Effects of River Flow on the Distribution of Chinook Salmon Redds

Abstract

The distribution of redds of chinook salmon Oncorhynchus tshawytscha was evaluated in October-November 1978–1983 on Vernita Bar, about 6.5 km downstream from Priest Rapids Dam on the Columbia River. Minimum flows of 1,020 m3/s (license minimum set by the Federal Energy Regulatory Commission) and 1,416 m3/s were maintained in six alternating years, but flows each day fluctuated widely above the minima because of power production; part of the spawning area was exposed for several hours on most days. We detected no effect of minimum flow regime on redd size. Chinook salmon spawned at depths as great as 7 m below the water surface as measured at a discharge of 1,020 m 3/s. The highest redds occurred near the waterˈs edge at the 1,982-m3/s flow elevation. Vertical range of spawning use was 8.5 m. Water velocities at 23 cm above the substrate (facing velocity) on the area used for spawning varied greatly with river discharge, but usually exceeded 0.67 m/s. Chinook salmon females completed redds in areas that were not covered by water for up to 8 h/d, and on other areas that had velocities near 2 m/s for part of each day. Chinook salmon spawned at Vernita Bar in areas with high fractions of cobble and low percentages of fines. They began spawning in early October below the 1,020-m3/s flow elevation and increasingly used the area between the 1,020- and 1,982-m3/s flow elevations until spawning ended about the third week of November. The percentage of redds above the 1,020-m3/s flow elevation correlated strongly with mean daily discharge during late October and early November over 7 years. We found no evidence that the minimum daily flow affected redd distribution in any way. Egg retention in ovaries of postspawning females, one measure of spawning success, was minimal in all years, as was the proportion of excavated redds that contained no embryos. Daily flow fluctuations from minima to 3,400–4,250 m3/s did not prevent females from completing redds. We concluded that minimum flows of 1,020 m3/s for up to 8 h/d do not determine availability of spawning habitat, and that use of spawning areas above the 1,020-m3/s flow elevation could be reduced by managing the river discharge with storage manipulation upstream to provide lower mean daily flows, thus easing minimum flow restrictions during posthatch incubation and emergence.