Grand Coulee at the higher side of the flexure. Encountering the steep slope of the monocline, the new river would have cascaded off the rim, down onto a broad plain where Coulee City and Dry Falls State Park now stand. Upper Grand Coulee began as an cascade just north of Coulee City. As the rush of water eroded the surface, it steepened into a waterfall. The falls continued to erode backward (northward) creating the canyon. When the falls reached the divide into Lake Columbia, i.e., preglacial Columbia Valley, it disappeared, leaving the elongated notch. Today, the waters of the Lake Roosevelt are
Coulee Dam, Washington Coulee Dam, Washington Coulee Dam is a town in Douglas, Grant, and Okanogan counties in the State of Washington. The Douglas County portion of Coulee Dam is part of the Wenatchee–East Wenatchee Metropolitan Statistical Area. The population was 1,098 as of the 2010 census. Coulee Dam was founded by the U.S. Bureau of Reclamation in 1933, to serve as headquarters for the construction of the Grand Coulee Dam. The Okanogan County portion was known as Mason City, location of the head contractor. The Portions in Douglas and Grant Counties was known as Engineers Town and was government owned. In 1942