Biological response of fish populations
to increased minimum flow,

Investigators: S.W. Wolff and T.A. Wesche, Wyoming Water Research Center, and D.D. Harris and W.A. Hubert, USDI, FWS/University of Wyoming Cooperative Fish and Wildlife Research Unit.

Purpose: Most research on minimum flows focuses on development of instream flow and habitat models but minimal reported evidence links biological response of fish populations to changes in minimum flows. This study assessed the biological significance of an increase in minimum flow to the brown trout (Salmo trutta) population in Douglas Creek, Wyoming.

Methods: Stage I of the Cheyenne water project included the construction of Rob Roy Reservoir, completed in 1963. As part of the use permit issued by the US Forest Service, a minimum flow of 1.0 cfs (cubic feet per second) was required downstream from the diversion structure built to carry to impounded water to Cheyenne. A proposal to enlarge this water project, Stage II, required further studies to assess instream flow needs of Douglas Creek fish downstream from Rob Roy Reservoir. These studies were initiated in the 1970s. Population and habitat data obtained during the period when minimum flow was 1.0 cfs (1972-76) were compared with data collected after the minimum flow was increased to 5.5 cfs (1988-89). Four reaches of the stream, divided into eight study sites, were sampled by electroshocking using removal methods.

Results: A four- to six-fold increase in brown trout standing stock was indicated between 1972 and 1988-89 in the 6.4 miles reach immediately downstream from the point of water diversion. Within this reach, the minimum low flow was 5.5 times greater than in the 1970s; habitat for adult fish was almost five times greater.

More than a twofold increase in brown trout standing stock between 1973 and 1988-89 was measured in a one mile reach between Rob Roy Dam and the point of water diversion. Within this reach, discharge was occasionally as low as 3.0 cfs before 1986, but not as severe as the 1.0 cfs downstream from the water diversion.

At sites more than 6.4 miles downstream from the water diversion structure, where the effect of reduced flow had been less because of the addition of water from tributary streams, there were no measurable changes related to the enhanced minimum flow.

Though additional work is necessary, this study supports the long-term value of increased minimum flow to fish populations.


Brown trout population and habitat changes associated with increased minimum low flows in Douglas Creek, Wyoming. US Department of Interior Fish and Wildlife Service, Biological Report 90(11).

For a complete list of publications available from the Wyoming Water Research center, or for further information on this or other research projects, contact the WWRC at (307) 766-2143, FAX: (307) 766-3718, or P.O. Box 3067, Laramie, WY 82071-3067.

WWRC RESEARCH BRIEFS are published by the Wyoming Water Research Center with funds provided in part by the US Geological Survey, Department of Interior, as authorized by the Water Resources Research Act of 1984. The research on which this report is based was financed in part by the US Fish and Wildlife Service, Department of Interior, and the Wyoming Water Research Center. The views expressed do not necessarily represent those of the Department of Interior or the WWRC. Persons seeking admission, employment, or access to programs at the University of Wyoming shall be considered without regard to race, color, national origin, sex, age, religion, political belief, handicap, or veteran status.


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