Northwest Fisheries Science Center

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Document Type: Journal Article
Center: NWFSC
Document ID: 7754
Title: Passage of juvenile fish through orifices in gatewells of turbine intakes at McNary Dam
Author: Wallace W. Bentley, Howard L. Raymond
Publication Year: 1969
Journal: Transactions of the American Fisheries Society
Volume: 98
Issue: 4
Pages: 723-727

Turbines in dams on the Columbia River are known to cause mortality among migrating juvenile salmon (Oncorhynchus spp.) and steelhead trout (Salmo gairdneri).  For example, in 1961, researchers reported 11% mortality of juvenile salmon that had passed through the turbines at McNary Dam.  More recently, additional mortalities from predation in the immediate vicinity of the draft-tube exit have been estimated to be as high as 33%.  Anadromous runs of salmon and steelhead trout pass 9 of the 11 dams now on the Columbia River.  It seems reasonable to assume that the losses of fish near turbines are significant.  We have also noted serious effects on the condition of fish trapped and delayed in turbine intake gatewells.

A number of methods of reducing or preventing mortality of juvenile salmon in turbines have been studied.  One method is to bypass fish around the turbines by means of the ice and trash sluiceways.  Screening the turbine intakes and lifting the forebay stoplogs to let water and fish pass from the forebay into the sluice and thence below the dam has also been suggested.  However, this suggestion was never carried out due to the prohibitive cost of screening the turbines.

The idea of bypassing fish around turbines was considered again after fingerlings were discovered in turbine intake gatewells in 1950.  A submerged orifice from the turbine intake gatewells to the ice and trash sluice was proposed to provide a means to pass fish safely.  In preliminary testing in 1962 at Bonneville Dam, the orifice passed 91% of the juvenile steelhead trout and 64% of the juvenile salmon.  Experiments with a siphon to simulate an orifice were conducted at Ice Harbor Dam in 1965. Researchers concluded that an orifice system showed promise and provided information on the most suitable location for a gatewell orifice.  On the basis of this work, orifice-pipe bypass systems were built at John Day, Lower Monumental, and Little Goose Dams.

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