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

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VOLUME II, CHAPTER 30
standard have additional effects. Primarily, they are simply layer within the soil that reduces infiltration of water into the
wider and affect more area per unit of length. A four-lane high- soil (see Poff 1996). Riparian zones that would not be harway
can occupy a substantial fraction of a small catchment, vested under current forest practices are often partially burned
Their impervious surface can create overland flow over large in intense fires. The combined effect of these changes is to
areas where it was nonexistent before construction. They are increase total water yield and overland flow. As the propordesigned
for more traffic at higher speeds and so tend to be tion of overland flow increases, streams receive more water
forced through the landscape, minimizing curvature and in less time than under prefire conditions, and peak flows
changes in grade instead of following the topography more may be increased. If a nearly continuous water-repellent (hyclosely.
In partial compensation for the greater hill-slope al- drophobic) layer is a few centimeters below the surface, the
teration, highways are better engineered than lightly used soil above that layer may become saturated and form shalroads.
Large investments are made in adequate drainage struc- tow debris flows. With bare soil, increased overland flow, and
tures, slope reinforcement, and revegetation. Nevertheless, lack of vegetation and litter, soil particles are more easily demitigation
for the sheer size and location of the highway tached and transported. As with other impacts, the proporprojects
is difficult at best. Major highways are immediately tion of a catchment that is modified by fire and the location
adjacent to portions of the Feather, North Yuba, South Yuba, of the burned area with respect to the channel largely deter-
Truckee, South Fork American, Merced, Walker, Kaweah, Tule, mine the effects on streams. A stream draining a watershed
and Kern Rivers. Within cities and towns, the storm water burned over 90% of its area will show much greater effects
drainage system for the entire road grid is often inadequate than a stream emanating from a similar watershed in which
during large storms because communities tend to develop in only the upper slopes and ridgetops were burned. Fire intena
piecemeal fashion, rather than having a complete road and sity is often highly variable over the landscape, and patches
drainage network planned from the start. Contaminants from of unburned or lightly burned vegetation (especially near
tire wear, fluid leaks, pet waste, and exhaust that accumulate streams) can reduce the adverse effects of upslope areas that
on the roadway are washed off into the nearest waterway,
Oils used for road dust abatement can also be problematic.
were intensely burned.
For example, contamination of Ponderosa Reservoir on the
South Fork Feather River with polychlorinated biphenyls
Water Yield
(PCBs) was traced to the use of transformer oil on forest roads Fires affect water yield primarily by killing vegetation. Inter-
(Plumas National Forest 1988). ception loss is decreased because of the loss of leaves, low
vegetation, and litter. Transpiration is virtually eliminated
Deicing Agents
wherever fire is intense. A daily cycle in stream flow reflecting
transpiration demand during daylight hours in a catch-
Chemicals used to remove snow and ice from roadways in ment in Washington came to an abrupt halt following a
winter can affect local water quality and roadside vegetation catastrophic fire (Helvey 1980). Annual runoff in this com-
(Hawkins and Judd 1972; Scharf and Srago 1975; Goldman pletely burned watershed increased by 10-47 cm during the
and Malyj 1990). During a heavy winter (1982/83), rock salt first seven years after the fire. Water yields in a small catch-
(sodium chloride) was applied to Interstate 80 near Donner ment in British Columbia that was burned over about 60% of
Summit in an average quantity of about 45 metric tons per its area increased by 25% on average for four years following
km (80 tons per mi) of roadway (Berg and Bergman 1984). the fire (Cheng 1980). Dramatic increases in flow of a small
Stream samples obtained about 0.5 km (0.3 mi) downstream spring and a creek in the Sierra Nevada were observed folfrom
the last highway crossing of the channel contained up lowing burning of riparian vegetation (Biswell 1989). A deto
100 times more chloride and 10 times more sodium than tailed modeling study for Pacific Northwest forests has
water obtained just upstream from the highway (Berg and suggested that a reduction in leaf area or basal area of about
Bergman 1984). 50% is necessary before annual water-yield increases exceed
about 50 mm (2 in) (Potts et al. 1989). Snow accumulation
and melt rates might be expected to increase from opening a
forest canopy by fire, and such effects have been observed in
F I R E S F I R E S U P P R E S S I O N, A N D
Washington and British Columbia (Helvey 1980; Cheng 1980)
but not in Idaho (Megahan 1983).
POSTFIRE TREATMENTS
Catastrophic fire can produce some of the most intensive and
Peak Flows
extensive changes in watershed conditions of any disturbance. Peak flows can be expected to increase following significant
Within areas of intense fire, most vegetation is killed and stops fires because of higher soil moisture resulting from reduction
transpiring, allowing soil moisture levels to remain high. Or- of transpiration, decreased infiltration, and higher rates of
ganic matter in the litter layer is volatilized and often forms a snowmelt. Infiltration is usually the most important influence,
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