The Missoula floods, also called the Spokane floods, the Bretz floods, or Bretz's floods, were a series of powerful floods that happened repeatedly in the area that is now eastern Washington, northern Idaho, and northern Oregon. These floods occurred at the end of the last ice age. They happened because an ice dam on the Clark Fork River, which formed Glacial Lake Missoula, suddenly broke. Each time the dam broke, water from the lake rushed down the Clark Fork River and the Columbia River, flooding large parts of eastern Washington and the Willamette Valley in western Oregon. After the water drained away, the ice reformed, creating Glacial Lake Missoula once more.
Early flood stories and postulates
Native American flood stories have been shared for many hundreds of years and may have been based on people who saw the ancient megafloods. The Kalapuya people told a story called "Panther, Coyote, Whale’s Daughter, The Flood, Obtaining the Fire," which describes a flood called "atswin." Evidence also comes from names given to land features. 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 events were Lt. Thomas William Symons’ 1882 expedition, which named glacial Lake Lewis, and T.C. Chamberlin’s 1885 exploration, which identified Glacial Lake Missoula as the source of the floodwater. In March 1917, Thomas Bonser wrote an article in The Spokesman-Review about ancient Spokane, correctly describing 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 about a large lake that once existed there. In 1922, Thomas Large wrote about his observations of glaciers and possible floods in Science and gave the hypothetical lake the name "Lake Spokane."
Flood estimates
Scientists have studied these floods since the 1920s. During the melting of glaciers after the end of the Last Glacial Maximum, geologists estimate that a cycle of flooding and lake forming occurred about every 55 years. These floods happened many times over 2,000 years between 15,000 and 13,000 years ago. U.S. Geological Survey hydrologist Jim O'Connor and scientist Gerardo Benito from Spain's Museo Nacional de Ciencias Naturales found evidence of at least 25 large floods. The largest flood released about 10 cubic kilometers of water each hour (2.7 million m³/s, 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 about 36 meters per second (130 km/h or 80 mph).
Within the Columbia River drainage basin, detailed studies of sediment deposits from glaciers, known as the Hanford formation, show that flood deposits from the Missoula floods are found in areas such as the Othello Channels, Columbia River Gorge, Channeled Scabland, Quincy Basin, Pasco Basin, and the Walla Walla Valley. Scientists used methods like magnetic layer analysis, light-based dating, and observations of certain rock structures to estimate that the oldest Missoula flood deposits are more than 1.5 million years old. However, because older sediment deposits have been removed by later floods, the exact number of ancient Missoula floods that occurred during the Pleistocene cannot be determined with certainty.
Flood hypothesis proposed
Geologist J Harlen Bretz first noticed signs of large, sudden floods, which he called the Spokane floods, in the 1920s. He studied areas such as the Channeled Scablands in Eastern Washington, the Columbia Gorge, and the Willamette Valley in Oregon. From the summer of 1922 to 1929, Bretz researched the Columbia River Plateau. He became interested in unusual erosion patterns in the area as early as 1910 after seeing a new map showing land shapes near the Potholes Cataract. In 1923, Bretz created the term "Channeled Scablands" to describe the area near the Grand Coulee, where strong erosion had removed layers of basalt rock. In 1923, Bretz wrote a paper stating that the Channeled Scablands were formed by powerful floods from long ago.
Bretz's idea, which suggested that sudden events shaped the land, opposed the common belief that geological changes happen slowly over time. Many scientists at first ignored his findings. In 1927, the Geological Society of Washington, D.C., invited Bretz to share his research during a meeting where other scientists presented different theories. A geologist named J.T. Pardee, who had worked with Bretz, shared evidence of an ancient glacial lake that supported Bretz's ideas. Bretz defended his theories, starting a long and heated debate about the Scablands' origin. Over the next 30 years, Pardee and Bretz collected and studied evidence that led them to conclude Lake Missoula was 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. He calculated the flood's water flow was 9 cubic miles per hour (38 km/h), greater than the combined flow of all rivers worldwide. More recent studies suggest the flood's flow was ten times the total flow of all rivers today.
The Missoula floods are also called the Bretz floods in honor of Bretz.
