Sediment delivery after a wildfire

Abstract

We use a record of sedimentation in a small reservoir within the Cerro Grande burn area, New Mexico, to document postfi re delivery of ash, other fi ne-grained sediment carried in suspension within fl oods, and coarse-grained sediment transported as bedload over a fi ve-year period. Ash content of sediment layers is estimated using fallout 137 Cs as a tracer, and ash concentrations are shown to rapidly decrease through a series of moderate-intensity convective storms in the fi rst rainy season after the fi re. Over 90% of the ash was delivered to the reservoir in the fi rst year, and ash concentrations in suspended sediment were negligible after the second year. Delivery of the remainder of the fi ne sediment also declined rapidly after the fi rst year despite the occurrence of higher-intensity storms in the second year. Fine sediment loads after fi ve years remained signifi cantly above prefi re averages. Deposition of coarse-grained sediment was irregular in time and was associated with transport by snowmelt runoff of sediment stored along the upstream channel during short-duration summer fl oods. Coarse sediment delivery in the fi rst four years was strongly correlated with snowmelt volume, suggesting a transport-limited system with abundant available sediment. Transport rates of coarse sediment declined in the fi fth year, consistent with a transition to a more stable channel as the accessible sediment supply was depleted and the channel bed coarsened. Maximum impacts from ash and other fi negrained sediment therefore occurred soon after the fi re, whereas the downstream impacts from coarse-grained sediment were attenuated by the more gradual process of bedload sediment transport.

Stream exports of coarse matter and phosphorus following wildfire in NE Victoria, Australia

“…Some estimates have been made by measuring the rate of sedimentation in water impoundments. These studies have been conducted in pine plantations in South Africa (Scott and Van Wyk, 1990), and coniferous forests in Colorado (Moody and Martin, 2001a,b) and New Mexico (Lavine et al, 2006;Reneau et al, 2007). During the first year after fire, Scott et al (1998) and Scott and Van Wyk (1990) reported a fourfold increase in bedload export, with 30% of the total sediment exported being bedload.…”

“…A large increase in P concentration was measured in North Carolinian river when increased flow produced turbulence, which resuspended stream bed material (Keup, 1968). As residence timing of coarse material in streams exceeds that of suspended matter (Reneau et al, 2007), increased P concentrations have the potential to be more sustained, extending the impact of wildfire.…”

Determining the effects of wildfire on sediment sources using 137Cs and unsupported 210Pb: the role of landscape disturbances and driving forces

“…To the best of our knowledge, this is the first study of its kind in North America, although FRNs have been used qualitatively in New Mexico, USA to investigate the effect of wildfires on ash redistribution and FRN depth profiles in soils (e.g. Johansen et al 2003;Reneau et al 2007). Recent Australian studies (e.g.…”

Soil-water dynamics and unsaturated storage during snowmelt following wildfire

“…Much of the snowmelt-derived sediment transport increases are the result of the sustained (i.e. 1 to 4 month) delivery of water to channels, which leads to transport of coarse-grained sediment as bedload (Moody and Martin, 2001;Malmon et al, 2004;Reneau et al, 2007). Elevated streamflow from snowmelt in burned areas can also impact stream channels by increasing nutrient export, removing periphyton, and altering benthic macroinvertebrate assemblages (Minshall et al, 2001).…”