ICE AGE FLOODS IN WASHINGTON
Missoula Floods
During the last ice age, between 15,000 and 13,000 radiocarbon years ago, the Purcell Trench Lobe of the Cordilleran Ice Sheet formed an ice dam in the Clark Fork valley, blocking the Clark Fork River and creating Glacial Lake Missoula. At its maximum, Glacial Lake Missoula stretched for over 200 miles from the current-day Idaho-Montana border to the southeast beyond present-day Missoula. The lake was greater than 2,000 feet deep. Over the course of 2,000 years, the ice dam of Glacial Lake Missoula failed repeatedly, rapidly emptying the contents of the lake.
The floods swept across the Spokane Valley in eastern Washington, spilling into Glacial Lake Columbia. After overflowing the shores of Glacial Lake Columbia, the floods spread out across the landscape and tore a myriad of channels across much of eastern Washington, creating the Channeled Scablands. The floodwaters then flowed across the Quincy and Pasco basins before damming up temporarily behind Wallula Gap, a topographic constriction at the border of modern-day Washington and Oregon. The dammed water formed Lake Lewis, which stretched from present-day Yakima to Walla Walla. As the waters flowed past Wallula Gap they dammed up behind the narrow Columbia River Gorge, forming Lake Condon. Westward of the gorge, the floods filled the Portland basin and began spilling into the Willamette Valley.
In their final phase, the floods filled the Willamette Valley, forming Lake Allison and flowed along the Columbia River corridor before reaching the Pacific Ocean. Ocean levels were much lower during this time period and the location of the ice-age coastline is shown on the map.
Channeled Scablands
Flood features are so huge, they can be seen from space.
The bedrock in the Channeled Scablands is made up of lava flows from the Columbia River Basalt Group (see the graphic above). These flood basalts cover an area of over 87,000 square miles. In some places their total thickness is estimated to be 16,000 feet (Citation 5). In the scablands the lava has been scoured and exposed along flood channels.
During the last ice age, the Okanogan Lobe of the Cordilleran Ice sheet blocked the Columbia River, forming Glacial Lake Columbia. During the greatest extent of the ice sheet, the outflow from Glacial Lake Columbia flowed through the Grand Coulee. The Missoula floods were also diverted through the same coulee, further eroding the massive landform.
Today, the upper section of the Grand Coulee holds Banks Lake, a reservoir that stores irrigation water pumped from the Grand Coulee Dam. The lower Grand Coulee includes Dry Falls, where the ice-age floodwaters formed a 3.5-mile wide and 400-foot tall waterfall. These cliffs are now part of Sun-Lakes-Dry Falls State Park and offer one of the most impressive vistas of ice-age floods evidence.
From Washington State Parks