ABSTRACT:
Annual sediment yields increased as much as 180 times above pre-fire levels after wildfire destroyed all vegetation on three forested watersheds in the Entiat Experimental Forest in the eastern Cascade Range of Washington. Sediment was transported in debris torrents, in suspension, and as bedload. Suspended sediment concentration correlated well with turbidity. Total N losses by erosion processes increased from a pre-fire average of 0.004 kg/ha/yr to 0.16 kg/ha/yr. Available P losses increased from 0.001 kg/ha/yr before the fire to 0.014 kg/ha/yr. The combined erosion loss of Ca, Mg, K, and Na increased from an average of 1.98 kg/ha/yr before the fire to 54.3 kg/ha/yr. Greatest nutrient losses occurred with mass soil movements (debris torrents). Material deposited in alluvial fans represented losses of 13.5 kg/ha of total N, 3.4 kg/ha available P, and 3,850 kg/ha of Ca, Mg, K, and Na combined. An unmeasured but certainly large quantity of soil and rock entered the river during the debris flows. Nutrient losses on eroded soil, although greater than solution losses, were insignificant to site productivity and stability compared with the physical effects of channel scouring associated with greater runoff, higher peak flows, and debris torrents following fire.
Footnotes
J. D. Helvey is the project leader for the Timber and Watershed Laboratory, Forest Service, U.S. Department of Agriculture, Parsons, West Virginia 26287. A. R. Tiedemann is a research range scientist and T. D. Anderson is a chemist with the Pacific Northwest Forest and Range Experiment Station, Forest Service, USDA, at La Grande, Oregon 97850, and Wenatchee, Washington 98801, respectively.
- Copyright 1985 by the Soil and Water Conservation Society
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