A 3.5-mile long and 400-foot high scalloped precipice known as Dry Falls is found in central Washington, on the opposite side of the Upper Grand Coulee from the Columbia River. As you could guess from the name, Dry Falls no longer carries water, but in the same time it was once the largest waterfall in the world, being five times the width of the Niagara Falls and more than twice its height.
The story starts nearly twenty thousand years ago, when glaciers moved south through North America and an ice sheet dammed the Clark Fork River near Sandpoint, Idaho. As a result, a large portion of western Montana flooded, giving birth to the enormous Lake Missoula. The water covering three thousand square miles of northwest Montana was blocked behind this glacial dam. After some time, the rising water in Lake Missoula broke through the ice dam, forming a cataclysmic flood that spilled into Glacial Lake Columbia and then down the Grand Coulee.
The Missoula Flood covered the Idaho panhandle, the Spokane River Valley, a large part of eastern Washington and Oregon, flooding the entire area that is now the city of Portland.
When the ice sheet melted, the river returned to its normal course, leaving the Grand Coulee and the falls dry.