Under the summer sun, Mineral County can be hot and dry. The snow disappears from all except the loftiest peaks, and only the most moisture-laden storms bring rain to the ground. In times of drought, the lack of moisture is magnified, and mountain streams can be reduced to a trickle. Fifteen thousand years ago, the scene couldn’t have been more different. The Clark Fork River Valley was covered by a massive inland sea known as Glacial Lake Missoula. When the ice dams holding it back failed, floods of unthinkable proportions shot through the region.

Glacial Lake Missoula formed after a massive ice sheet advanced into the Idaho Panhandle, blocking the Clark Fork River drainage. The lake is believed to have reached depths of 2,000 feet and contained as much water as Lake Erie and Lake Ontario combined. Mountains became islands. Repeated ice dam failures sent floodwaters ripping down the valleys, perhaps in excess of 60 miles per hour, carrying a torrent of water and debris at remarkable force. These events left marks visible today in bench lines on mountainsides, giant sediment waves along prairies, narrow canyons entirely stripped of soil, and deep holes in the river bedrock.

In recent decades, local organizations based in the former lake bottom have investigated their Ice Age heritage and found remarkable places to observe the powerful effects of the lake and icy torrents. Take, for example, an overlook high above the town of St. Regis. Local Ice Age Flood expert Glenn Koepke calls it the “mixing bowl,” a place where water that had backed up in the various gulches was released by the floods only to collide “like the spokes on a wheel meeting in the center.” The massive flow created a swirling motion in the basin that is clearly visible thousands of years later. Then there’s Alberton Gorge and Cinderella Mountain, where the floodwaters sliced a gouge in the mountain’s face and sent massive boulders downstream to where the river widens. Indeed, as Koepke explained, “The signs of these Ice Age Floods are readily visible throughout Mineral County, even from I-90.”

For more information on Glacial Lake Missoula and the Ice Age Floods visit http://www.iceagefloodsinstitute.org/.

Credits and Sources:

Alt, Dave. “Glacial Lake Missoula.” Montana Magazine (Spring 1976): 35–37.

Devlin, Sherry. “Hard work is paying off for a group seeking to designate the flood route Glacial Lake Missoula took through the Northwest around 14,000 years ago.” Missoulian, August 26, 2004.

The Glacial Lake Missoula Chapter of the Ice Age Floods Institute. “Glacial Lake Missoula: Points of Interest in and around the Missoula Valley.” Interpretive Pamphlet, 2015.

Ice Age Floods Institute. “Ice Age Floods.” Interpretive Pamphlet, 2015.

Koepke, Glenn. “Signs of Ice Age Flood visible from I-90.” Poor Coop’s Almanac (2004).

Contemporary photographs courtesy Historical Research Associates, Inc.