Glacial Lake Columbia Glacial Lake Columbia Glacial Lake Columbia was the lake formed on the ice-dammed Columbia River behind the Okanogan lobe of the Cordilleran Ice Sheet when the lobe covered of the Waterville Plateau west of Grand Coulee in central Washington state during the Wisconsin glaciation. Lake Columbia was a substantially larger version of the modern-day lake behind the Grand Coulee Dam. Lake Columbia's overflow – the diverted Columbia River – drained first through Foster Coulee, and as the ice dam grew, through first Moses Coulee, and finally, the Grand Coulee. The Cordilleran ice sheet also blocked the Clark Fork River and
Grand Coulee volume of water flowing out of the upper coulee. The distribution begins in the uncanyoned basin below Dry Falls and expanded to over a before reaching Quincy Basin. One cataract (Unnamed Coulee) is high and had three alcoves over more than a . There is no channel as the water arrived in a broad sheet. The gravel deposits of Quincy Basin represent only a third or a fourth of the estimated 11 cubic miles of rock excavated from the Grand Coulee and its smaller other related coulees (Dry, Long Lake, Jasper, Lenore, and Unnamed). Most of the debris was carried