Glacial Lake Missoula

It is almost unimaginable to think that long before what is now known as the city of Missoula, Montana, this valley sat deep under water.

Approximately 15,000 years ago, towards the end of the last ice age, a massive body of water was dammed up in the area where Missoula sits today. The lake was later identified as the water source of the catastrophic ice age floods that swept through the Pacific Northwest Region.

There is evidence to believe that a finger broke off from the Cordilleran Ice Sheet, floated down into Idaho, and formed an ice dam along the Clark Fork River. The ice dam then created the formation of the 2,000-foot deep lake now known as Glacial Lake Missoula. The lake grew to a combined size of Lake Erie and Lake Ontario.

The waters continued to rise and build up behind the ice dam, reaching a maximum height of 4,200 feet of elevation.

Eventually, the pressure on the dam became too powerful and the water began rupturing beneath the ice dam at extreme rates causing massive flooding. The floodwaters traveled through parts of eastern Washington and down the Columbia River Gorge at speeds between 30 and 50 mph. The floodwaters took a historic 430-mile journey before reaching the Pacific Ocean.

Glacial Lake Missoula and the ice age floods that drastically changed the landscape throughout western Montana and eastern Washington have been researched thoroughly since the 1920’s.

The Glacial Lake Missoula high water marker on Mount Jumbo.

The Glacial Lake Missoula high water marker on MountJumbo.

Geologist J. Harlen Bretz was the first to discover evidence of catastrophic flooding after conducting field research for seven years on the Columbia River Plateau. Bretz, along with another geologist, J.T. Pardee, spent another 30 years uncovering evidence of the floods, which led them to conclude Glacial Lake Missoula as the cause.

Geologists have estimated that Glacial Lake Missoula resided as a body of water for approximately 55 years and went through numerous cycles of flooding and reforming.

Visitors to Missoula can hike to specific sites on Mount Sentinel and Mount Jumbo known by locals as the Glacial Lake Missoula high water mark.

The Mount Sentinel high water mark can be accessed at the trailhead located off South Avenue and Maurice Avenue. Follow the Mount Sentinel Fire Road until reaching a sign pointing to Pengelly Ridge Trail. Along the Pengelly trail on Mount Sentinel, the high water mark is positioned at an evaluation of 4,200 feet to the west of the trail and symbolized with a beautifully engraved stone overlooking Missoula to the west.

The hike to the Mount Jumbo high water mark begins at the saddle trailhead just off Upper Lincoln Hills Drive in the Rattlesnake. Park your car at the trailhead on the west side of the road, cross the street, and follow the trail up through the forest. Once you reach the junction with the larger, flatter saddle trail, take a right and head south. You’ll wind up through the woods and find the stone high water marker near a west-facing vista on your right.

Read more about historic Glacial Lake Missoula on the The Ice Age Floods Institute and Montana Natural History Center.