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Sediment Impact Analysis for the Proposed Hemlock Dam Removal Project

Hemlock Dam

Hemlock Dam

The U.S. Forest Service (USFS), Mount Adams Ranger District, is proposing to remove Hemlock Dam, a 26-foot high structure on Trout Creek, which is a tributary to the Wind River in southwest Washington. The purpose of the proposed action is to improve fish passage and aquatic conditions in Trout Creek. Trout Creek supports Lower Columbia River Steelhead, a fish that is federally listed as threatened under the Endangered Species Act. The concrete arch dam and associated fish ladder were originally constructed in 1935-1936 by the Civilian Conservation Corps (CCC) to provide water for hydroelectric power generation, and was modified in 1958 to provide irrigation water to the Wind River Tree Nursery. The hydropower was terminated in the 1950s, and the diversion that used to supply the nursery is not currently in use. The dam currently has a fish ladder that maintains a fish attraction flow of 18.5 cfs. The Wind River Tree Nursery was closed in 1997 and the dam is no longer used to divert water.

Hemlock Delta

Reservoir Delta Upstream of Hemlock Dam

Hemlock Dam is on the National Register of Historic Places as the only surviving concrete dam constructed by the CCC on the west coast. The fish ladder is one of the earliest fish ladders in the Pacific Northwest. The dam replaced an earlier timber splash dam located between 70 and 140 feet upstream. A picnic and camping area with small boat launch was first established upstream of the dam on the left bank in 1935. Hemlock Dam impounds a small, shallow lake, which is currently about 6.5 acres in size and the lake is used by an unknown number of local residents for recreation.

The Technical Service Center of Reclamation was requested by the USFS to perform a sediment impact analysis of the sediment management alternatives and to provide plans and cost estimates for the proposed dam removal.

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