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NW Fishletter #373, September 5, 2017

[7] Scientists Press for Columbia/Snake Spill Up to 125 Percent TDG

Saying the groundwork has been laid for increasing spill, 46 regional scientists urged Northwest policymakers to expand spring and summer water releases to levels that reach up to 125 percent total dissolved gas.

The scientists said in their Aug. 16 expansion of spill as called for in March by a U.S. District Court in Portland.

"Modeling the effects of increased spill levels up to 125 percent TDG predicted the potential for significant improvement in juvenile fish travel times, in-river survival, ocean/marine survival, smolt-to-adult returns, and life-cycle survival of Snake River spring/summer Chinook and steelhead," wrote the scientists.

The modeling analysis has been done by the U.S. Fish and Wildlife Service and the Fish Passage Center.

Spill over Columbia and Snake river dams helps juvenile salmon, steelhead and lamprey move more quickly through the hydro system with less mortality than if the fish pass through powerhouses or bypass mechanisms.

Currently, the State of Washington allows spill to 115 percent TDG in forebays and 120 percent in tailrace monitors, while the State of Oregon allows spill to 120 percent TDG in tailrace monitors.

Efforts are underway to align spill levels in time for the 2018 out-migration with a uniform 120-percent TDG limit, according to the scientists' letter.

The group also said a monitoring structure is already in place: "current fish marking/tagging levels appear sufficient to monitor the effects of experimental spill management on Snake River spring/summer Chinook and steelhead."

The letter said that regardless of future decisions about dam removal, "increased spill holds immediate potential to provide substantial survival benefits for Snake and Columbia river salmon." -L. B.

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