Sediment delivery after a wildfire

Sediment delivery after a wildfire

28 January 2007

published by

Geological Society of America  — Wildfires have become increasingly common and destructive in the American West, aided by the buildup of fuel associated with land-use changes and possibly by global climate changes.

Impacts to society have also intensified as the urban-wildland interface encroaches on fire-prone terrain. Reneau et al. document five years of post-fire sediment delivery to a small reservoir near Los Alamos, New Mexico, providing a relatively long and detailed record of erosion and sediment transport from a watershed that was burned in the May 2000 Cerro Grande fire. The transport of ash from burned slopes can have particularly negative effects on aquatic communities and water quality, and this study is the first to estimate ash content in post-fire sediment, using the fallout radionuclide 137Cs as a tracer. 

The watershed experienced major and rapid geomorphic responses to the fire, and erosion rates in the first year increased about 500-fold compared to pre-fire conditions. Maximum impacts from ash and other fine-grained sediment carried in suspension in floods occurred soon after the fire, declining rapidly after the first year. 

In contrast, coarse-grained sediment transported as bedload was first deposited along the channel close to the source in short-duration summer floods and subsequently remobilized by longer-duration snowmelt runoff. Substantial transport of coarse-grained sediment was still occurring five years after the fire, although at decreasing rates as the channelstabilized.

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