The effects of wildfire on the sediment yield of a coastal California watershed

TitleThe effects of wildfire on the sediment yield of a coastal California watershed
Publication TypeJournal Article
Year of Publication2012
AuthorsWarrick J.A, Hatten J.A, Pasternack G.B, Gray A.B, Goni M.A, Wheatcroft R.A
JournalBulletin of the Geological Society of America
Pagination1130 - 1146
Date Published2012///
ISBN Number0016-7606

The occurrence of two wildfi res separated by 31 yr in the chaparral-dominated Arroyo Seco watershed (293 km2) of California provides a unique opportunity to evaluate the effects of wildfi re on suspended-sediment yield. Here, we compile discharge and suspended- sediment sampling data from before and after the fi res and show that the effects of the postfi re responses differed markedly. The 1977 Marble Cone wildfi re was followed by an exceptionally wet winter, which resulted in concentrations and fl uxes of both fi ne and coarse suspended sediment that were ~35 times greater than average (sediment yield during the 1978 water year was 11,000 t/km2/yr). We suggest that the combined 1977–1978 fi re and fl ood had a recurrence interval of greater than 1000 yr. In contrast, the 2008 Basin Complex wildfi re was followed by a drier than normal year, and although suspended-sediment fl uxes and concentrations were signifi cantly elevated compared to those expected for unburned conditions, the sediment yield during the 2009 water year was less than 1% of the post–Marble Cone wildfi re yield. After the fi rst postfi re winters, sediment concentrations and yield decreased with time toward prefi re relationships and continued to have signifi cant rainfall dependence. We hypothesize that the differences in sediment yield were related to precipitationenhanced hillslope erosion processes, such as rilling and mass movements. The millennialscale effects of wildfi re on sediment yield were explored further using Monte Carlo simulations, and these analyses suggest that infrequent wildfi res followed by fl oods increase long-term suspended-sediment fl uxes markedly. Thus, we suggest that the current approach of estimating sediment yield from sediment rating curves and discharge data— without including periodic perturbations from wildfi res—may grossly underestimate actual sediment yields.

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