In the middle of eastern Washington, in a desert that gets less than eight inches of rain a year, stands what was once the largest waterfall in the world. It is three miles wide and 400 feet high—ten times the size of Niagara Falls—with plunge pools at its base suggesting the erosive power of an immense flow of water. Today there is not so much as a trickle running over the cataract’s lip... Dry Falls is not the only curiosity in what geologists call the Columbia Plateau. Spread over 16,000 square miles are hundreds of other dry waterfalls, canyons without...