During the last retreat of the glaciers about 15,000 years ago, a large ice dam blocked the Clark Fork River in the Idaho Panhandle, creating Glacial Lake Missoula. This lake was massive. It is estimated is was 2,500 feet deep from rim to bottom. As the lake filled, it would back up to the Canadian border covering 3,000 square miles of Western Montana and containing more than 500-600 cubic miles of water which today is more water than Lake Erie and Lake Ontario combined.
The ice dams at Glacial Lake Missoula routinely failed, resulting in periodic gigantic catastrophic glacial floods filled with ice, dirt-filled water, sheared rocks and gravel, leaving a lasting mark on the landscape where it flowed. The floods swept across Northern Idaho, through the Spokane Valley, southwestward across Eastern Washington, and down the Columbia River Gorge, enroute to the Pacific Ocean.
The Glacial Lake Missoula floods are the largest known floods on Earth during the last two million years. When the ice dam exploded, the flow of water was ten times the combined flow of all the rivers of the world. The floods had such height and velocity that it scoured everything in its path. From speeds approaching 65-80 miles per hour, the Glacial Lake Missoula would have drained in as little as 48 hours. In terms of potential energy, the equivalent of 4,500 megatons of TNT was released by each flood.
The cumulative effect of the Glacial Lake Missoula floods was to excavate 50 cubic miles of very fine grained silt or clay, sediment and basalt, largely coming from the Channeled Scablands of Eastern Washington and transport it downstream through the Columbia Gorge. Near present day Kalama, Washington, a backwater lake was formed known as Lake Allison.
Lake Allison filled the area we now know as the Willamette Valley. At the time of the floods, the measured height of the lake at Oregon City was 400 feet and in Eugene at 380 feet. Floods deposited boulders the size of houses and enormous gravel beds along the way. As velocity slowed, the silt-laden water deposited its cargo on the valley floor, forming some of the richest farming soil in the country. Here, the Glacial Lake Missoula floods deposited thick layers of deep, well-drained soils, in some places up to 300 feet, making the Willamette Valley one of the most fertile agricultural lands in the United States.
The last Glacial Lake Missoula flood occurred about 13,000 years ago. And thence, to this land came the Kalapuya Indians that settled between the Rivers of the Santiam, nestled in the fertile Willamette Valley nearly 10,000 years ago.