Lake Pend Oreille was glacially formed during the ice age. It is also believed that the eastern side of the lake was in the path of the ancient Missoula Flood. The lake sits at the south end of the Purcell Trench, carved by glaciers moving south from Canada. The eastern side of the glacier is believed to have formed the dam for the Missoula flood, at the point where the Clark Fork river enters the lake between the Cabinet and Bitterroot mountains. The lake is made slightly larger by the dam at Albeni Falls, just east of Oldtown; the dam is high and operated by the U.S. Army Corps of Engineers. Along with Crater Lake, Lake Tahoe, Lake Chelan, and Lake Superior, Lake Pend Oreille is among the five deepest lakes in the United States.