The Effects of Wildfire on the Sediment Yield of a Coastal California Watershed
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The effects of wildfi re on the sediment yield of a coastal California watershed J.A. Warrick1,†, J.A. Hatten2, G.B. Pasternack3, A.B. Gray3, M.A. Goni4, and R.A. Wheatcroft4 1U.S. Geological Survey, Pacifi c Coastal and Marine Science Center, Santa Cruz, California 95060, USA 2College of Forest Resources, Mississippi State University, Mississippi State, Mississippi 39762, USA 3Department of Land, Air and Water Resources, University of California, Davis, California 95616, USA 4College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331, USA ABSTRACT approach of estimating sediment yield from sediment discharge and sedimentation in sediment rating curves and discharge data— downstream channels, reservoirs, and coastal The occurrence of two wildfi res separated without including periodic perturbations landforms, which can alter landform morphol- by 31 yr in the chaparral-dominated Arroyo from wildfi res—may grossly underestimate ogy and aquatic habitats (e.g., Florsheim et al., Seco watershed (293 km2) of California pro- actual sediment yields. 1991; Reneau et al., 2007; Malmon et al., 2007; vides a unique opportunity to evaluate the Warrick et al., 2008). Postfi re erosion also re- effects of wildfi re on suspended-sediment INTRODUCTION sults in increased export of carbon and nutrients yield. Here, we compile discharge and sus- from burned watersheds, which can infl uence pended-sediment sampling data from before Wildfi re alters the physical conditions of veg- rates of primary production and carbon preser- and after the fi res and show that the effects of etation and soil, and these changes can modify vation in depositional settings (Johnson et al., the postfi re responses differed markedly. The the hydrologic and geomorphic processes within 2004; Murphy et al., 2006; Hunsinger et al., 1977 Marble Cone wildfi re was followed by the burned landscape (Shakesby and Doerr, 2008). Because of the rates and patterns of ero- an exceptionally wet winter, which resulted 2006). There are two primary hydrogeomorphic sion following a fi re, both hillslope morphology in concentrations and fl uxes of both fi ne effects of wildfi re: (1) an increase in runoff, pri- and sedimentary deposits within the geologic and coarse suspended sediment that were marily through increased overland fl ow from the record will be infl uenced by periodic wild- ~35 times greater than average (sediment combined effects of reduced water infi ltration fi re (Meyer et al., 1995; Mensing et al., 1999; yield during the 1978 water year was 11,000 through soil hydrophobic layers, reduced sur- Pierce et al., 2004; Roering and Gerber, 2005; t/km2/yr). We suggest that the combined face roughness, and a reduction in evapotranspi- Shakesby and Doerr, 2006). 1977–1978 fi re and fl ood had a recurrence in- ration (DeBano and Krammes, 1966; Swanson, The rate of erosion following a wildfi re can terval of greater than 1000 yr. In contrast, the 1981; Brown, 1972; Rice, 1974; DeBano, 2000; vary widely, and differences have been at- 2008 Basin Complex wildfi re was followed Doerr et al., 2000; Martin and Moody, 2001; tributed to prefi re vegetation, landscape slope, by a drier than normal year, and although Neary et al., 2005), and (2) an increase in ero- wildfi re burn intensity, postfi re soil conditions, suspended-sediment fl uxes and concentra- sion through several mechanisms including dry and precipitation rates (Shakesby and Doerr, tions were signifi cantly elevated compared to ravel, rain splash erosion and transport, rilling 2006; Malmon et al., 2007). The prefi re vegeta- those expected for unburned conditions, the resulting from surface-water fl ow, and mass tion types and conditions will have important sediment yield during the 2009 water year movements (Osborn et al., 1964; Wells, 1981; infl uences on the burn intensity and postfi re was less than 1% of the post–Marble Cone Scott and Williams, 1978; Scott and Van Wyk, soil hydrophobicity. For example, combustion wildfi re yield. After the fi rst postfi re winters, 1990; Inbar et al., 1998; Moody et al., 2005; of fi re-prone chaparral produces marked water sediment concentrations and yield decreased Shakesby and Doerr, 2006). Although these two repellency in soils because of the low vegetation with time toward prefi re relationships and effects can be pronounced—runoff and erosion height and high burn temperatures (Rice, 1982; continued to have signifi cant rainfall depen- can increase by up to several orders of magni- Wells, 1981). dence. We hypothesize that the differences in tude following wildfi re—they commonly last Vegetation also inhibits downslope sediment sediment yield were related to precipitation- only 3–8 yr and decay quickly over this time transport during the years before a wildfi re, enhanced hillslope erosion processes, such