The big glaciers of the last ice age covered all the northern fringe of Washington and Idaho.

In Washington, these glaciers advanced down the western part of the Okanogan Valley about as far south as Chelan. In Idaho, a finger of the ice sheet came down to Sandpoint, where it blocked the mouth of the Clark-Fork River. In so doing, an ice dam almost one-half mile high was created.

This ice dam resulted in the creation of Lake Missoula, which was 2,000 feet deep and stretched eastward some 200 miles into Montana. Approximately the size of Lake Ontario and Lake Erie combined, the dam holding this vast inland sea first burst 15,000 years ago. Torrents of ice- and dirt- filled water rushed down the Columbia River toward the Pacific Ocean. The floods cut deep canyons, also called coulees, into the underlying bedrock. Over approximately 2,500 years, the cycle of nature creating an ice dam, forming of a lake behind the dam, the dam failing, and catastrophic flooding would repeat itself up to seventy times.

The floods carved out cubic mile after cubic mile of earth, forming deep channels and digging out riverless canyons such as Dry Coulee, Spring Coulee and Moses Coulee in a matter of days. And the floods widened and deepened the Grand Coulee to a depth of 900 feet.

These catastrophic floods and the formation of Glacial Lake Missoula are referred to as the Ice Age Floods.