Glacial Lake Missoula and the Great Floods Lex Blood, D.Sc., Founder, Crown of the Continent Ecosystem Education Consortium (COCEEC) Professor Emeritus, Flathead Valley Community College

oday the waters of the Flathead River and its tributaries flow into Flathead TLake near Bigfork and leave at Polson on their way southwesterly to join the Clark Fork River near Paradise. From this junction, the Clark Fork flows westerly to cross into Idaho, emptying into Lake Pend d’Oreille near the small town of Clark Fork.

Source: Foster, Tom 2009 While there is much study and interpreta- tion yet to be done, it is clear that near the end of the last major period of glacia- tion—the Wisconsin—the Flathead lobe of the Cordilleran Ice Sheet extended Glacial Lake Missoula and the channeled Scablands 15,000 years ago into the Flathead Valley across the area now occupied by Flathead Lake, and a Pacific Ocean, carrying rock debris plucked Possibly the most spectacular features in similar lobe extended into northern Idaho from Montana and along the entire route. the Flathead Watershed are a series of through the Purcell Trench across what is immense ripples in Camas Prairie formed today Lake Pend d’Oreille. Incredible erosional and depositional fea- during the emptying of the lake. Fast mov- tures were created, both within the lake ing waters carrying rock fragments carved During a period extending from about area and across Idaho and Washington from the nearby mountain passes formed 17,000 to 12,000 years before the pres- to the Pacific Ocean. Geologists believe these features, which are as much as 30 ent, the Purcell lobe blocked the flow of the that in this several-thousand-year period feet high and 100 feet from crest to crest, Clark Fork River just east of the Montana- Glacial Lake Missoula may have been so widespread and immense that they are Idaho border. This damming action caused dammed, emptied and refilled between easily visible on satellite images. the river’s flow, swollen by continuous melt- 40-100 times and that in previous glacial ing of the glaciers, to progressively fill the periods of the approximately 2 million-year- Less notable features in the glacial lake valleys of the Clark Fork, Flathead, Bitter- long Pleistocene Epoch or “Ice Age,” this area are huge piles of sediment deposited root and Blackfeet Rivers, creating what is process may have been repeated many by the emptying water in areas where flow now known as Glacial Lake Missoula with a hundreds of times. rates diminished, similar to the formation of surface area twice the size of Rhode Island. sand and gravel bars in modern river chan- Within the Flathead Watershed today there nels. These are notable west of Missoula At its maximum depth at the ice dam near are numerous relic features formed by Gla- and Plains. Similar hydrodynamics have the Montana-Idaho line, Glacial Lake Mis- cial Lake Missoula. High on hillsides above left sediment terraces perched high in the soula was over 2000 feet deep and covered many of the valleys are long horizontal side channels of tributaries to the main river 3000 square miles of western Montana. ridges, known as strand lines, that mark gorge. These are particularly notable along Eventual rupture of the dam by overtopping the ancestral shore lines of the lake. The the Flathead River from Perma to Paradise. and/or undercutting released the contained highest of these are at 4250 feet elevation, 500 cubic miles of water, equal to about indicating that Missoula would have been While there were several other significant half the volume of Lake Michigan, within a under approximately 950 feet of water at glacial lakes in the Pacific Northwest dur- matter of days. The pent-up waters surged the maximum filling. ing the Pleistocene, Glacial Lake Missoula westerly across central Washington at up and the area scoured by the huge floods to 65 miles per hour and a flow rate equal Glacial meltwaters carried fine clay and which came from the lake have left the to 10 times the combined flow of all earth’s silt sediment into Lake Missoula, which most impressive legacy across today’s rivers today. Confined to the narrow Co- settled out as multiple layers of tan sedi- landscape of northwest Montana and the lumbia Gorge, the waters backed up the ment, known as varves (layers of sediment), Pacific Northwest. Snake River to the Idaho line, flooded the which tell us a great deal about the history Willamette Valley and the site of Portland of the lake and conditions in the surround- to a depth of 300 feet, and gushed into the ing region.