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

875
Hydrology and Water Resources
overland flow, predominance of subsurface flow, and rela- a portion of a slope fails all at once. Movement may be catatively
continuous vegetation coven The sources and pathways strophic in seconds or progressive over years. Mass wasting
of sediments supplied to stream channels are not completely may be important in providing a material supply to channels
understood. The channel system itself is an obvious candi- slowly through soil creep or suddenly when a debris flow
date as a source for most of the sediment (King 1993). During reaches a stream, but it is not regarded as a major erosive
persistent rainfall and peak snowmelt, the network of very agent in most of the Sierra Nevada (Seidelman et al. 1986).
small channels becomes rather extensive, mobilizing sediment Mass movement typically occurs when most of the pores in
from a large fraction of a watershed. Such sediment probably the material become filled with water. The positive pressure
does not move very far but may be made available for trans- of the pore water and its added mass may exceed the strength
port by a high-magnitude runoff event. The sequence of events of the material, and failure of part of the slope may occur.
of different magnitudes can determine the net sediment trans- Unusually high rates of water input to previously wet soils
port over long time periods (Beven 1981). In the Sierra Ne- can lead to large numbers of landslides in the Sierra Nevada
vada, the greatest potential for overland flow to occur appears (De Graft et al. 1984). Disturbance of slopes accelerates the
to be below the snow zone in woodland-grassland communi- natural occurrence of landslides (Sidle et al. 1985). Excaties
between 300 and 900 m (1,000 and 3,000 ft) (Helley 1966). vations across slopes for roads intercept water flowing
The maximum rates of sediment production have been ob- downslope through the soil and increase pore water pressure
served in this same altitude range (Janda 1966). The wood- at the exposed seepage face. In granitic portions of the Sierra
land zone also was the primary sediment source in part of Nevada, ground-water flow is often at a maximum at the inthe
American River Basin with annual erosion of about 150 terface between the porous coarse-grained soils and underlym3/km
2 (0.3 AF/mi 2) (Soil Conservation Service 1979). ing relatively impermeable bedrock (De Graft 1985). Exposure
of this layer can bring large quantities of water to the surface
Accelerated Erosion
(Seidelman et al. I986). Such excavations also reduce the me-
Human activities often disrupt the natural geomorphic prochanical
support for adjacent parts of the slope. Tree roots are
often important in maintaining the integrity of a slope. Minicesses
and accelerate erosion or destabilize hill slopes. Mod- mum strength occurs about ten years after fire or timber
eling erosion in the Camp and Clear Creek Basins suggests harvesting when roots from young trees have not yet cornthat
disturbance, especially roads, can increase erosion many pensated for the progressive loss of old roots (Ziemer 1981).
times above natural rates (McGurk et al. 1996). When soil loss Most opportunities to minimize mass wasting as a conseand
sediment transport occur at unusually high rates in re- quence of road construction and forest harvesting involve
sponse to some human disturbance, erosion and sedimen- commonsense approaches to avoiding accumulation of subtation
become issues of concern. Accelerated soil loss is surface water on steep slopes (Sidle 1980; McCashion and Rice
primarily a problem in terms of losing productivity for grow- 1983).
ing vegetation (Poff 1996). Excessive sedimentation can dam- In years of high precipitation with large individual storms,
age terrestrial plants and aquatic organisms. High levels of the number, extent, and size of mass movements increase well
sediment deposition can also reduce the utility of facilities above those of years with modest precipitation. Landslides
for water storage and diversion and hydroelectric produc- were particularly active during the wet years of 1982 and 1983.
tion. At the extreme, hydraulic mining for gold on the west In both those years, springs and seeps appeared in places they
slope of the Sierra’Nevada intentionally eroded entire hill- had not been noticed before, including many road cuts and
sides. The resulting sedimentation in downstream river chan- fills. More than $2 million in damage occurred to roads on
nels left deposits tens of meters thick. Sediment yield in the national forests in the Sierra Nevada during 1982, and addi-
Yuba River was up to twenty-five times greater than natural tional damage estimated at more than $1 million occurred in
rates (Gilbert 1917) and led to a legal decision effectively halt- 1983 (De Graft 1987). A landslide in the American River caning
hydraulic mining. Activities that purposefully move soil,-~ yon blocked U.S. 50 for April, May, and June of 1983. Sus-
such as construction of roads and structures, have the great- I
tained high levels of soil moisture and ground water occurred
est potential for increasing erosion. Activities that reduce veg- ! throughout the winter and spring of each year. Additional
etative cover and root strength can also increase erosion rates, water input from rainfall, combined rainfall and snowmelt,
Activities in and near stream channels have the greatest po- and snowmelt alone triggered the unusual number of failtential
for altering sediment delivery and storage as well as ures (Bergman 1987; De Graft 1987). However, there seems to
channel form. For example, destruction of riparian vegeta- be relatively little interaction between high flows and initiation
can lead to massive streambank erosion, or dams can trap tion of landslides within the inner gorges of Sierra Nevada
sediment from upstream while causing channel incision or streams (Seidelman et aL 1986). Landslides can also be initinarrowing
downstream, ated by earthquakes (Harp et al. 1984) and summer thunder-
Processes involving movement of large units of soil or rock storms (Glancy 1969). An extraordinarily intense storm
rather than individual particles are collectively known as mass occurred in the headwaters of the South Fork of the Ameriwasting.
Landslide activity is a typical mass failure in which can River on June 18, 1982 (Kuehn 1987). About 100 mm (4 in)
D--048987
D-048987