Lake Missoula and Its Floods

Abstract

The city of Spokane lies astride the Spokane River, some 40 miles from its origination near Coeur d’Alene, Idaho. The convention center and hotel, site of the 65th annual meeting of SRM, are located right along the river, creating a beautiful and even idyllic setting. These qualities make it even harder to imagine a long-ago day when a miles-wide wall of water, measuring as much as 500 feet high, bore down on this site. Hours before, a great ice dam on glacial Lake Missoula (present-day Clark Fork River) had failed. The dam was as much as 2,500 feet in height, impounded a lake up to 2,000 feet deep, extended east in the intermontane valleys for 200 miles, and covered some 3,000 square miles. It probably contained over 500 cubic miles of water, approximating half the volume of Lake Michigan. It is now generally agreed that catastrophic fl oods, in accordance with prevailing ice age conditions, repeatedly issued from Lake Missoula, probably on the order of several tens of times. Geologists estimate that the latest and last of the fl oods occurred about 13,000 years ago, creating the observable effects on the ground today, while largely obscuring the effects of earlier fl oods. Most students of the fl oods support that this particular event and its effects should be named “Bretz’s Flood” after the early geologist J Harlen Bretz, who fi rst recognized in 1919 the effects of catastrophic fl ooding on the Columbia drainage of southeastern Washington, as well as the Columbia River Gorge, the Portland-Vancouver Basin, and the Willamette Valley of Oregon. Over the 2 million years of the last ice age, four major glaciations advanced into what is now the northwestern United States. The last of these, the Wisconsin glaciation, left clear evidence of its effects, while likely obscuring most of the evidence of earlier fl ooding. It is near-certain, however, that each of the many fl oods inundated thousands of square miles in what is now central and southeastern Washington to depths of hundreds of feet. Later study also showed that the many fl oods likely shared the same provenance, that is, the same cause and effect. Piecing together the available evidence, it appears that the full glacial climate was cold and dry, with strong winds that redistributed soil materials to the southeastern quadrant of the Columbia Basin. These processes created what is today called the Palouse, an area of wind-deposited silts (loess) over the underlying basalt that had literally oozed out of the earth over previous untold centuries. The winds created dune-like deposits of variable orientation and depth that today make up fertile farmland in southeastern Washington and adjacent Idaho. Bretz’s Flood followed the path of the many earlier fl oods and recreated the effects over thousands of square miles in the Columbia Basin. These can briefl y be considered in four major categories: 1) the channeled scablands, 2) the river canyon, 3) the Grand Coulee, and 4) Lake Lewis.