The Missoula floods, also called the Spokane floods, Bretz floods, or Bretz's floods, were powerful floods that happened repeatedly in the area that became eastern Washington, northern Idaho, and northern Oregon. These floods occurred at the end of the last ice age. They happened when the ice dam on the Clark Fork River broke suddenly, creating Glacial Lake Missoula. Each time the dam broke, the water from the lake rushed down the Clark Fork and Columbia River, flooding parts of eastern Washington and the Willamette Valley in western Oregon. After the lake emptied, the ice formed again, creating Glacial Lake Missoula once more.
Early flood stories and postulates
Native American flood stories have been shared for thousands of years and may have come from people who saw ancient floods firsthand. The Kalapuya people told a story called "Panther, Coyote, Whale’s Daughter, The Flood, Obtaining the Fire," which includes a description of a flood called "atswin." Evidence also comes from the names of local places. The Sahaptin name for Rattlesnake Ridge near Hanford, Washington, is "Laliik," which means "stands above the water." However, there is no water near Rattlesnake Ridge today.
Some early explorers, soldiers, teachers, and scientists believed the Scablands were shaped by large amounts of water. Two important examples are the 1882 expedition by Lt. Thomas William Symons (U.S. Army Corps of Engineers), which named glacial Lake Lewis, and the 1885 work by T.C. Chamberlin, who found that the floodwater came from Glacial Lake Missoula. In March 1917, Thomas Bonser wrote an article for The Spokesman-Review that described what is now called Glacial Lake Columbia. In 1910, J. Pardee published a paper that identified Glacial Lake Missoula.
Thomas Large, Alonzo Pearl Troth, Thomas Bonser, Joseph McMacken, and others studied the area around Spokane. Their work helped scientists and the public learn more about the region’s geology and the study of ancient plants. In 1922, Thomas Large shared his observations about glaciers and possible floods in Science and gave the lake the name Lake Spokane.
Flood estimates
Scientists have studied these floods since the 1920s. During the time after the last ice age, which ended about 15,000 to 13,000 years ago, geologists believe the lake flooded and refilled in a cycle that lasted about 55 years on average. Over 2,000 years, these floods happened many times, possibly dozens of times. Researchers Jim O'Connor from the U.S. Geological Survey and Gerardo Benito from Spain's Museo Nacional de Ciencias Naturales found evidence of at least 25 large floods. The biggest flood released about 10 cubic kilometers of water each hour, which is 13 times the flow of the Amazon River. Other estimates suggest the largest flood could have released up to 17 cubic kilometers of water per hour. The fastest water flow reached 36 meters per second, or about 130 kilometers per hour.
In the Columbia River drainage basin, scientists studied deposits left by the Missoula floods, called the Hanford formation. These deposits are found in areas like the Othello Channels, Columbia River Gorge, Channeled Scabland, Quincy Basin, Pasco Basin, and Walla Walla Valley. By examining layers of rock and soil, including signs of ancient soil and other dating methods, scientists estimate the oldest Missoula floods occurred more than 1.5 million years ago. However, because older flood deposits have been removed by later floods, it is difficult to determine exactly how many ancient Missoula floods happened during the Pleistocene era.
Flood hypothesis proposed
Geologist J Harlen Bretz first noticed signs of powerful floods, which he called the Spokane floods, in the 1920s. He studied the Channeled Scablands in Eastern Washington, the Columbia Gorge, and the Willamette Valley in Oregon. Starting in the summer of 1922, Bretz researched the Columbia River Plateau for seven years. He became interested in unusual erosion patterns in the area as early as 1910 after seeing a new topographic map of the Potholes Cataract. In 1923, Bretz created the term "Channeled Scablands" to describe the area near the Grand Coulee, where large erosion had carved through layers of basalt. That same year, Bretz wrote a paper stating that the Channeled Scablands in Eastern Washington were formed by large floods from long ago.
Bretz’s idea, which suggested a sudden and powerful event shaped the land, contradicted the widely accepted belief that geological changes occur slowly over time. His views were not accepted at first. In 1927, the Geological Society of Washington, D.C., invited Bretz to share his research at a meeting where other scientists presented different theories. A fellow geologist, J.T. Pardee, had worked with Bretz and found evidence of an ancient glacial lake, which supported Bretz’s ideas. Bretz defended his theories, beginning a long and heated debate about the Scablands’ origin. Over the next 30 years, Pardee and Bretz collected and studied more evidence, leading them to identify Lake Missoula as the source of the Spokane flood and the cause of the Channeled Scablands.
