Glacial Lake Missoula was bigger than all five Great Lakes put together. The huge lake, created by 45-mile wide tongue of ice known as the Purcell Lobe, had a surface area of more than 3,000 square miles. When the ice dam collapsed, the ensuing flood raced across the landscape at several hundred miles an hour, draining the lake in just a matter of days.
As the flood rolled down the Columbia River it reached the Wallula Gap in southern Washington, which created a bottleneck. As the flood squeezed through the bottleneck, water levels rose to 800 feet and backed up, temporarily forming Lake Lewis in the Pasco Basin behind it. The flood eventually dropped ice-rafted boulders as far south as the Willamette Valley in Oregon.
When I originally read the accounts of the flood and the bottleneck created by the Wallula Gap, I pictured a very narrow passageway. However, when I actually visited the area, I discovered that the Wallula Gap is 1.2 miles (2 km) wide, not exactly the constricted channel that I had envisioned.
This video was originally published by Nick Zentner on Jun 14, 2015. It shows evidence for the lake including strandlines above the city of Missoula, Montana, along with giant current ripples at Camas Prairie and striking rhythmites along Interstate 90 at Nine Mile Road near Missoula.
Nick Zentner is the science outreach and education coordinator for the Department of Geological Sciences at Central Washington University. He has produced more than 40 short videos about Central Washington geology.
I’ll be posting more fascinating episodes of “Nick on the Rocks” in the days to come.