Alder Dam is a concrete arch dam on the Nisqually River in the U.S. state of Washington. When construction was completed in 1945, Alder Dam was among the tallest dams in the United States, although this title has already 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).