HYDROLOGICAL RESPONSE OF SMALL WATERSHEDS FOLLOWING THE SOUTHERN CALIFORNIA PAINTED CAVE FIRE OF JUNE 1990

Following the Painted Cave Fire of 25 June 1990 in Santa Barbara, Cali fornia which burned 1214 ha, an emergency watershed protection plan wa s implemented consisting of stream clearing, grade stabilizers and con struction of debris basins. Research was initiated focusing on hydrolo gical response and channel morphology changes on two branches of Maria Ygnacio Creek, the main drainage of the burned area. Research results support the hypothesis that the response of small drainage basins in chaparral ecosystems to wildfire is complex and flushing of sediment b y fluvial processes is more likely than by high magnitude debris flows . During the winter of 1990-1991, 35-66 cm of rainfall and intensities up to 10 cm per hour for a five-minute period were recorded with a se asonal total of 100% of average (normal) rainfall (average = 63 cm/yea r). During the winter of 1991-1992, 48-74 cm of rainfall and intensiti es up to 8 cm per hour were recorded with a seasonal total of 115% of normal. Even though there was moderate rainfall on barren, saturated s oils, no major debris flows occurred in burned areas. The winter of 19 92-1993 recorded total precipitation of about 170% of normal, annual a verage intensities were relatively low and again no debris flows were observed. The response to winter storms in the first three years follo wing the fire was a moderate but spectacular flushing of sediment, mos t of which was derived from the hillslopes upstream of the debris basi ns. The first significant storm and stream flow of the 1990-1991 winte r was transport-limited resulting in large volumes of sediment being d eposited in the channel of Maria Ygnacio Creek; the second storm and s tream flow was sediment-limited and the channel scoured. Debris basins trapped about 23 000 m(3), the majority coming from the storm of 17-2 0 March 1991. Sediment transported downstream during the three winters following the fire and not trapped in the debris basins was eventuall y flushed to the estuarine reaches of the creeks below the burn area, where approximately 108 000 m(3) accumulated. Changes in stream morpho logy following the fire were dramatic as pools filled with sediment wh ich greatly smoothed longitudinal and cross-sectional profiles. Major changes in channel morphology occur following a fire as sediment deriv ed from the hillslope is temporarily stored in channels within the bur ned area. However, this sediment may quickly move downstream of the bu rned region, where it may accumulate reducing channel capacity and inc reasing the flood hazard. Ecological consequences of wildfire to the r iparian zone of streams in the chaparral environment are virtually unk nown, but must be significant as the majority of sediment (particularl y gravel necessary for fish and other aquatic organisms) entering the system does so in response to fires. (C) 1997 by John Wiley & Sons, Lt d.