Missoula Floods

Glacial Lake Missoula formed when the Purcell Trench lobe of the last-glacial Cordilleran ice sheet dammed the Clark Fork valley in northwestern Idaho with ice a kilometer thick. Behind this ice dam, water rose 2,000 feet (610 m), filling the valleys to the east and creating a lake 200 miles long. The only outlet of the lake was through its ice dam. In Waitt's 1983 USGS report, calculations show that each time the lake rose to about 600 m deep, it made the glacier buoyant and engendered a catastrophic discharge along the glacier bed, known as a jökulhlaup. In Waitt's 1983 USGS report, a reconstructed water budget suggests that the lake refilled in 3 to 6 decades after a complete draining, and varve beds indicate a mean period of about 4 decades between floods, shorter during the last several floods. The 2020 USGS review finds intervals possibly as long as 100 years early in the lake's history, shrinking to just one or two years for the last few floods. Volcanic ash layers help date the sequence: the Mount St. Helens Set S tephra of about 16 ka is commonly interbedded within flood deposits, and tephra from the 13.7 to 13.4 ka eruption of Glacier Peak overlies all glacial Lake Missoula and Missoula flood deposits, defining an end time. The 2020 USGS review concludes that glacial Lake Missoula was producing floods for at least 3 to 4 thousand years during 20 to 14 ka (thousand years ago), and that the maximum extent of the Okanogan and Purcell Trench lobes and many Missoula floods came about 17 to 15 ka. Cosmogenic 10Be dating by Balbas and others (2017) places the last floods from glacial Lake Missoula at 14.7 ± 1.2 ka. The National Park Service places the end of the last Ice Age, when a wall of ice blocked the Clark Fork River near the Idaho-Montana border, at 18,000 to 13,000 years ago.

• Ice dam: the Purcell Trench lobe of the Cordilleran ice sheet, across the Clark Fork valley in northwestern Idaho, with ice a kilometer thick
• Lake: water rose 2,000 feet (610 m) behind the dam; the lake was 200 miles long
• Failure: calculations show that at about 600 m deep the lake made the glacier buoyant and engendered a catastrophic discharge along the glacier bed (a jökulhlaup; Waitt 1983)
• Refill: a reconstructed water budget suggests 3 to 6 decades after a complete draining (Waitt 1983); intervals possibly as long as 100 years early and one or two years for the last few floods (2020 USGS review)
• Dating: floods during 20 to 14 ka; last floods from glacial Lake Missoula at 14.7 ± 1.2 ka (Balbas and others 2017); Glacier Peak tephra of 13.7 to 13.4 ka overlies all Missoula flood deposits

When the dam failed, the lake released up to 600 cubic miles (~2,500 km³) of water, the volume of Lake Ontario and Lake Erie combined, in as little as two days, in the National Park Service's figures; Waitt's 1983 USGS report gives the lake as 2000 km3. Flow analyses in the 2020 USGS review put peak discharge at about 20 million m3/s near the Lake Missoula outlet. The wall of water, ice and debris raced westward over 16,000 square miles (41,440 km2) through present-day Montana, Idaho, Washington and Oregon. In eastern Washington the floods carved the Channeled Scabland, a 30,000-km2 landscape of coulees and cataracts cut into the region's loess-covered basalt. Cosmogenic 10Be dating by Balbas and others (2017) places one of the largest floods at 18.2 ± 1.5 ka and finds that Upper Grand Coulee fully opened as a flood route after 15.6 ± 1.3 ka. Downstream, all flood routes converged onto Pasco Basin and flowed through Wallula Gap and the Columbia River Gorge into the Pacific Ocean. Wallula Gap, a two-mile-wide (3.2 km) gap in the Horse Heaven Hills, funneled all floodwaters crossing the Channeled Scabland, and water backed up behind it into a temporary hydraulic pond 900 feet (274 m) deep that flooded Pasco Basin and its tributaries. Depositional landforms include current dunes and gravel bars, and a fan deposit near Ephrata, Washington. Features deposited by wind in the floods region include dune fields (sand) and loess hills (silt). The landscape spurred J Harlen Bretz's vigorously disputed flood hypothesis in the 1920s. First known as the Spokane flood, it was renamed the Missoula flood once it was understood that the water came from glacial Lake Missoula, and Bretz's flood evidence eventually swayed consensus for cataclysmic flooding. Congress established the Ice Age Floods National Geologic Trail in the States of Montana, Idaho, Washington, and Oregon in Public Law 111-11 of March 30, 2009.

