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<br />Data Sources
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<br />loads. Hydraulic mining peaked in the late 1870s, and ended in 1884 due to federal
<br />legislation.
<br />Gilbert (1917) estimated that hydraulic mining in the Feather, Yuba, Bear,
<br />and American River basins yielded nearly 1.1 billion cubic meters of debris,
<br />primarily mud, sand, and gravel. This enormous sediment load and the absence of
<br />any environmental controls led to severe aggradation of the streams draining the
<br />mines, with amounts ranging from several to as much as 30 meters (Gilbert, 1917).
<br />The first major flood to transport this sediment occurred in water year 1862.
<br />Transport continued in subsequent tloods. The sediment moved primarily in pulses
<br />during winter floods, with amounts gradually decreasing after the curtailment of
<br />hydraulic mining. By 1988, American River channels near the mining district were
<br />largely free of mining sediments, except terrace sediment, and appeared to be at or
<br />near their pre-mined grade (James, 1988). 1\'RC (1995) and James (1997) concluded
<br />that the lower American River reaches still have substantial mining sediment
<br />remaining and that cyclical patterns of aggradation and degradation occur, but that
<br />the net trend appears to be slow channel degradation and increasing channel flow
<br />capacity.
<br />Vast areas of Sierra forests were cut in the mid-1800s to support the mining
<br />industry (Beesley, 1996). Lumber was needed for fuel and construction of camps,
<br />towns, water flumes, mining structures, tunnels, and railroads. In the l840s, the
<br />estimated annual lumber production in California was about 20 million board feet per
<br />year. In less than 30 years, annual lumber production increased to nearly 700 million
<br />board feet, primarily to support activities related to gold mining (Mount, 1995).
<br />Based on data from various sources, Beesley (1996) concluded that about one-third
<br />of the trees (primarily yellow and sugar pine) in the mining area in the Sierra Nevada
<br />had been harvested by about 1885. In the early 1900s, logging diminished, enabling
<br />the forest ecosystem to substantially recover, although pine was replaced primarily
<br />by white fir (Beesley, 1996). As a result of the rapid population growth of California
<br />after World War II, timber harvesting rapidly increased, reaching 6 billion board feet
<br />per year by 1960.
<br />Grazing of domestic livestock, primarily sheep and cattle, has probably
<br />affected a larger proportion of the Sierra Nevada than any other human activity
<br />(Menke et aI., 1995). Grazing was minimal prior to about 1860, then increased
<br />dramatically until the early 1900s. The effects of unmanaged grazing included
<br />increases in runoff and sediment yields and localized gully formation (Gilbert, 1917).
<br />With the advent of regulation by the U.S. Forest Service in 1905, better management
<br />practices were instituted, reducing the overall watershed impacts (Beesley, 1996).
<br />Wildfire can produce extensive changes in streamflow and sediment yield
<br />(Florsheim et aI., 1991; Meyer et a1., 1995; Weise and Martin, 1995). Hydrophobic
<br />conditions often develop after a wildfire, as combustion of vegetation and organic
<br />matter produces aliphatic hydrocarbons that move as vapor through the soil and
<br />substantially reduce infiltration. Hydrophobic soils, decreased vegetation cover, and
<br />reduced surface storage following wildfire dramatically increase the potential for
<br />extreme flooding and soil erosion. Favorable runoff conditions may remain for
<br />several years to decades until burned areas sufficiently recover to pre-burn conditions
<br />(Evanstad and Rasely, 1995). Native Americans, who have inhabited California for
<br />at least 10,000 years, modified the Sierra Nevada landscape by burning and various
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