THE MISSOULA FLOOD OVERVIEW

The Okanogan lobe of the Cordilleran ice sheet advanced into the Columbia River valley, creating Lake Columbia and diverting the Columbia River into the Grand Coulee. Another lobe of the ice sheet advanced southward down the Purcell Trench to the present Lake Pend Oreille in Idaho and dammed the Clark Fork River. This created an enormous Lake Missoula, about 4 times the size of Lake Erie [530 mi3 or 2200 km3], that had no outlet [Fig. 2]. When the dammed water got deep enough, it started floating the glacier, and the tremendous surge of water under the ice immediately broke up the ice dam, leading to the cataclysmic dumping of Lake Missoula. A wall of water close to 2000 ft [700 m] high surged through the breached dam and poured across eastern Washington at speeds of up to 100 mph [45 m/s]. Discharge was about 17 million cubic m3/s, about 20 times the size of the Bonneville Flood and a rate that would drain Lake Erie dry in about 8 hours. This was, perhaps, the largest flood discharge known [see The Altai Flood]. Figure 1—Columbia River drainage as the Cordilleran ice sheet advanced southward. Figure 2—Lakes Columbia and Missoula created by ice dams. Dry Falls in Grand Coulee, Washington, is the largest waterfall in the world, but it is currently inactive because it is on an intermittent stream that is not expected to flow again until the next Missoula Flood. Height of falls is 385 ft [117 m]. Flood waters were about 260 ft deep [80 m] above the top of the falls, so a more appropriate name might be Dry Cataract