|
Table 4-Natural watershed sediment losses in the USA teased on published literature.
<br />Location Watershed Conditions Sediment Loss Reference
<br />(t ac ~) (Mg ha ~)
<br />USA Geologic Erosion
<br />Natural Rate, Lower Limit
<br />Natural Rate, Upper Limit
<br />Eastern USA Forests, Lower Baseline
<br />Forests, Upper Baseline
<br />Western USA Forests, Lower Baseline
<br />Forests, Upper Baseline
<br />Sediment yields 1 year after prescribed borne and
<br />wildfires range from very low, in flat terrain and in the
<br />absence of major rainfall events, to extreme, in steep
<br />terrain affected by high intensity thunderstorms
<br />(table 5). Erosion on burned areas typically declines in
<br />subsequent years ae the site stabilizes, but the rate
<br />varies depending on burn or fire severity and vegeta-
<br />tion recovery. Soil erosion after fires can vary from
<br />under 0.4 to2.6taclyrl(O.1to6Mghalyrl}in
<br />prescribed burns and from 0.2 to over 49 t ac 1 yr 1
<br />(0.01 to over 110 Mg ha 1 yr 1) in wildfires (Megahan
<br />and Molitor 1975, Noble and Lundeen 1971, Robichaud
<br />and Brown 1999) (table 5). For example, Radek (1996)
<br />observed erosion of 0.1 to 0.8 t ac 1 {0.3 to 1.7 Mg ha 1)
<br />from several large wildfires that covered areas rang-
<br />ing from 375 to 4,370 ac (200 to 1,770 ha) in the
<br />northern Cascades mountains. Three years after these
<br />fire, large erosional events occurred from spring rain-
<br />storms, not from anowmelt. Moat of the sediment
<br />produced did not leave the burned area. Sartz (1953}
<br />reported an average soil loss of 1.5 in (37 mm) after a
<br />wildfire on anorth-facing elope in the Oregon Cascades.
<br />Raindrop splash and sheet erosion accounted for the
<br />measured soil lose. Annual precipitation was 42 in
<br />(1070 mm), with a maximum intensity of 3.5 in hr 1
<br />(90 mmhr-1). Vegetation covered the site within 1 year
<br />after the burn. Robichaud and Brown (1999) reported
<br />Fret-year erosion rates after a wildfire from 0.5 to
<br />1.1 t ac 1(1.1 to 2.5 Mg ha-1) decreasing by an order of
<br />magnitude by the second year, and to no sediment by
<br />the fourth, in an unmanaged forest stand in eastern
<br />Oregon. DeBano and others (1996) found that follow-
<br />ing awildfire in ponderosa pine, sediment yields
<br />from a low severity fire recovered to normal levels
<br />after 3 years, but moderate and severely burned
<br />watersheds took 7 and 14 years, respectively. Nearly
<br />all fires increase sediment yield, but wildfires in ateeg
<br />terrain produce the greatest amounts (12 to 165 t ae ,
<br />28 to 370 Mg ha 1) (table 6). Noble and Lundeen (1971)
<br />reported an average annual sediment production rate
<br />of 2.5 t ac 1(5.7 Mg ha 1) from a 900 ac (365 ha) burn
<br />on steep river breaklands in the South Fork of the
<br />Schumm and Harvey 1982
<br />0.3 0.6
<br />7 15
<br />0.05 0.1 Patric 1976
<br />0.1 0.2
<br />0.0004 0.001 8iswell and Schultz 1965
<br />2 6 DeByle and Packer 1972
<br />Salmon River, Idaho. This rate was approximately
<br />seven times greater than hillslope sediment yields
<br />from similar, unburned lands in the vicinity.
<br />Sediment Yield and Channel Stabllity-Fire-
<br />related sediment yields vary, depending on fire fre-
<br />quency, climate, vegetation, and geomorphic factors
<br />such as topography, geology, and soils (Swanson 1981).
<br />In some regions, over 60 percent of the total landscape
<br />sediment production over the long-term iafire-related.
<br />Much of that sediment loss can occur the first year
<br />after a wildfire (Agee 1993, DeBano and others 1996,
<br />DeBano and others 1998, Rice 1974, Robichaud and
<br />Brown 1999, Wohlgemuth and others 1998). Conse-
<br />quently, BAER treatments that have an impact the
<br />first year can be important in minimizing damage to
<br />both soil and watershed resources.
<br />After fires, suspended sediment concentrations in
<br />streamflow can increase due to the addition of ash and
<br />silt-to-clay sized soil particles in streamflow. High
<br />turbidity reduces municipal water quality and can
<br />adversely affect fish and other aquatic organisms. Itie
<br />often the moat easily visible water quality effect of
<br />fires (DeBano and others 1998). Leas is known about
<br />turbidity than sedimentation in general because it is
<br />difficult to measure, highly transient, and extremely
<br />variable.
<br />A stable stream channel reflects a dynamic equilib-
<br />rium between incoming and outgoing sediment and
<br />atreamflow (Roagen 1996). Increased erosion after
<br />fires can alter this equilibrium by transporting addi-
<br />tional sediment into channels (aegradation). How-
<br />ever, increased peakflowa that result from fires can
<br />also produce channel erosion (degradation). Sediment
<br />transported from burned areas as a result of increased
<br />peakflowa can adversely affect aquatic habitat, recre-
<br />ation areas, roads, buildings, bridges, and culverts.
<br />Deposition of sediments alters habitat and can fill in
<br />lakes and reservoirs (Rinne 1996, Reid 1993).
<br />Mass Wasting-Mesa wastingincludea slope creep,
<br />rotational slumps, debris flows and debris avalanches.
<br />Slope creep is usually not a major poatfire source of
<br />USDA Forest Service Gen. Tech. Rep. RMRS-GTR-63. 2000
<br />
|