Raging Ice Age floodwaters carved spectacular features throughout eastern Washington, creating unique landscapes. The powerful floods cut steep-walled canyons called coulees, including Frenchman Coulee, Moses Coulee, Potholes Coulee, and the largest, Grand Coulee. Rocks, plants, and landscape features provide clues about the Ice Age floods and other stories of the area’s past. Small lakes called plunge pools at the base of cliffs are deep depressions scoured out by water tumbling over a cliff, and today these lakes and ponds are fed by groundwater. Grand Coulee is up to 800 feet deep and more than one mile wide in some places. Ice Age floods also carved Steamboat Rock, a streamlined basalt mesa in Grand Coulee about 30 miles north of here.