The waters of Lake Roosevelt fill an immense valley and gorge.

During the ice age floods, glaciers from the north descended south and created large valleys. Huge lakes extending well into Montana formed behind the ice. When the ice dams collapsed, there were torrents of flooding that have no parallel in size and scope today. These floods further shaped the gorge. Indeed, vertical walls more than 800 feet high were created.

Beyond the ice age floods, the physical characteristics of the lands reflect the crossroads of three distinct geographic provinces: the Okanogan Highlands, the Kootenay Arc, and the Columbia Plateau. As described by the Washington State Department of Natural Resources:

The Okanogan Highlands province is situated east of the Cascade Range and north of the Columbia Basin. To the east and north, the highlands extend into northern Idaho and southern British Columbia, respectively. They are characterized by rounded mountains with elevations up to 8,000 feet above sea level and deep, narrow valleys. The Columbia River divides the Okanogan Highlands into two geographic regions: to the east of the river are the Selkirk, Chewelah, and Huckleberry Mountains; to the west are the Kettle, Sanpoil, and other mountains.

The eastern portion of the Okanogan Highlands contains the oldest sedimentary and metamorphic rocks of the state. Precambrian Belt Supergroup, Windermere Group, and Deer Trail Group metasedimentary rocks extend from British Columbia south to the Columbia River. The nation's second largest magnesium operations are located near Addy, in Stevens County.

All Paleozoic and some younger rocks have been repeatedly folded into a northeast-trending regional structure called the Kootenay Arc, which extends northeastward for 150 miles into British Columbia and contains numerous lead-zinc mines. In Stevens County, Cambrian Addy Formation quartzite is mined for its silicon content, and near Northport, Devonian argillite includes interbedded barite.

When the Columbia reaches the confluence of the Spokane River, it turns sharply west. This is where it meets the flood basalts of the Columbia Plateau. As the river moves toward the dam, the scablands of the Columbia Plateau dominate. The geology in this area is also interesting because on one side of the river you can see where the basalts were deposited on top of the granites and then later eroded away by the Ice Age floods.