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Streamflow from the river's three forks is controlled by a series of dams culminating at Folsom Dam near Sacramento, California. Elevations reach 3050 m (10 000 feet) along the crest on the eastern flank of the basin. Gradients are between 10 and 40 percent for 78 percent of the slopes, while for 16 percent of the slopes, gradients are in excess of 40 percent. Westerly and southerly aspects are evenly divided and comprise 75 percent of the slopes. The climate of the American River Basin is conditioned by the general maritime climate of California. Summer and winter temperatures are mild, Summers are dry, except for occasional small thunderstorms. Vegetation must exist on winter-deposited precipitation. Approximately 90 percent of precipitation at 1525 to 2440 m (5000 to 8000 ft) elevation occurs as snow. Snow normally falls at air temperatures between about - 4 and 3 0 C (25 and 37 0 F). Soils in the area have little or no strongly developed profiles or textures finer than loams. Soils are very porous, with high waterflow rates. The amount of water held by the soils available for plant use ranges from moderate to low. Excess water above field capacity drains from all but the longest, deep soil, north slope areas within 3 to 6 weeks after snow disappears. Because of the warm winter climate, the entire pack remains at the freezing point (0 0 C (32 0 F)) throughout most winter seasons. Snowmelt continues throughout the winter, providing water to replenish the soil moisture. The excess becomes streamflow. Heavy rainstorms occur quite often in the fall and early winter, frequently producing floods. Water reaching the streams from early-season snowmelt and rainfall is normally retained in the reservoirs for use in producing electricity. However, in times of flood or heavy spring runoff, this water would be released unused from the reservoirs. Snowmelt in the basin is significantly affected by vegetation. A full forest canopy can protect the snow on a south slope from solar radiation in early- and mid-winter to the extent that melting time is comparable to that for an open north slope snowpack. Because of the warm winter temperatures, snow melts during the day and the snowpack surface refreezes at night. This process creates ice lenses which are buried by subsequent snowfall. These lenses become barriers to further water penetration, Large changes occur in the melt rate and water-holding capacity of snowpack as a result of forest removal. Evaluation of the ecological effects of weather modification becomes very complex because of the climatic conditions in the American River Basin. As a result of snowmelt throughout the winter, increased snowfall may not delay the time of snow meltOut. Further, as a result of this all-winter melt, stream peak flows in spring may not be increased. Precipitation may be increased, but midwinter snowmelt and water movement to streams through the porous soils may not affect plants very much because the water leaves the soil prior to major plant water use. This is in direct contrast to the situation in the cold, continental snowpacks of the Rocky Mountains. In the Rockies, additional snowpack or precipitation lengthens snowpack duration, decreases the growing season, and increases streamflow and the amount of water for plant use later in the summer. PRECIPITATION Data Source . Precipitation records from four different meteorological stations for 76 years (1899-1974) have been used to compile a climatic history of the CSSL site at Soda Springs, California. These sta- 1-3