Estimation of stream discharges preferred by steelhead trout for spawning and rearing in western Washington

Abstract

Determined during the study of selected stream reaches used for spawning and rearing by steelhead trout were (1) stream discharges that cover the greatest areas of the streambeds with water at both the depths and the velocities preferred by spawning steelhead; (2) discharges that cover selectively reduced streambed areas with water at both the depths and velocities preferred by spawning steelhead; (3) discharges that cover the greatest streambed areas with water at velocities preferred by spawning steelhead; (4) rearing discharges that cover the streambed, but not the banks of the channel, with water; and (5) average wetted perimeters of the channels at water stages corresponding to the rearing discharges. These discharges and wetted perimeters were determined by measurements at 54 study reaches on 18 streams representative in size and location of those used by steelhead for spawning and rearing. Using multiple-regression techniques and the measured discharges and wetted perimeters, equations were developed for estimating the spawning and rearing discharges and the wetted perimeter for rearing at unmeasured stream sites. The independent parameters used were drainage area, mean altitude of the basin, reach altitude, reach slope, and average width of reach at toe of bank. Standard errors of estimate for the equations ranged from 26 to 50 percent for discharges corresponding to the greatest areas of preferred depths and velocities, from 32 to 46 percent for discharges corresponding to reduced areas of preferred depths and velocities, from 44 to 68 percent for discharges corresponding to the greatest areas of preferred velocities, from 46 to 57 percent for rearing discharges, and from 20 to 41 percent for the rearing wetted perimeter.