Defining the northern limit of the WRB, the Columbia River has reestablished its course through multiple dammings by slides, basalt flows, and ice. Reaching their maximum 13 mya, basalt floods from the enormous Grande Ronde volcano in northeast Oregon flowed down the Columbia channel all the way to the river's mouth and into the Willamette Valley. In the Pliocene (3 to 11 mya), western Oregon was a desert and gravels produced during that time eroded into and covered the spreading valley floor. Work performed by J. Harlan Bretz (as cited in Branscomb, 2002) in the 1920s revealed that between 15,500 and 13,000 years ago ice periodically dammed the Clark Fork in Idaho east of Spokane, Washington, forming a lake in the present-day Missoula Valley of Montana covering some 3100 square miles (8000 km2) and containing 500 cubic miles (2000 km3) of water. In a typical release occurring over less than two weeks, a quantity of water equal to half the present volume of Lake Michigan, more than the modern annual volume of all the world's rivers, poured down the Columbia channel (Loy, Allen, Patton, and Plank as cited in Branscomb, 2002). Backwater from these floods carried ice-rafted "erratic" rocks from Montana repeatedly into the Willamette Valley, filling it to about 400 ft (122 m) above current sea level (Orr, Orr, and Baldwin as cited in Branscomb, 2002). The sediments left by these floods form much of the present valley floor.