This is the head of the Lower Grand Coulee where a three and a half mile-long precipice known as Dry Falls exists. Ten times the size of Niagara, Dry Falls is thought to be the greatest known waterfall that ever existed. Geologists speculate that during the last ice age catastrophic flooding channeled water at 65 miles per hour through the Upper Grand Coulee and over this 400-foot (120 m) rock face. At this time, it is estimated that the flow of the falls was ten times the current flow of all the rivers in the world combined.
Nearly twenty thousand years ago, as glaciers moved south, one ice sheet plugged the Clark Fork of the Columbia River, which kept water from being drained from Montana. Eventually, enough pressure accumulated on the ice dam that it gave way.
This sudden release put parts of Idaho, Washington, and Oregon under hundreds of feet of water in just a few days. These extraordinary floods created the Grand Coulee and Dry Falls in a short period. Similar glacial flooding on a smaller scale kept the falls flowing for several thousand years.
Once the ice sheets that obstructed the Columbia melted, the river returned to its normal course leaving the Grand Coulee and the falls as it exists today.