Hydrogeologic response of small watersheds to wildfire
Following the Santa Barbara, CA. Painted Cave Fire of June 25, 1990 an emergency watershed protection plan was implemented consisting of stream clearing, grade stabilizers, debris basins and research focusing on streambed changes on two different branches of Maria ygnacio Creek, the main drainage of the burned area.
During the winter of 1990-1991, between 35 and 66 cm of rainfall and intensities up to 10 em per hour for a five minute period were recorded. During the winter of 1991-1992, between 48 and 74 cm of rainfall and intensities up to 8 cm per hour were recorded. Even though there was moderate rainfall on barren, saturated soils, no major debris flows occurred in burned areas. The winter of 1992-1993 recorded total precipitation of about 170% of normal. Intensities were relatively low and no debris flows were observed. The response to winter storms in the first three years following the fire was a spectacular flushing of sediment, most of which was derived from the hillslopes. The debris basins trapped 30,000 m3, the majority coming from the storm of March 17- 20, 1991. Sediment transported downstream during the three winters following the fire and not trapped in the debris basins was eventually flushed to the estuarine reaches of the creeks below the burn area, where approximately 108,000 m3 accumulated.
Changes in stream morphology following the fire were dramatic as pools filled with sediment which greatly smoothed longitudinal and cross sectional profiles. Major changes in channel morphology do occur following a fire as sediment derived from the hillslope is temporarily stored in channels within the burned area. However this sediment may quickly move downstream of the burned region, where it may accumulate reducing channel capacity and increasing the flood hazard.