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 upstream …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 upstream with a capacity of 241,950 acre-feet. With 28 miles of shoreline, the lake is a popular recreation spot close to Mount Rainier National Park.