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Research Documents Dalles Dam Dangers for Fish by Mike O'Bryant Columbia Basin Bulletin - November 22, 2002

Juvenile salmon have less of a chance of surviving their passage through The Dalles Dam than any other federal Columbia River basin dam.

Several studies presented by scientists at this year's Anadromous Fish Evaluation Program review in Portland found that the higher mortality is a direct result of the conditions the fish experience during their passage.

Mike Langeslay, who hosted the passage studies at the U.S. Army Corps of Engineers-sponsored review, said there are several reasons why mortality is high at the dam: the design and depth of the spillway and stilling basin, along with a high rate of predation by small mouth bass and Northern pikeminnow are high on the list of the most direct causes.

A relatively large proportion of juvenile salmon that pass the dam, especially during the summer, are preyed on or injured at a higher rate than other dams, according to Paul Heisey of Normandeau Associates, Inc., which this year looked specifically at the direct mortality of spillway passage on juveniles. Summer passage often results in damage to fish, such as a loss of equilibrium, hemorrhaged eyes, torn or bent gill covers and scrapes resulting from fish sliding along concrete.

Predation also rises in the summer to 13 percent or more (predation at Bonneville Dam in the summer is 5 percent), while spring and fall predation is about 1 percent. The higher the water temperature, he said, the higher the incidence of predation.

According to Normandeau's study (also contributing is Mid Columbia Consulting, Inc.), survival rates differ among spillbays. Survival rates in May through spillbay 13 was 91.7 percent, while spillbay 4 was 97.4 percent and spillbay 9 was 95.6 percent. Another indicator that something isn't quite right at The Dalles is the incidence of passage-related injury, which ranged from 4.8 percent at spillbay 9 to 5.3 percent at spillbay 4. Spillbay 13 was the highest at 8.9 percent.

Predation and injury are related to the time a juvenile spends in the stilling basin, according to Marshall Richmond of Battelle, Pacific Northwest National Laboratory. Using sensor fish (electronic sensors) that can measure pressure and acceleration during spill that the real thing would feel, Richmond and Thomas Carlson attempted to characterize what the spill environment at The Dalles Dam is like for juvenile salmon.

They found that "changes in spill patterns result in changes in stilling basin dynamics." Richmond said that with low flow conditions there is a higher tendency for lateral flow -- flow across the dam -- that keeps juveniles in the stilling basin longer where they are subject to adverse conditions. With high flow, juveniles are retained in the stilling basin environment for a shorter period of time, but a backroller still traps some fish, accelerates their movement and pushes them back towards the dam.

The reason Bay 13 has the highest mortality and injury rates could be because lateral flow moves those fish towards spillbay 1, exposing them for a longer time to the stilling basin, according to John Beeman of the U.S. Geological Survey.

Related Links:
U.S. Army Corps of Engineers: AFEP