• Travel time faster with spill and structural improvements

    March 15, 2015:Survival through the eight federal dams and reservoirs on the lower Snake and Columbia rivers was about average for juvenile chinook and the highest ever for juvenile steelhead and sockeye in 2014, the Northwest Fisheries Science Center reported.Snake River steelhead survival was estimated at 77.1 percent, sockeye at 71.3 percent, and Chinook at 49.7 percent. See the report here.


    In a year where flow, spill and water temperature were all within average ranges, juvenile fish travel times through the hydrosystem continued to be shorter than before all eight dams had surface passage routes installed. NOAA scientists say that surface passage and spill have improved travel times. The surface flow is attractive and the fish spend less time in the forebay searching for the best route to pass. In the reservoirs, higher flows translate into faster water velocities.


    Increased spill and use of surface passage may be helping the fish get past more quickly and safely, but the scientists cautioned that it is also making survival estimates more uncertain. Fish are tagged with devices that can only be detected if the fish go through the bypass system at the dams. Fewer and fewer fish are using these routes, in favor of spill and surface passage. 



     

     

     

     

     

    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.


    See pictures of types of surface passages.

     

    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.