Foster Coulee Cordilleran Glacier moved down the Okanogan River valley and blocked the ancient route of the Columbia River, backing up water to create Glacial Lake Columbia. Initially, floodwater discharged from Glacial Lake Columbia by running up through the head of Grand Coulee and down through Foster Coulee to rejoin the Columbia River. After the floods, meltwater from the glacier reversed course and flowed east through the coulee, forming a hanging valley above the west rim of Grand Coulee and a debris fan on the Grand Coulee floor that can be seen today. As the glacier moved further south, Foster Coulee was
Grand Coulee Dam to oversee the dam project, and Reclamation was selected to oversee construction. On July 16, 1933, a crowd of 3,000 watched the driving of the first stake at the low dam site, and excavation soon began. Core drilling commenced that September while the Bureau of Reclamation accelerated its studies and designs for the dam. It would still help control floods and provide for irrigation and hydroelectricity, though at a reduced capacity. Most importantly, it would not raise its reservoir high enough to irrigate the plateau around the Grand Coulee. However, the dam's design provided for future raising and upgrading. Before