After prescribed burns at three locations and one wildfire, rainfall simulations studies were completed to compare postfire runoff rates and sediment yields on ash-cap soil in conifer forest regions of northern Idaho and western Montana. The measured fire effects were differentiated by burn severity (unburned, low, moderate, and high). Results indicate that this dry, undisturbed ash-cap soil exhibits high runoff rates and is naturally water repellent at the surface. However, the unburned, undisturbed ash-cap soil is not highly erodible due the protective duff layer on the surface. When ash-cap soil was exposed to prolonged soil heating (high severity burn), surface water repellency was destroyed and a strong water repellent layer occurred a few centimeters beneath the soil surface. With the simulated rainfall, the non-water repellent surface layer became saturated; thus making the soil above the water repellent layer highly erodible-especially during high intensity rainfall.