More than 15,000 years ago the largest and most powerful scientifically documented freshwater flood to occur on earth happened in the Pacific Northwest.[ http://foco.us/bDiE ] The floodwaters, moving up to 60 mph, created the largest waterfall to ever exist–Dry Falls almost 400 ft tall and taller than Niagra Falls.. Lake Lenore is a remnant of this flood. Nearby is Grand Coulee Dam.