Steelhead Emigration in a Seasonal Impoundment Created by an Inflatable Rubber Dam

Abstract

AbstractWhile large, permanent dams and reservoirs are known to impede smolt movements, investigations of small, temporary impoundments are scarce. Inflatable rubber bladder dams are used to create temporary impoundments worldwide, yet downstream fish passage at these structures has not been evaluated. To examine smolt emigration in a seasonal water supply reservoir and passage at a rubber dam, we tracked 110 radio‐tagged steelhead Oncorhynchus mykiss smolts through a 4.5‐km free‐flowing reach and an adjacent 5.1‐km‐long impoundment on the Russian River, California, during 2001, 2002, and 2004. Unlike the results of studies in other impoundments, median travel rates in the free‐flowing (0.6–0.8 km/h) and reservoir (1.0–1.4 km/h) reaches did not differ significantly within or among years. Smolts slowed significantly, however, in the dam forebay. Forebay travel rates (0.02–0.1 km/h) were more than an order of magnitude lower than rates in free‐flowing and reservoir reaches. Although delayed, more than 75% of the fish detected in the forebay successfully passed the dam. More than 80% of the passing fish traveled over the dam crest as opposed to through ladders and flow bypasses. To determine whether increased spill depth and velocity would reduce forebay delay, we deformed the crest of the inflatable dam into a notched configuration throughout the 2004 study year. The notched configuration increased crest depth and velocity approximately 20‐fold over the normal fully inflated condition. Smolts moved significantly faster through the forebay when the dam crest was notched in 2004 (median = 2.4 h) than when the dam was fully inflated in 2001 (6.3 h). Our finding that steelhead traveled through free‐flowing and impounded reaches at similar rates but slowed dramatically in the forebay should encourage others to examine forebay conditions in small reservoirs. Concentrating spill by deforming the crest of an inflatable rubber dam is a simple and effective way of reducing forebay delay.