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Relationship of Discharge Reductions to Available Trout Habitat for Recommending Suitable Streamflows


The approach taken in this study to determine suitable streamflows to maintain trout habitat has focused on defining the changes observed for physical stream characteristics as streamflow was reduced. To relate these changes to reductions in available trout habitat, criteria were developed defining the cover preferences of trout, while for food production and spawning areas, criteria from the literature were applied.

Trout cover has been defined as instream rubble - boulder areas and overhanging bank cover in association with a water depth of at least 0.50 feet (0.15 meters). Using these cover preferences, a rating system has been developed allowing comparisons of available trout cover to be made for the same stream section at various discharge levels and for different stream reaches at approximately the same level of flow, based upon the average daily flows (ADF) for the sections. Also, verification of the rating system as an indicator of the standing crop of trout present has been initiated in an effort to quantify the biological significance of instream dewatering in regard to trout populations.

The primary study area, Douglas Creek below Pelton Creek had an average daily flow of 78.7 cfs (2.23 cu m/sec) and was intensively investigated in 1973 at 100%, 71%, 51%, 38%, 27%, and 11% ADF. Available trout habitat was found to decrease at the greatest rate for the discharge reduction interval from 27% to 11% ADF. These findings verify the results found by Wesche (1973) on two stream sections having smaller average dally flows. As a minimum flow to maintain trout habitat, a discharge in the 25%, ADF range will avoid the flow range for which the rate of habitat decrease is greatest.

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