On a smaller scale, the Columbia River basin appears to have been
scoured by massive floods about every 50-60 years at the end of the last ice age 13-15,000 years ago. General estimates put humans in North America between 15-20,000 years ago (evidence is growing for more waves going back to 30,000 years or more). The lack of archeological sites in Oregon and Washington more than about 8-10,000 years ago probably has to do with this prolonged and repeated flooding of the river systems where longer term camps and shelters would have been (tribes on the Columbia River spent the most time near the river to harvest and process fish and trade along the water routes).
The floods were caused by melting glaciers which formed a lake behind the glacier that extended to near Missoula, Montana. As water pressure increased, the glacial dam failed, scouring the interior valleys clean. Estimates of the water flow indicate a lake about half the volume of Lake Michigan. After each flood, the glacier would reform and water would build up again. Rinse and repeat.
Early people in the Pacific Northwest would have witnessed a similar flood every 1-2 generations for thousands of years.