Executive Summary The purpose of this study was to assess the biological significance of an increase in minimum flow to the brown trout (Salmo trutta) population in Douglas Creek, Wyoming. Douglas Creek is a regulated stream that underwent a substantial increase of the required minimum flow in 1986 from 1.0 to 5.5 cubic feet/second (cfs) after 23 years at 1.0 cfs. Population and habitat data obtained during the period when minimum flow was 1.0 cfs (1972-1976) were compared to data collected after the minimum flow was increased to 5.5 cfs (1988-1989).
More than a two-fold increase in brown trout abundance was measured between 1973 and 1988-1989 in a reach between Rob Roy Dam and the point of water diversion. Within this reach discharge was occasionally as low as 3.0 cfs prior to 1986, but the low flow was not as severe as downstream of the water diversion.
A four- to six-fold increase in brown trout abundance was indicated between 1972 and 1988-1989 in a reach immediately downstream from the point of water diversion. Within this reach the minimum low flow was 5.5 times greater than in the 1970's; wetted width at low flow was doubled; and weighted usable area for adult fish was almost five times greater.
At sites more than 6.4 mi downstream from the water diversion structure, where the impact of reduced flow had been less due to addition of water from tributary streams, no measurable changes related to the enhanced minimum flow were identified.
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