Finally, more and more evidence pointed to the existence of a large lake in western Montana and Bretz’s theory began to fit together. About 12,000 to 20,000 years ago during the Ice Age, a lobe of an ice sheet moved south from Canada blocking the Clark Fork River near what is now Sandpoint, Idaho. Water backed up behind this ice dam creating Glacial Lake Missoula, a body of water larger than Lakes Erie and Ontario combined. Super-cooled water under tremendous pressure created friction and heat enough to destabilize and collapse the dam, releasing 2.5 trillion tons of water.
|Steamboat Rock in Grand Coulee|
The lake waters surged across eastern Washington with a force ten times greater than all the rivers of the world combined. Traveling at speeds of up to 80 mph, the flood waters scoured the top soil, carved canyons, blasted out deep potholes, created colossal waterfalls, carried boulders the size of houses, sliced through the Columbia Gorge, filled the Willamette Valley, and transformed the face of the Pacific Northwest forever.
The legacy of the floods (there were several such reoccurring events during the Ice Age) can be found throughout the area, but the sites in the scablands region are the most dramatic and provide a greater understanding of the power of the surging waters. Most locations are easily accessible by car and a good place to begin is by following the largest of all the flood channels, Grand Coulee. From Electric City, Highway 155 parallels the eastern side, running the full 50-mile length of this formidable canyon. Today, water from behind Grand Coulee Dam is pumped into the coulee to form Banks Lake, a holding place for irrigation water, but the immense size of this channel is still apparent. Some of the most interesting features along the way are Steamboat Rock, a monumental, flat island of layered basalt, and steep cliffs of columnar basalt on the east side of the canyon.
At the end of the Grand Coulee, just west of Coulee City on Highway 17, is Dry Falls. Situated in the middle of desert land, this barren 400-foot high cliff was once the site of a massive waterfall five times wider than and twice as high as Niagara Falls. The Interpretive Center here provides an instructive museum and a dramatic overlook of this geologic wonder.
Continuing north on Highway 17 and west on 172, the route crosses remnants of a glacier moraine and around the town of Mansfield, passes through fields strewn with gigantic boulders. These rocks, known as erratics, were carried here by the floods, often imbedded in glacial icebergs. When the ice melted, the rocks, originating as far away as Canada and weighing as much as 400 tons, remained scattered about the plateau. (There are also many erratics delivered by the floods in the Willamette Valley; the most famous being the Willamette Meteorite.)
Another waterfall, this one with actual water flowing over it, is Palouse Falls located 25 miles south of Washtucna on Highway 261. The Palouse River appears, seemingly from nowhere, and flows over a semicircle of volcanic rock down 200-feet into a deep plunge pool. From here, it travels through a steep-walled, rocky canyon another ten miles to the Snake River. The view of the falls from the canyon rim is nothing short of spectacular. Hiking trails lead down to the base of the falls and through the canyon, but only for the sure-footed.
While there are many more fascinating geologic sites in the region, these few provide good insight into the story of the Ice Age Floods and their role in shaping the Northwest.