Networks of valleys with steep, amphitheater-shaped headwalls are prominent features on the surfaces of Earth and Mars. These landforms are commonly used as diagnostic indicators of undermining and headwall retreat by groundwater-seepage erosion. In this presentation, I question the link between seepage erosion and canyon form, and present an alternate hypothesis for canyon formation: waterfall retreat during large-scale flooding. To support this hypothesis I will discuss three interrelated studies and some ongoing work. First I investigated several canyons in Idaho, which have long been thought to be formed by groundwater sapping because they contain some of the largest springs in the United States. 4He and 14C dates, plunge pools, and sediment transport modeling indicate, however, that these canyons most likely formed during a catastrophic flood about 48,000 years ago. Second, Canyon Lake Gorge, Texas, was examined as a rare modern example of bedrock canyon formation during a single flood event. Results show that the combination of well jointed rock and high flow discharges caused block plucking, waterfall formation, and a rate of erosion that was limited only by the ability of the flow to transport sediment. Finally, theoretical modeling and flume experiments are used to show that steep headwalls can persist during canyon formation due to waterfall-induced toppling in fractured rock.