Fish Lake Dam, located 35 miles northeast of Medford, Oregon, on the North Fork of Little Butte Creek is an earth and rockfill structure with a height of 50 feet and a crest length of 960 feet.
Rehabilitation at Fish Lake Dam by the Bureau of Reclamation in 1955 included constructing a new spillway, riprapping the upstream face of the dam, and placing fine fill and grading work on the dam crest. The total capacity is 7,900 acre-feet (active 7,850 acre-feet).
In 1996 and 1997, further work was done on Fish Lake Dam under Reclamation’s Safety of Dams Program. This included installing a rockfill berm on the downstream face of the embankment, modification and extension of the outlet conduit, and constructing a new auxiliary spillway on the right abutment of the dam.
Fish Lake Dam and reservoir are located within the high cascades physiographic province near the crest of the mountain range. Upstream of the dam the North Fork Little Butte drainage basin flows west from the crest of the Cascades and is defined by Mt. McLaughin to the north of the reservoir and Brown Mountain, a small shield volcano located to the southeast of Fish Lake. The left abutment of Fish Lake Dam consists of blocky, andesite lava flow erupted from Brown Mountain (Qbm) subsequent to the retreat of the Pleistocene glaciers. The andesite is dark gray with fine grained ground mass and scattered vesicles. The lack of notable phenocrysts within the ground mass distinguishes the Brown Mountain andesite from the underlying Fish Lake lavas (Qfi). The Brown Mountain andesite is divided into dense central unit of the flow and the upper and lower breccia. The central zone is fresh to slightly weathered, intensely to slightly fractured, hard and dense. The breccia (Qbmb) is an intensely to moderately weathered, moderately hard to soft, vesicular andesite that is moderately to intensely fractured.