Flood initiation
The Cordilleran ice sheet created an ice dam across the Clark Fork valley, forming a large lake southeast of present-day Sandpoint. The dam failed multiple times, causing sudden glacial outburst floods called jökulhlaups. Scientists disagree about why the dam failed. One theory from the book Cataclysms on the Columbia: The Great Missoula Floods suggests the lake may have floated the dam, allowing water to flow underneath and weaken it. A 2020 review article concluded that the dam likely failed due to either water overflowing the dam, water flowing through tunnels in the dam, or heat melting the ice.
Flood events
As the water flowed out of the Columbia River gorge, it collected again at the narrow area near Kalama, Washington, which is about 1 mile (1.6 km) wide. This created temporary lakes that rose over 400 feet (120 meters) high, flooding the Willamette Valley all the way to Eugene, Oregon, and beyond. Evidence of these events includes large rocks carried by icebergs and signs of erosion. Sediments left behind by the floods have made the Willamette and Columbia Valleys rich in soil, which helps farming. Layers of glacial deposits covered by wind-blown soil (called loess) have formed steep, south-facing dunes in the Columbia Valley, creating ideal conditions for growing fruit trees and grapes in areas farther north.
After much study and debate, scientists now believe there were 40 or more separate floods. However, the exact source of the water is still not certain. The strongest flood is estimated to have moved 27 cubic kilometers (6.5 cubic miles) of water every hour. The fastest water flow reached about 36 meters per second (130 km/h or 80 mph). Each flood released up to 1.9×10 joules of energy, equal to 4,500 megatons of TNT. For comparison, this is 90 times more powerful than the most powerful nuclear bomb 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 proposed by R.B. Waitt Jr. in 1980. Waitt suggested there were 40 or more floods. His conclusion was based on layers of sediment found in Ninemile Creek and Burlingame Canyon. His strongest evidence was the discovery of two layers of volcanic ash (called tephra) from Mount St. Helens, separated by thin layers of wind-blown dust in the Touchet beds. These layers suggest the floods happened over long periods. Waitt argued that since 40 similar layers were found in Burlingame Canyon, they likely represented floods separated by similar time intervals.
For many years, scientists debated whether the landforms in the Channeled Scablands were shaped by many floods or by a single huge flood from Glacial Lake Missoula or an unknown source in Canada. In 1999, some geologists, including Shaw, studied the Touchet beds and suggested that the layers did not necessarily mean the floods happened decades or centuries apart. Instead, they proposed that water from Glacial Lake Missoula may have flowed into the lake from British Columbia. They also suggested that a large underground reservoir in central British Columbia, including the Rocky Mountain Trench, may have released water through multiple paths, including through Lake Missoula. This would have created much larger flood volumes. They also said the rhythmic layers in the Touchet beds might have formed from repeated bursts of water during a single flood.
In 2000, a team led by Komatsu used a 3D computer model to simulate the floods. They based their model on the estimated water flow from Glacial Lake Missoula, which was previously thought to be up to 17×10 m/s. They assumed modern-day land shapes and found that their model predicted water depths in most areas to be much lower than what was found in the field. For example, they calculated water depths in the Pasco Basin–Wallula Gap area to be about 190 meters, but field evidence showed depths of 280–300 meters. They concluded that a flood with a flow rate of about 10 m/s could not have created the observed high-water marks.
In response to Komatsu’s study, Brian Atwater and others pointed out strong evidence for multiple large floods. This includes cracks in mud and animal burrows in lower layers that were later filled with sediment from later floods. They also found signs of floods spreading over many centuries along side channels of Glacial Lake Columbia. They noted that the water from Lake Columbia changed over time, first flowing across the Waterville Plateau into Moses Coulee, and later through the Grand Coulee after the Okanagan lobe blocked the earlier route. Komatsu’s study did not consider the large amount of erosion that occurred during the floods. They also said that using modern-day land shapes to model flood behavior might not be accurate, as narrower areas like the Wallula Gap and the Columbia Gorge would have created stronger resistance to water flow, leading to larger floods.
Studies using paleomagnetism later supported Waitt’s idea that the layers of volcanic ash from Mount St. Helens were deposited about 30–40 years apart, suggesting floods occurred every 30–40 years. However, this could also mean up to 60 years between floods. Offshore deposits near the mouth of the Columbia River show 120 meters of material spread over thousands of years, matching the multiple floods seen in the Touchet Beds. Based on Waitt’s count of 40 floods, this would mean an average of about 50 years between each flood.