as (Rowe et al., 1954; LACFCD, 1959; Swanson, resulting in hillslope storage of sediment (Fig. rilling and mass movements. The millennial- 1981; Brown et al., 1982; Cerdà, 1998; Cerdà 1A; Rice, 1982; Florsheim et al., 1991). After a scale effects of wildfi re on sediment yield and Lasanta, 2005; Reneau et al., 2007; War- wildfi re, this hillslope sediment will be released were explored further using Monte Carlo rick and Rubin, 2007). Even so, wildfi re will downslope as dry ravel on slopes greater than simulations, and these analyses suggest that increase long-term erosion rates from the land- a critical angle of repose (Fig. 1B). Dry ravel infrequent wildfi res followed by fl oods in- scape if the effects are marked and fi re recur- is recognized as an important postfi re sediment crease long-term suspended-sediment fl uxes rence is suffi ciently frequent (Swanson, 1981; transport process in both wet and dry climates markedly. Thus, we suggest that the current Lavé and Burbank, 2004). (e.g., Florsheim et al., 1991; Roering and Ger- Increased runoff and erosion from burned ber, 2005), and Wells (1981) reported that an- †E-mail: [email protected] landscapes often cause increased suspended- nual net dry ravel transport rates increased GSA Bulletin; July/August 2012; v. 124; no. 7/8; p. 1130–1146; doi: 10.1130/B30451.1; 16 fi gures; 3 tables; Data Repository item 2012140. 1130 For permission to copy, contact [email protected] © 2012 Geological Society of America Downloaded from https://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/124/7-8/1130/3406643/1130.pdf by California State Univ user on 22 January 2020 The effects of wildfi re on the sediment yield of a coastal California watershed an ideal opportunity to evaluate the effects of A Prefire B Postfire C1 Light rainfall wildfi re on hydrologic fl uxes from a water- shed. This is largely due to a river gauging and sampling program by the U.S. Geological Erosion of Dry ravel dry ravel and Survey (USGS) that provided discharge and Chaparral Barren loose soil suspended-sediment data, which we supple- C2 Heavy rainfall mented with additional suspended-sediment Veg Soil bic sampling from 2008 to 2010. Using these data, we investigated whether the wildfi res produced Dry ravel Hydropholayer signifi cant changes in water and suspended- 1 m 1 m Rills, gullies, debris flows sediment discharge rates. Our primary goals were to: (1) characterize the postfi re changes Figure 1. Illustration of the effects of wildfi re on sediment yield from a steep, chaparral in water and sediment yields, (2) use the two landscape. (A) Before a wildfi re, the dense chaparral vegetation (veg) and organic debris wildfi res and postfi re hydrologic conditions retain sediment that had been mobilized downslope by diffusive processes. (B) During and to compare and contrast postfi re effects, and immediately after a wildfi re, the combustion of vegetation and organic debris above the (3) use these data to provide insights into long- ground reduces surface roughness and releases retained soil as dry ravel, which accumu- term (millennial) dynamics of watershed-scale lates as talus in colluvial hollows, hillslope toes, and stream channels. The high tempera- denudation. ture of chaparral fi re also creates a hydrophobic layer beneath the soil surface. (C1) and (C2) Sediment erosion and transport processes during postfi re rainfall are highly dependent STUDY SITE upon rainfall intensity. Whereas light rainfall will result in the erosion of loose soil and dry ravel talus, heavy rainfall will generate overland fl ow at rates that can cut rills and gullies The Arroyo Seco watershed is a steep, into the soil and potentially generate debris fl ows. 790 km2 basin within the second largest water- shed of California’s coastal ranges, the Salinas River (11,000 km2). Here, we focus on the ~30-fold during the fi rst year following wild- 1998; Moody and Martin, 2001), the results of upper Arroyo Seco watershed that drains into fi res in southern California chaparral. which cannot be scaled directly to watersheds USGS gauging station 11151870 (site 1A Once the postfi re sediment supply has in- (Shakesby and Doerr, 2006; Walling, 2006). in Fig. 2; Table 1) and has a drainage area of creased by dry ravel, the fate of this sediment Perhaps the largest drainage basin with exten- 293 km2. The Arroyo Seco drains the steep and further erosion of hillslope soils will de- sive pre- and postfi re sampling of suspended- Santa Lucia Range (maximum elevation 1784 m), pend largely on the timing and intensity of rain- sediment discharge is the ephemeral mountain which trends southeast from Monterey Bay to fall (LACFCD, 1959; Keller et al., 1997; Lavé stream draining a 7 km2 burn sampled by Mal- San Luis Obispo and forms the rugged Big Sur and Burbank, 2004; Shakesby and Doerr, 2006; mon et al. (2007), in which suspended-sediment coastal setting. The Santa Lucia Range is part Malmon et al., 2007).