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
Grand Coulee Dam MWAK constructed Mason City in 1934. Mason city contained a hospital, post office, electricity and other amenities along with a population of 3,000. Three-bedroom houses in the city were rented for $32 a month. Of the two living areas, Engineer's City was considered to have the better housing. Several other living areas formed around the construction site in an area known as Shack Town, which did not have reliable access to electricity and the same amenities as the other towns. Incorporated in 1935, the city of Grand Coulee supported workers as well and is just west of the dam on