McNary Dam, 7,265 ft (2,214 m) long and 183 ft (56 m) high, on the Columbia River between Oregon and Washington, near Umatilla, Oreg.; built 1947–56 by the U.S. Corps of Engineers. Located at the head of the slack-water pool created by John Day Dam, it provides slack-water navigation 61 mi (98 km) upstream to the mouth of the Snake River; locks permit vessels to pass around the dam. McNary Dam is one of the largest hydroelectric power facilities in the United States.