. Any evaluation to <br />determine the suitability of forage fish habitat in the central <br />Platte River must take into account this relationship between <br />water temperature and discharge. <br /> <br />Data collected by the Service during the summers of 1988-1990 was <br />used with the model by Theurer et al. (1984) to predict changes <br />in water temperatures as a result of an increase or decrease in <br />discharge for the central Platte River. Results from this effort <br />indicate a relationship exists between daily maximum water <br />temperature and discharge throughout central Platte River. As <br />flow increases, the wetted width and water depth increases, and <br />the velocity of the water tends to speed up slightly. Hence, <br />heat supplied to the water surface by the sun or by warm air is <br />absorbed by a larger volume of water which accordingly results in <br />a lower maximum water temperature (Figure B3) and less <br />fluctuation around the mean (Figure B4). <br /> <br />Increased flows during summer months can reduce the frequency and <br />duration of daily maximum water temperatures in excess of 350C <br />throughout the central Platte River (Figure B5 and B6). Flows of <br />400 cfs at the Grand Island gage provided little to no additional <br />protection to the central Platte River fish community. A flow of <br />800 cfs at the Grand Island gage reduced the average daily <br />maximum water temperatures and the number of days when <br />temperatures were in excess of 350C throughout the central Platte <br />River. A flow of 1200 cfs further reduced average daily maximum <br />water temperature at all sites and reduced the number of days <br />when maximum water temperatures where in excess of 350C. <br /> <br />Results indicate that to reduce the frequency and duration of <br />potentially lethal maximum water temperatures and to maintain and <br />protect an abundant and diverse assemblage of fish species so as <br />to provide a reliable food source for listed bald eagles and <br />interior least terns, flows of sufficient quantity and quality <br />must be provided. By providing adequate flows the frequency of <br />lethal temperatures can be reduced throughout the central Platte <br />River. <br /> <br />11 <br />