D--048966 - CALFED Bay-Delta Program - State of California

895
Hydrology and Water Resources
and it is decreased in two ways. Removal of vegetation and Nutrient Yield
the litter layer exposes bare mineral soil to raindrop impacts,
which can physically force the soil particles closer together Fires provide an opportunity for nutrients that have been
and disperse soil aggregates into surface pores, thereby re- stored in vegetation and soils to move into streams. Materiducing
the infiltration capacity. Fires also vaporize organic als that are not volatilized and lost to the atmosphere are left
compounds in the litter layer, some of which move into the in ash on and near the soil surface in forms that are readily
soil until the vapor condenses and forms a layer that is water mobile. A variety of studies throughout the West have demrepellent,
or hydrophobic (De Bano 1981). These layers tend onstrated that concentrations of nitrates and other ions in
to be more coherent in coarse-textured soils (e.g., decomposed streams usually increase dramatically after fires (Tiedemann
granitics), under very hot fires, and where a thick litter layer et al. 1979). However, the background concentrations of these
and/or organic horizon was present (De Bano 1981; Poff constituents in streams draining healthy forests are typically
1989b). The continuity of such layers, which may be a func- so low that the relative increases following fires appear to be
tion of fire intensity and litter distribution, determines their huge even though the absolute amounts often remain almost
overall impact on hill-slope water movement. Additionally, negligible or at least below water quality standards. Neverlarger
macropores from roots and animals allow some water theless, there is potential for a nutrient flush to dramatically
movement through the hydrophobic layers (Booker et al. increase algae in streams, which can have additional conse-
1993). Although the water-repellent layers tend to break down quences. There is also the potential for large nutrient losses
within a year or two, those formed in soils that are somewhat associated with physical erosion of soil particles that often
hydrophobic even without burning may be more persistent carry nutrients with them (Tiedemann et al. 1979). A study of
(Poff 1989b). Under some conditions, a hydrophobic layer the chemistry of Sagehen Creek north of Truckee following
forms on the surface of the soil and acts as a binder and seal- the Donner Burn in 1960 did not detect any change in the
ant, maximizing overland flow while minimizing erosion (see ionic composition of the stream relating to the fire, which did
Poff 1996). As usual, there is a lack of measured hydrologic not burn the riparian zone (Johnson and Needham 1966). The
response to fire in the Sierra Nevada. A variety of studies else- inevitable fires in urban intermix zones have the potential to
where in the western United States have demonstrated dra- release a variety of chemicals and combustion products into
matic increases in peak flows following wildfire (Tiedemann the aquatic environment. Reconstruction can keep soils bare
et al. 1979). and disturbed for years.
Sediment Yield Aquatic Effects
In general, sediment yields increase markedly after fires, par- Studies of the aquatic effects of a fire on the Plumas National
ticularly if riparian vegetation was burned. Most of the sedi- Forest demonstrate how both physical and biological features
ment response seems to be from the channels themselves. In of the stream change over time (Roby 1989; Roby and Azuma
the absence of streamside vegetation, soil particles move into 1995). The lower two-thirds of this catchment, including rithe
channels from dry ravel erosion, and the banks become parian vegetation, was thoroughly burned. Initially, the chanless
stable. Increases in total discharge and peak flows result nel widened in response to presumed higher flows of water
in channel erosion. Debris torrents may scour streams if ex- and sediment. However, as vegetation became established and
treme climatic events follow the fire (Helvey 1980; Kuehn the watershed recovered, the cross sections of the channel
1987). If the fire is particularly hot, woody debris that helped returned to their prefire areas within six years of the burn.
stabilize the channel may be destroyed. Erosion from the gen- Partial recovery of the invertebrate community seemed to
eral land surface usually increases, but it may not always be have occurred relatively quickly. No differences in commuas
important a delivery mechanism as has been assumed nity similarity were noted between burned and unburned
(Booker et al. 1993). Erosion from plots in brushland near reaches one year after the fire, and density and taxa richness
North Fork in the San Joaquin River Basin increased by 200 were comparable within three years. However, significant
to 400 times after repeated burning (Lowdermilk and Rowe (though declining) differences in a species-diversity index
1934). In Dog Valley in the eastern Sierra Nevada near Reno,
a single storm produced about 600 m3/km
between the burned and unburned reaches remained through-
2 (1.3 AF/mi2) of
sediment from a burned catchment while an adjacent un-
out eleven years of monitoring (Roby and Azuma 1995).
burned area yielded only a trace of sediment (Copeland 1965).
Fire Suppression
Under extraordinary rainfall, gully erosion, sheet erosion, and
a debris torrent removed more than 19,000 m3 (15 AF) of ma- Fire suppression during this century has created forests with
terial from a burned catchment of about 0.8 km2 (0.3 mi2) in greater density of vegetation than in the past (Chang 1996;
the headwaters of the South Fork of the American River in Skinner and Chang 1996; Weatherspoon I996). This forest
1982 (Kuehn 1987). structure has current and potential hydrologic consequences.
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