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

Abstract

The occurrence of two wildfi res separated by 31 yr in the chaparral-dominated Arroyo Seco watershed (293 km 2) 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/km 2 /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 datawithout including periodic perturbations from wildfi res-may grossly underestimate actual sediment yields.

Suspended sediment behavior in a coastal dry-summer subtropical catchment: Effects of hydrologic preconditions

“…The provenance of storm waters above the lower Salinas was not a significant factor in determining CSSf for low to moderate flows, which indicates that upper basin wash load signals attenuate before reaching the lower Salinas stations. This is counterintuitive, as the Arroyo Seco and the upper Salinas are on average very different subbasins in terms of relief, area, vegetation and soil characteristics, and fire regimes (Farnsworth and Milliman, 2003;Warrick et al, 2012); and one would expect wash load signatures to differ, as more moderate differences in subbasin characteristics have been shown to cause significant differences in suspended sediment behavior (e.g., Ankers et al, 2003).…”

“…Transition to a primarily gravel bed occurs high in the upper Salinas, below the mainstem dam, while the Arroyo Seco transitions to a sand bed just before its confluence with the Salinas. Sediment export from the Arroyo Seco has been shown to be dominated by the convergence of wildfire and subsequent large 9 u n c o r r e c t e d , a c c e p t e d precipitation events (Warrick et al, 2012). The flashy nature of discharge in the Arroyo Seco leads to large flows produced rapidly relative to the upper Salinas, which can lead to lower Salinas discharge events that are primarily expressions of Arroyo Seco runoff.…”

Remobilization of trace elements by forest fire in Patagonia, Chile

“…However, these contaminants are released in more labile forms when vegetation and soil organic matter are burned and volatilized by fire (Young and Jan 1977;Biswas et al 2007;Wiedinmyer and Friedli 2007). In addition, wildfires increase erosion rates by as much as two orders of magnitude, which subsequently increases the transport rates of remobilized contaminants to water bodies (Cohen et al 2005;Stein et al 2012;Warrick et al 2012). While the impacts of wildfires on Hg influxes to aquatic systems have been relatively well documented (e.g., Kelly et al 2006), there is much less information on other trace elements (e.g., Pb and Zn) mobilized by wildfires (Erel et al 1997;Flegal 2011, 2014).…”

Effects of antecedent hydrologic conditions, time dependence, and climate cycles on the suspended sediment load of the Salinas River, California

“…More recent studies have also incorporated temporal dependence in suspended sediment rating curves. Interannual to inter-decadal patterns of sediment behavior due to the effects of large flooding events (Kelsey, 1980;Klein and Anderson, 2012;Warrick et al 2013), wildfire (Shakesby and Doerr, 2006;Warrick et al, 2012) urbanization (Warick and Rubin, 2007) and combined land use changes (Pasternack et al, 2001) have all been shown to significantly affect decadal to inter-decadal scale suspended sediment flux.…”

“…Samples were collected by the authors as per Warrick et al (2012), with the following modifications. Samples were retrieved from the water surface at cross-channel stations of ~ onequarter, one-half, and three-quarters wetted channel width.…”