In Central Washington at the egress of the Grand Coulee on the Columbia River resides a three and a half mile crescent shaped precipice known as Dry Falls. Ten times the size of Niagara, Dry Falls is thought to be the greatest waterfall of all time. Geologists speculate that during the last ice age catastrophic flooding channeled water at 60 miles per hour through the Grand Coulee and over this 400-foot rock face. At this time, it is estimated that the flow of the falls was ten times greater than 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. Consequently most of Montana flooded forming the gigantic Lake Missoula. Eventually, enough pressure accumulated on the ice dam that it gave way.
This sudden release put most of Idaho, Washington, and Oregon under hundreds of feet of water in just a few days. This event, some consider to be one of the most extraordinary known to man, created the Grand Coulee and Dry Falls in one fell swoop. Similar glacial flooding, though not as impressive as the fore mentioned, kept the falls flowing for the several thousand years.
Once the ice sheets that obstructed the Columbia melted, the river returned to it normal course leaving the Grand Coulee and the falls desiccated. Today, this massive cliff can be viewed from the Dry Falls Interpretive Center, a state park located on highway 17 near the town of Grand Coulee. Admission is free.
Sources:
http://www.bpa.gov/Power/pl/columbia/4-geology.htm
http://www.nps.gov/laro/geology.htm
http://www.contemposcribe.com/cbcwash/centwashgeology.htm