The U.S. Army Corps of Engineers operates eight mainstem hydroelectric dams on the Columbia and Lower Snake Rivers. Adult and juvenile anadromous and resident fish passage at these 8 dams primarily includes several species including Chinook (O. tshawytscha), sockeye (O. Nerka), coho (O.kisutch) salmon, steelhead (O. mykiss), Pacific lamprey (Entosphenus tridantatus), and bull trout (Salvelinus confluentus). Responding to a March 2017 U.S. District Court order, gas cap spill operations were implemented in the spring of 2018. Gas cap is defined as the spill discharge resulting in 120% saturation of Total Dissolved Gasses (TDG) in a given dam tailrace or 115% nitrogen saturation in the forebay of the next downstream dam. Prior to this implementation, the Corps’ Northwestern Division including the Walla Walla and Portland Districts performed extensive preparations. These preparations included physical hydraulic modeling to develop operating configurations to optimize and balance tailrace flow conditions for juvenile fish tailrace egress, and adult fish upstream attraction and passage. Juvenile Chinook salmon and steelhead were implanted with acoustic transmitters at Lower Granite and McNary dams to study reach specific and total hydrosystem survival (Lower Granite to Bonneville dam). Adult Chinook salmon were implanted with modified acoustic transmitters at Ice Harbor dam to study detailed 3-D dam approach and passage behavior at Little Goose dam. These studies represent the first ever total hydrosystem survival study with acoustic telemetry tags and the first ever 3-D tailrace behavior and passage study in an acoustically noisy environment.
This presentation will describe the methods and results of unintended consequences of gas cap spill on the survival of seaward migrating juvenile salmon and steelhead and the survival and behavior of adult salmon migrating towards natal tributaries. A companion presentation will describe the extensive preparation and coordination conducted to implement gas cap spill in the Snake and Columbia Rivers.