After studying the Flathead River canyon, Pardee estimated that floodwaters moving at speeds faster than 45 miles per hour (72 km/h) were needed to move the largest boulders left by the flood. He calculated the water flow was 9 cubic miles per hour (38 km/h), more than the combined flow of all rivers in the world. Later studies showed the flood’s flow was ten times greater than the total flow of all rivers today.
The Missoula floods are also known as the Bretz floods, named in honor of Bretz.
Flood initiation
The Cordilleran ice sheet created a glacial dam across the Clark Fork valley, forming a large lake just southeast of present-day Sandpoint. This dam repeatedly failed, causing sudden floods known as jökulhlaups, which are types of glacial outburst floods. Scientists disagree about why the dam failed. One theory from the book Cataclysms on the Columbia: The Great Missoula Floods suggests that the lake’s water may have lifted the dam, allowing water to flow underneath and break it apart. A 2020 review article concluded that the dam likely failed due to either water overflowing the top of the dam or water creating tunnels through it, possibly combined with heat melting the ice.
Flood events
As the water flowed out of the Columbia River gorge, it collected again at the narrow part of the river near Kalama, Washington, which is about 1 mile (1.6 kilometers) wide. This caused temporary lakes to form, rising over 400 feet (120 meters) high. These lakes flooded the Willamette Valley, reaching as far as Eugene, Oregon, and beyond. Evidence of these events includes large rocks carried by ice and signs of erosion. Sediments left behind by the floods have helped make the Willamette and Columbia Valleys rich in soil for farming. Layers of glacial deposits covered by windblown soil (called loess) created steep, sloping dunes in the Columbia Valley. These dunes are well-suited for growing fruit trees and grapes in areas farther north.
Geologists now believe there were 40 or more separate floods, though the exact source of the water is still debated. The strongest flood is estimated to have moved 27 cubic kilometers (6.5 cubic miles) of water each hour. The fastest water flow reached about 36 meters per second (130 kilometers or 80 miles per hour). Each flood released up to 1.9×10 joules of energy, equal to 4,500 megatons of TNT. This is 90 times more powerful than the most powerful nuclear weapon ever tested, the 50-megaton "Tsar Bomba." Together, the floods removed 210 cubic kilometers (50 cubic miles) of soil, sediment, and basalt from the Channeled Scablands in eastern Washington and carried it downstream.
The idea that there were multiple floods was first suggested by R.B. Waitt Jr. in 1980. He studied layers of sediment from Ninemile Creek and Burlingame Canyon and found evidence of 40 or more floods. His strongest argument was that two layers of volcanic ash (called tephra) from Mount St. Helens were separated by thin layers of windblown dust in the Touchet bed deposits. This pattern repeated in 40 layers at Burlingame Canyon, leading him to conclude the floods happened over long periods of time.
Scientists debated whether the Channeled Scablands were shaped by many smaller floods or one massive flood from Glacial Lake Missoula or another source. In 1999, some geologists argued that the Touchet beds did not necessarily mean many floods happened over decades or centuries. Instead, they suggested the sediments came from sudden bursts of water from Glacial Lake Missoula and a large underground water source in British Columbia. This water might have flowed through multiple paths, including through Lake Missoula.
In 2000, a team led by Komatsu used a computer model to simulate the floods. They estimated the water flow from Glacial Lake Missoula based on previous studies. However, their model showed water depths in most areas were much lower than what geologists observed in the field. For example, their model predicted water depths of about 190 meters in the Pasco Basin–Wallula Gap area, but evidence shows depths were closer to 280–300 meters. They concluded that a flood with a flow rate of 10 cubic meters per second could not have caused the observed flooding.
Other scientists, like Brian Atwater, found evidence supporting multiple large floods. They noted signs like mud cracks and animal burrows in older layers that were later filled with sediment from later floods. They also found evidence that water from Glacial Lake Columbia flowed through different paths over centuries. They pointed out that Komatsu’s model did not consider how much the landscape changed during the floods. Narrower areas like Wallula Gap and the Columbia Gorge would have created stronger water resistance, leading to higher flood levels.
Studies using paleomagnetism later supported Waitt’s idea that the 40 layers of ash from Mount St. Helens were deposited about 30–40 years apart, with a possible gap of up to 60 years. Offshore deposits near the mouth of the Columbia River show 120 meters of material spread over thousands of years, matching the 40 floods seen in the Touchet Beds. This suggests an average of about 50 years between each flood.