WWRC 90-14
Brown Trout Population and Habitat Changes Associated with Increased Minimum Low Flows in Douglas Creek, Wyoming
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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