GEOMORPHIC CONSEQUENCES OF WILDFIRE

Wildfires increase the erodibility of soils but decrease the infiltration of rainfall into the soil. When high intensity rainfall follows wildfire the changes in erodibility and infiltration produce erosional and depositional features that may alter the trajectory of landscape evolution on geological time scales. The length of time that these wildfire-related features affect landscape evolution depends upon the magnitude of the wildfire and the climate. These features probably persist longer in arid and semi-arid climates than in humid climates. This type of feature may persist throughout the world from past geological periods. Soil erodibility can be defined by the critical shear stress require to initiate the significant entrainment of sediment. This stress is greater for cohesive unburned forest soils held together by organic matter than for burned forest soils in which the organic matter has been consumed on ignition. For example, hillslope erosion (kg/m of hillslope per mm of rain) after the 1996 Buffalo Creek Fire in steep mountainous topography of Colorado was about 100 times greater on burned hillslopes where runoff was greater and erodibility was less than on unburned hillslopes. Hillslope erosion coupled with channel erosion produced sediment during the first summer after this fire. Four years after the wildfire, the sediment transport rates out of the watershed had decreased significantly and approximately 74% of the initial sediment remained within the burned watersheds as depositional features with an estimated residence time of 240 years. Estimates of the mean wildfire recurrence interval in this system range from 20 to 50 years. Thus, depositional and erosional features may persist and become initial conditions within the landscape affecting subsequent sediment transport in response to the next wildfire disturbance. By examining combinations of wildfire recurrence intervals, soil erodibility, topographic slope, and the rainfall intensity for the western United States, maps were produced to estimate areas where wildfire-related geomorphic features may exist in the Holocene record. Evidence exists that wildfires also occurred during the Carboniferous and Cretaceous periods; these wildfires may have created similar geomorphic features that have also influenced landscape evolution over geological time.