USDA Forest Service
 

Pacific Southwest Research Station


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Pacific Southwest
Research Station

800 Buchanan Street
Albany, CA 94710-0011
(510) 883-8830
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Research Topics Fire Science

Post-fire erosion

Erosion following a postfire in-channel treatment.
An example of a failure of check dams with straw bales and wattles following an extreme rain event. U.S. Forest Service photo by Jan Beyers.

Following a fire, the loss of canopy and ground cover, along with changes in soil physical characteristics, leaves landscapes susceptible to the forces of erosion: gravity, running water, and wind. Fire increases erosion in the short term, but predicting the amount of post-fire erosion depends on a complex combination of site conditions, fire characteristics, and rainfall patterns.

Surface erosion actually begins while the fire is still burning, as soil and rock material are exposed. A pulse of "dry ravel," or flow of material, is greater in steep terrain and may continue for weeks after a fire. A second pulse of hillslope erosion begins with the start of the rainy season and the resulting increased runoff from bare hillsides. With little or no vegetation to intercept rainfall and transpire (take up water), more water is available for runoff. Changes in soil porosity (open spaces that hold water) – either by water repellency or surface sealing by fine sediments or ash – restrict soil’s ability to absorb water, promoting overland flow.
Post-fire hillslope erosion can be significantly greater than pre-burn rates. As vegetation regrows and easily mobilized surface material is depleted, hillslope erosion decreases and returns to pre-fire levels within 2-3 years in some environments.

Impacts on aquatic ecosystems

Eroded material from the hillsides initially fills adjacent stream channels. Sediment deposited in the stream channels fills pools, making streambeds smoother and less diverse for aquatic life.

Storms later in the season can cause post-fire debris flows, which are slurries of water and sediment that have substantial erosive power to scour stream channels and deposit sediment downstream. Debris flows form when sediment stored in the channel is mobilized by the increased flow of water from the bare hillsides.

Sediment generated from burned watersheds can exceed pre-fire levels by 35-50 times. Because of temporary storage of sediment in the stream channels and subsequent re-mobilization during major storms, sediment production from burned watersheds can remain elevated for up to 10 years after fire.

Publications and references:
  • Ahlgren, I.F.; Ahlgren, C.E. 1960. Ecological effects of forest fires. Botanical Review. 26: 483-533.
  • Anderson, H.W. 1949. Does burning increase surface runoff? Journal of Forestry. 47: 54-57.
  • Assouline, S.; Ben-Hur, M. 2006. Effects of rainfall intensity and slope gradient on the dynamics of interrill erosion during soil surface sealing. Catena. 66: 211-220.
  • DeBano, L.F. 1981. Water repellent soils: a state-of-the-art. Gen. Tech. Rep. PSW-46. Berkeley, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Forest and Range Experimental Station. 21 p.
  • Florsheim, J.L.; Keller, E.A.; Best, D.W. 1991. Fluvial sediment transport in response to moderate storm flows following a chaparral wildfire, Ventura County, southern California. Geological Society of America Bulletin. 103: 504-511.
  • Heede, B.H.; Harvey, M.D.; Laird, J.R. 1988. Sediment delivery linkages in a chaparral watershed following a wildfire. Environmental Management. 12: 349-358.
  • Keller, E.A.; Valentine, D.W.; Gibbs, D.R. 1997. Hydrological response of small watersheds following the southern California Painted Cave Fire of June 1990. Hydrological Processes. 11: 401-414.
  • Krammes, J.S. 1960. Erosion from mountain side slopes after fire in southern California. Research Note PSW-171. Berkeley, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Forest and Range Experiment Station.
  • Neary, D.G.; Ryan, K.C.; DeBano, L.F., eds. 2005. Wildland fire in ecosystems: effects of fire on soils and water. Gen. Tech. Rep. RMRS-GTR-42-vol.4. Ogden, UT: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 250 p.
  • Rice, R.M. 1974. The hydrology of chaparral watersheds. In: Rosenthal, M., ed. Proceedings of a symposium on living with the chaparral. San Francisco, CA: Sierra Club: 27-34.
  • Rowe, P.B.; Countryman, C.M.; Storey, H.C. 1954. Hydrologic analysis used to determine the effects of fire on peak discharge and erosion rates in southern California watersheds. U.S. Department of Agriuculture, Forest Service, California Forest and Range Experiment Station. 49 p.
  • Wells, W.G., II. 1981. Some effects of brushfires on erosion processes in coastal southern California. In: Proceedings of a symposium on erosion and sediment transport in Pacific Rim steeplands. Publication No. 132. Christchurch, New Zealand: International Association of Hydrological Sciences: 305-342.
  • Wells, W.G., II. 1987. The effects of fire on the generation of debris flows in southern California. Geological Society of America, Reviews in Engineering Geology. 7: 105-114.
  • Wohlgemuth, P.M.; Beyers, J.L.; Wakeman, C.D.; Conard. S.G. 1998. Effects of fire and grass seeding on soil erosion in southern California chaparral. In: Gray, S., ed. Proceedings of the 19th forest vegetation management conference. Redding, CA: 41-51.
  • Wohlgemuth, P.M.; Hubbert, K.R.; Arbaugh, M.J. 2006. Fire and physical environment interactions. In: Sugihara, N.G., Van Wagtendonk, J.W., Shaffer, K.E., Fites-Kaufman, J., and Thode, A.E., eds. Fire in California’s Ecosystems. Berkeley, CA: University of California Press: 75-93. Chapter 6.
Last Modified: Dec 19, 2016 03:03:47 PM