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Sediment delivery after a wildfire

¹ Environmental Geology and Spatial Analysis Group, MS D452, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
² Water Stewardship Program, MS M992, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
³ USDA Forest Service, 475 20th Street, Suite B, Los Alamos, New Mexico 87544, USA
⁴ Geomatrix Consultants, Inc., 2101 Webster Street, 12th Floor, Oakland, California 94612, USA
⁵ Western Earth Surface Processes Team, 345 Middlefield Road, MS 973, U.S. Geological Survey, Menlo Park, California 94025, USA

We use a record of sedimentation in a small reservoir within the Cerro Grande burn area, New Mexico, to document postfire delivery of ash, other fine-grained sediment carried in suspension within floods, and coarse-grained sediment transported as bedload over a five-year period. Ash content of sediment layers is estimated using fallout ¹³⁷Cs as a tracer, and ash concentrations are shown to rapidly decrease through a series of moderate-intensity convective storms in the first rainy season after the fire. Over 90% of the ash was delivered to the reservoir in the first year, and ash concentrations in suspended sediment were negligible after the second year. Delivery of the remainder of the fine sediment also declined rapidly after the first year despite the occurrence of higher-intensity storms in the second year. Fine sediment loads after five years remained significantly above prefire 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 floods. Coarse sediment delivery in the first 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 fifth 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 fine-grained sediment therefore occurred soon after the fire, whereas the downstream impacts from coarse-grained sediment were attenuated by the more gradual process of bedload sediment transport.

Key Words: sediment load • sedimentation rates • erosion rates • fires • cesium • New Mexico