a large dam (= barrier built across a river) on the Columbia River in the US state of Washington. It was completed in 1942 and made larger in the 1980s. It was built to produce electricity from water power. The dam is 550 feet/168 metres high and 4 173 feet/1 273 metres long. It holds back the waters of Franklin D Roosevelt Lake, one of the largest US reservoirs (= places where water is stored for use).