The 120-foot tall, 80-foot wide, 1,000-ton fish passage device was to have been installed at Lower Monumental by mid-April. However, the Corps delayed installation until fall to avoid affecting the safety of juvenile salmon migrating downstream. (HNN 5/22/07)
The weir left Portland, Ore., on Oct. 20 attached to two grain barges to improve maneuverability while being moved upriver by a towboat and tugboat. The Corps said the weir was being moved upstream at about four to five miles per hour.
The weir will be fitted into a spillway bay to allow juvenile salmon and steelhead to pass the dam near the water's surface. Once installed at Lower Monumental, near Kahlotus, Wash., it will be the Walla Walla District's third removable weir on the lower Snake River.
The Corps said the spillway weir design has proven an effective means of providing a surface bypass route for migrating fish, while maintaining navigation, power production, irrigation, and flood-risk-management benefits.
The Corps' 810-MW Lower Granite and 603-MW Ice Harbor dams feature the other two removable spillway weirs, which are lowered to the bottom of the dam forebay when not in use. Testing at Lower Granite and Ice Harbor noted averages of 96-98 percent survival for fish passing through the weirs, which the Corps describes as fish slides.
The Corps said the new weir demonstrates its commitment to improving migration conditions for fish listed under the Endangered Species Act. It said it now has surface bypass improvements at six of its eight dams on the Columbia and Snake rivers: 1,092.9-MW Bonneville, 1,807-MW The Dalles, 980-MW McNary, Ice Harbor, Lower Monumental, and Lower Granite. The Corps said it eventually plans to add surface bypass improvements at the other two dams, 2,160-MW John Day and 810-MW Little Goose.