US federal agencies operating the Columbia river dams have announced revised spring and summer hydro operations which aim to increase fish survival.

The announcement came after formal consultations between the Bonneville Power Administration, the US Army Corps of Engineers, Bureau of Reclamation and the National Marines Fisheries Service. While the complete biological opinion on operation of the federal dams is to be released later this year, the federal agencies were in agreement that these revised operations should be implemented immediately to maximise their contribution to survival and recovery of critically depressed salmon and steelhead runs.

Accordingly, dam operators are changing the volume and the duration of water spilled at four federal dams — Bonneville, The Dalles, John Day and Lower Monumental. Operations at four other projects — McNary, Ice Harbor, Little Goose and Lower Granite — will remain unchanged.

The revised operations are meant to increase salmon survival, as most scientists are of the opinion that spilling fish over the dams is the safest and most effective way to move migrating threatened and endangered fish through the hydro system. Fish survival increases because fewer fish are subjected to passage through the hydroelectric turbines. Spilling does increases the risk of gas bubble trauma, but moderating the volume of water spilled mitigates that threat.