Most Columbia River Basin juvenile anadromous salmon and steelhead tend to stay in the upper 10 to 20 feet of the water column as they migrate downstream to the ocean. Juvenile fish passage routes at the Corps’ lower Columbia and Snake river dams, because of the dams’ configurations, cause the juvenile fish to dive to depths of 50 to 60 feet to find the passage routes. Engineers and biologists for the past several years have been pursuing new technologies that would provide more surface-oriented, less stressful, passage routes for juvenile fish.

A prototype removable spillway weir, or RSW, was installed at Lower Granite Dam on the lower Snake River in 2001. A second spillway weir was installed at Ice Harbor Dam in 2005, with a third weir planned for installation at Lower Monumental Dam in 2007. The RSW allows juvenile salmon and steelhead to pass the dam near the water surface under lower accellerations and lower pressures, providing a more efficient and less stressful dam passage route. Other surface oriented passage systems are being considered at the remaining Lower Snake and Lower Columbia River hydropower sites.

The design of the RSW is different from existing spillways whose gates open 50 feet below the water surface at the face of the dam and pass juvenile fish under high pressure and high velocities. The RSW passes juvenile salmon and steelhead over a raised spillway crest, similar to a waterslide. Juvenile fish are safely passed over the weir more efficiently than with conventional spill while reducing migration delays at the dam.

The RSW structure also is designed to be "removable" by controlled descent to the bottom of the dam forebay. This capability permits returning the spillway to original flow capacity during major flood events.

Fish passing through dam with SW. (avi 650k)

Fish passing through dam without SW. (avi 650k)

Lower Monumental Lock and Dam

• (LM) Fact Sheet September 21, 2006

Ice Harbor Lock and Dam

• (IH) Fact Sheet May 20, 2005

Lower Granite Lock and Dam