• Improvements to the dams on the lower Snake and Columbia help fish survival

    The Northwest Fisheries Science Center has released preliminary 2013 estimates of juvenile Snake River chinook and steelhead through the through the seven dams and reservoirs (Lower Granite Dam tailrace to Bonneville Dam tailrace) on the lower Snake and Columbia rivers.

    The survival rate for spring chinook was 62 percent. This is the third highest survival rate for spring chinook in the past 15 years.

     

     

    Steelhead survival for the same reach was estimated at 50 percent, which is relatively low compared to recent years. NWFSC noted that steelhead survival between Lower Granite and McNary Dams decreased through the last few weeks of the season, possibly due to increased predation and increasing temperatures during that period.

     

    Other findings from the study:

     

    Spring flow in the Snake River was the fifth lowest in the past 21 years.

     

    Spill percentage on the lower Snake dams was 33.5 percent of flow.

     

    q Low flow conditions may have increased the percentage of fish that passed the dams at the spillways. “Lower water velocities allow fish more time to react to conditions and may increase the attractiveness of the surface bypass collectors,” the memo said. (See more about surface bypass in the right-hand box on this page.)

     

    q The estimated percentage of fish transported was among the lowest known estimates over the past 20 years.

     

     

     

     

    Improved fish passage installed at all lower Snake and Columbia federal dams

    Click here for larger image.

     

    Surface passage routes such as spillway weirs are yielding survival rates of 95 percent or better at every one of the eight federal dams on the lower Snake and Columbia Rivers.

     

     

  • Surface passage provides a route where the fish swim

    Surface passage structures such as spillway weirs and the Bonneville corner collector provide more natural river passage conditions.

     

    Most juvenile salmon tend to travel in the upper 10 to 20 feet of the water column as they migrate downstream to the ocean. When approaching the dams without surface passage structures, juvenile fish need to dive to depths of 50 to 60 feet to access passage routes such as a spillbay opening or a guidance screen that will guide them into a juvenile bypass channel.

     

    Surface passage structures allow downstream migrating fish to pass the dam at the surface. This reduces juvenile fish passage delay, improves water quality, makes more efficient use of spill and improves juvenile fish survival.

     

    Read more at the Corps of Engineers Walla Walla District web page.

    The Dalles Dam spill wall

    The Corps added a spill wall at The Dalles Dam (above, center) that steers juvenile fish out from the shallower parts of the river where they were more vulnerable to predators. Tests in 2010 showed a 4 to 7 percent survival improvement over 2004 and 2005 tests: 96 percent for juvenile spring chinook; 94 percent for fall chinook and 95 percent for steelhead. Read more here.