THE US ARMY CORPS OF Engineers (USACE) is testing the use of a removable spillway weir (RSW) to facilitate fish passage at the 77m high Lower Granite dam on the Snake river in Washington, US.

Preliminary testing has already started on the US$11.5M device installed at the dam. It is designed with ballast tanks to submerge in the river during high floods. During 2002’s fish migratory season, technicians will track migrating fish, and test the RSW for two years, prior to assessing its suitability.

The RSW is a possible solution to an easier passage for fish across dams as it does not induce the kind of stress endured by juvenile salmon when they pass over normal spillways. The weir allows fish to slide down the face of the dam.

The weir is essentially a man-made waterfall that cascades from the top of the reservoir to the tailrace below. The concept proved attractive during a five-year test at Lower Granite with another prototype device, the surface bypass collector. The bypass collector confirmed that the surface flow conditions were the most effective, as young fish prefer to travel near the top of the river surface, and when possible, stay there.

The bypass collector showed that 30% to 40% of the fish approaching the dam would follow just 3% or 4% of the flow over the dam.

Lower Granite dam, with a capacity of 810MW, is one of the four dams on the Snake River that environmentalists groups have targeted for removal.