Tuesday, September 26, 2023


Roadside Geology: The Missoula Floods and the wonders of North Idaho

An RVer traveling on US-95 in Northern Idaho from the town of Athol to Sandpoint, or eastbound on State Highway 200 near Clark Fork, Idaho, will see sweeping and dramatic evidence of a geological event dating from the Miocene Epoch of geologic time: The Missoula Floods.

The Missoula Floods

About 17,000 years ago, the Cordilleran Ice Sheet covered everything from the Arctic through Western Canada, its southern tip extending into Northern Idaho. As the glacier pushed southeastward, it split, with the south lobe blocking the Clark Fork River and the western lobe scouring out what is now Lake Pend Oreille. The split, created by the Green Monarch mountains, effectively created a giant dam, with glacial ice standing 2,000 feet deep above what are now the towns of Clark Fork and Bayview, Idaho. As the ice melted about 12,000 years ago, the ice dam broke, and the outwash created the Rathdrum Prairie, crossed by Interstate 90 between Spokane, Washington, and Coeur d’Alene, Idaho.

According to the article “Glacial Lake Missoula and the Ice Age Floods” on the Montana Historical Center website:

“About 12,000 years ago, the valleys of western Montana lay beneath a lake nearly 2,000 feet deep. Glacial Lake Missoula formed as the Cordilleran Ice Sheet dammed the Clark Fork River just as it entered Idaho. The rising water behind the glacial dam weakened it until water burst through in a catastrophic flood that raced across Idaho, Oregon, and Washington toward the Pacific Ocean. Thundering waves and chunks of ice tore away soils and mountainsides, deposited giant ripple marks, created the scablands of Eastern Washington, and carved the Columbia River Gorge. Over the course of centuries, Glacial Lake Missoula filled and emptied in repeated cycles, leaving its story embedded in the land.”

The Cordilleran Ice Sheet as it descended into Northern Idaho.

The lake was 2,000 feet deep

Glacial Lake Missoula was 2,000 feet deep and stretched eastward about 200 miles. It contained more water than Lake Ontario and Lake Erie combined. A modern-day remnant of this enormous body of water is Lake Pend Oreille.

Photo by Bruce Bjornstad The site of the ice dam of Glacial Lake Missoula is visible in this stunning drone photo, taken just east of Farragut State Park.

Parts of this geological story can be seen all around the campgrounds at Farragut State Park (I recently wrote about that park here), east of Athol, Idaho; Sam Owen Park near Hope, Idaho; and commercial campgrounds such as Beyond Hope, adjacent to the Forest Service’s great lakeshore Sam Owen Park. There is also a terrific—and free—boondocking site on a peninsula at the mouth of the Clark Fork River near East Hope, Idaho (shown on the Boondocking app as Clark Fork River Free Camping).

The American West displays a panorama of fantastic geography—and geology—right along the roadside.

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Randall Brink
Randall Brink
Randall Brink is an author hailing from Idaho. He has written many fiction and non-fiction books, including the critically acclaimed Lost Star: The Search for Amelia Earhart. He is the screenwriter for the new Grizzly Adams television series and the feature film Goldfield. Randall Brink has a diverse background not only as a book author, Hollywood screenwriter and script doctor, but also as an airline captain, chief executive, and Alaska bush pilot.


  1. I believe I’ve seen this. In the desert, about 60 miles north of Moses Lake. WA., lays a dry waterfall from that event. Its width is several Niagaras end to end. It is the start of a miles long approximately 800 foot deep canyon that today holds several oasis along its route. Some are state parks. Quite the oddity in the middle of a desert at the surface.

    • Yes indeed–all part of the “Channeled Scablands” that were awash from the outflow of the repeated Missoula Floods in Pleistocene time.

    • There is a “property development” (land platted out as lots that was to become a development years ago) just outside of Ephrata WA called Rimrock Meadows that backs onto the coulee from that. It was very interesting to hike along it. If you own property there you can stay on it or you can use the campground for $10 to $15 a night. Nice campground with clubhouse and large pool. I don’t plan to develop my lots but I love to spend weeks there in the summer as I pass through the area.


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