Wildfires can produce significant changes in the hydrology of forest, woodland, and grassland watersheds. Next to the combustion of forest vegetation during a wildfire, the most destructive impact of a wildfire comes from post-fire flood peak flows. These flows can severely affect stream physical conditions, aquatic habitat, aquatic biota, cultural resources, and human health and safety. Flood flows in forest, woodland, and rangeland watersheds after wildfires can increase dramatically due to factors such as combustion of vegetation and forest floor cover, development of water repellant layers in the soil, and accelerated development of post-fire thunderstorms. Increases in storm flows of 1.5 to 2,300 times the measured pre-fire flood peaks, well beyond observed ranges of variability in managed watersheds, have been documented. These flows need to be understood in order to safely manage watershed and cultural resources, and human health and safety in a post-fire environment.