• Release: up to 600 cubic miles (~2,500 km³) in as little as two days (National Park Service); floods as large as 2,500 cubic kilometers (USGS Open-File Report 2003-408)
• Peak discharge: about 20 million m3/s near the Lake Missoula outlet (2020 USGS review); some floods exceeded 20 million cubic meters per second (USGS Open-File Report 2003-408)
• Flood-carved landforms (NPS list): Grand Coulee, Moses Coulee and Frenchman Coulee (coulees); Dry Falls and Palouse Falls (cataracts); Drumheller Channels (scabland)
• Dry Falls: the Great Cataract Group was 3.5 miles (5.6 km) wide and 400 feet (122 m) tall, four times larger than Niagara Falls; viewpoint at the Dry Falls Visitor Center off Washington State Route 17 between Coulee City and Sun Lakes-Dry Falls State Park
• Wallula Gap: two miles (3.2 km) wide in the Horse Heaven Hills; floodwater ponded 900 feet (274 m) deep behind it, flooding Pasco Basin
• Depositional features: current dunes, gravel bars and a fan deposit near Ephrata, Washington

The rhythmic Touchet Beds in the Walla Walla and lower Yakima valleys resulted from many separate backfloodings by hydraulically ponded glacial Lake Missoula water. The evidence that the Touchet Beds rhythmites came from brief backfloodings includes up-valley thinning and fining of locally derived bedload, up-valley paleocurrents, and up-valley transport of erratics derived from Cordilleran ice. Waitt (1980) found that a lengthy nonflood environment followed each of about 40 Touchet Beds rhythmites; a Mount St. Helens "set S" tephra couplet sits atop one rhythmite, and the thickest section is 30 m. The lack of weathering or soil within the Touchet Beds suggests that all the rhythmites are late Wisconsin. In the bottom sediment of glacial Lake Missoula in Montana, the same study reads forty superposed rhythmites as a record of about 40 late-Wisconsin fillings and emptyings of the lake. Waitt's 1983 USGS report adds rhythmic successions of 40 or more graded beds in backflooded tributaries of the lower Columbia River. The 2020 USGS review counts dozens, likely more than a hundred, separate Missoula floods during the last glacial period, with at least about 75 floods before the Mount St. Helens Set S tephra and 30 or more after it. Rhythmites are evidence for a repeated series of floods.

• Touchet Beds: rhythmic deposits in the Walla Walla and lower Yakima valleys from many separate backfloodings of ponded glacial Lake Missoula water
• About 40 Touchet Beds rhythmites, each followed by a lengthy nonflood interval (Waitt 1980); the thickest section is 30 m
• A Mount St. Helens set S tephra couplet sits atop one rhythmite
• Flood count: about 40 (Waitt 1980); dozens, likely more than a hundred (2020 USGS review)

Floods flowing from Wallula Gap traveled down the Columbia River and covered the Willamette Valley with hundreds of feet of flood deposits. One valley constriction, Kalama Gap northwest of Portland, backed water 120 to 150 meters high in the Portland basin and backflooded 200 km south into the Willamette Valley. Dozens of floods backed up into the Willamette Valley, eroding scabland channels and depositing giant boulder gravel bars in areas of vigorous currents as well as bedded flood sand and silt in backwater areas. The water inundated the region from Portland down to Eugene up to 400 feet above present-day sea level. Large chunks of ice from the breached glacier dam rafted hundreds of erratic rocks into the valley and left them scattered among the flanking foothills and valley bottom; a USGS database lists approximately 400 of these ice-rafted erratics. Separately, the National Park Service gives the Bellevue Erratic as a 90-ton rock (a photo caption in its 2014 foundation statement gives 160-ton), six miles west of McMinnville just off Highway 18, at Erratic Rock State Natural Site. It was floated as much as 500 miles in an iceberg. The boulder is banded argillite, which can be traced back 400 miles or more to the 1.5 billion-year-old Belt Supergroup.

• Kalama Gap, northwest of Portland: backed water 120 to 150 meters high in the Portland basin and backflooded 200 km south into the Willamette Valley
• Flood stage: water inundated the valley from Portland down to Eugene up to 400 feet above present-day sea level
• Deposits: giant boulder gravel bars where currents were vigorous; bedded flood sand and silt in backwater areas
• Erratics: approximately 400 ice-rafted erratics in a USGS database; separately, the National Park Service gives the Bellevue Erratic as a 90-ton banded argillite rock six miles west of McMinnville

Primary sources (U.S. Geological Survey and National Park Service, Ice Age Floods National Geologic Trail): https://pubs.usgs.gov/publication/ofr83671 (Waitt 1983, USGS Open-File Report 83-671) https://www.usgs.gov/publications/about-forty-last-glacial-lake-missoula-jokulhlaups-through-southern-washington (Waitt 1980, Journal of Geology) https://www.usgs.gov/publications/missoula-and-bonneville-floods-a-review-ice-age-megafloods-columbia-river-basin (O'Connor and others 2020, Earth-Science Reviews, USGS review) https://pubs.usgs.gov/publication/70194547 (Balbas and others 2017, Geology 45(7)) https://pubs.usgs.gov/publication/ofr2003408 (Minervini, O'Connor and Wells 2003, USGS Open-File Report 2003-408) https://www.nps.gov/iafl/learn/management/upload/IAFL_FD_2014-508s.pdf (NPS Ice Age Floods National Geologic Trail Foundation Statement 2014) https://www.nps.gov/iafl/planyourvisit/park-brochure.htm (NPS Ice Age Floods park brochure) https://www.nps.gov/places/erratic-rock-state-natural-site-bellevue-erratic.htm (NPS Erratic Rock State Natural Site, Bellevue Erratic) https://www.nps.gov/iafl/planyourvisit/oregon-state.htm (NPS Ice Age Floods, Oregon) https://www.nps.gov/iafl/learn/photosmultimedia/ice-age-floods-animation.htm (NPS Ice Age Floods animation transcript) Last verified 2026-11-09