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Alder Dam
Alder Dam.JPG
A front view of the Alder Dam.
Location Pierce / Thurston counties, Washington
Coordinates 46°48′05″N 122°18′37″W / 46.8015°N 122.3104°W / 46.8015; -122.3104Coordinates: 46°48′05″N 122°18′37″W / 46.8015°N 122.3104°W / 46.8015; -122.3104
Construction began 1942
Opening date 1945
Dam and spillways
Impounds Nisqually River
Height 330 ft (100 m)
Length 1,600 ft (490 m)
Width (base) 120 ft (37 m)
Reservoir
Creates Alder Lake
Total capacity 241,950 acre⋅ft (298,440,000 m3)
Catchment area 286 sq mi (740 km2)
Surface area 3,065 acres (12.40 km2)
Power station
Turbines 2 x 25 MW
Installed capacity 50 MW
Annual generation 197,830,000 KWh

Alder Dam is a concrete thick arch dam on the Nisqually River in the U.S. state of Washington. The construction began in 1942 and was completed in 1945. At this time Alder Dam was among the tallest dams in the United States, although this title has since been surpassed. The impounded water behind the dam forms Alder Lake, stretching about 7 miles (11 km) upstream with a capacity of 241,950 acre feet (0.29844 km3). With 28 miles (45 km) of shoreline, the lake is a popular recreation spot close to Mount Rainier National Park.

Water from Alder Lake is sent into two generators at the base of the dam, each of which produces 25 Megawatts for a total nameplate capacity of 50 Megawatts. Two miles downstream is LaGrande Dam, site of the first dam in the area, dating from 1912, and rebuilt in 1945 along with Alder's construction. Most of the energy produced at the dam is sent to the city of Tacoma, about 25 miles (40 km) north. Both Alder and LaGrande dams are owned and operated by Tacoma Power.

The name of the lake and the dam recalls the former small town of Alder, which was flooded in 1945 by the impounded water of the lake and disappeared.

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