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. snowpack is gone, soils and subsurface material contain so little water and drain so rapidly that base flow drops rapidly to low values. In short, storage volume in the basin is small compared with the volume available directly from precipitation and snowmelt. Reservoirs regulate peak and low flows on the major tributaries. The effect of flood control structures on the South Fork American is apparent from analysis of flow-duration curves at recessive downstream sites (fig. 3). Flows in Alder Creek at 1340 m (4400 ft) and South Fork American River at 1340 m near Kyburz, exceed 280 m3 Is (1O 000 ft3 Is) for half the record with peaks up to 565 m3 Is (20000 ft3 Is). These streams meet near Kyburz and flow into Slab Creek Reservoir. The record for station 1351 m (4435 ft), South Fork American River near Camino, shows that flow below the dam also exceeds 280 m3/s for half the record, but peak flows do not exceed 565 m3/s; whereas median flow at that point, in the absence of the reservoir, would be at least the sum of median flows at the two tributary stations upstream. The record below Lotus at 1358 m (4455 ft) indicates further at- tenuation of peak flows to less than 280 m3/s and median flow to about 135 m3/s (4800 ft3 Is) by Chili i Bar Reservoir. In summary, water supplied by the snowmelt dominates the streamflow. Some melting occurs throughout the winter especially in the middle zone and from south-facing slopes. With the ap- proach of spring, the air warms appreciably, increasing the melting rate. The pack gradually disappears from the' lower areas first and later at higher elevations. Some snow remains over summer at the highest elevations. 4 ~ Water Quality.- The chemical composition of water in the Sierra Nevada has been in- vestigated, but detailed studies of the American River Basin are lacking. Feth et al. [10] investigated springs in the Sierra Nevada. A summary of these data is shown in table 9. Waters are generally a calcium bicarbonate (CaC03) type with a substantial fraction of sodium. High silica concentrations are observed throughout the Sierra, indicating considerable weathering of primary alumin~-silicate rocks. Dissolved solid concentrations are generally less than 100 mglL, but values as high as 162 mglL are observed. The pH of most water is neutral to slight- ly acidic. Dissolved oxygen concentrations in steep channels at higher elevations are nearly always close to saturation. tow concentrations can occur at night in pools containing attached algae. Sagehen Creek near Truckee is representative of waters draining the Sierra Nevada (table 10). The same general features are demonstrated both in the analyses of Sagehen Creek water and the analyses of spring waters. Calcium and bicarbonate composition dominate, and sodium is a substantial fraction of the cations. Chemical data on the American River and its tributaries between the headwaters and the valley are scarce. Irwin and Lemon [12], however, have summarized data on the American River at Sacramento. Excerpts from that summary are shown in table 11. The water appears to undergo few, if any, changes of substance from the headwaters to the valley. Composition: is dominated by calcium and bicarbonate with significant fractions of sodium also present. Magnesium fs a substantial minor constituent. Bicarbonate concentration was slightly lower tqan that found in mountain springs. These data reflect mixing of large volumes of very dilute snowmelt which flows directly to streams with smaller volumes of water flowing from the soi~ to the streams in the American River Basin. The latter (also termed base flow) presumably coritains higher amounts of chemical constituents per unit of volume. All available data indicate water quality throughout the American River Basin is excellent for even the most demanding of beneficial uses. However, very little data are available on nitrogen and phosphorus concentrations, which may indicate the tropic status of the river. IV -17