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<br />~ <br /> <br />found at higher elevation. Pack water content increases to a maximum on about April 1 (usually <br />in late March). <br />From about the 1680-m (5500-ft) elevation to the crest in a "normal" year, these packs will <br />usually hold all but the most massive of rainstorms and seldom contribute to floods. Packs at this <br />elevation normally last from late fall to well into spring. Tables 5 through 8 present water content <br />information from several Yuba River and American River Basin sites above 1680 m (5500 ft) <br />elevation. <br />The basin's rainfall zone, from 1220 to 1525 m (4000 to 5000 ft) elevation, can be expected to <br />lose most or all its snowpack to a rain event at any time. <br />The 1525- to 1680-m (5000- to 5500-ft) rainlsnow zone can be expected to absorb and hold <br />minor rains, but large amounts of the snowpack will melt under sustained rains. <br />Major floods in the basin occur as the result of rain melting snow in the 1220- to 1680-m (4000- <br />to 5500-ft) elevation zone. <br /> <br />i <br /> <br />" <br /> <br />Hydrology.- The hydrology of the American River Basin is described by Hannaford (Elliott et <br />al. [9]). <br />The basin slopes moderately upward to the east-northeast from Folsom Reservoir at about 150 <br />to nearly 3050 m (500 to 10 000 ft) at the highest peaks. <br />The upper portions of the basin consist largely of bare and poorly weathered granitic rocks <br />deeply incised by glacial action and forming many steep-sided canyons. Alluvial fills are rare, but <br />when found, are usually small, and extremely porous with poor water storage capacity. Winter <br />snow is the most prominent precipitation above 1830 m (6000 ft). Summer storms are frequent <br />but account for only a small portion of the more than 127 cm (50 in) average annual precipitation. <br />Since storage capacity is limited, stream flows occur as the snowpack melts and decrease rapidly as <br />the snowpack disappears. <br />Streams rise rapidly when summer storms occur, but taper off rapidly as storms cease. <br />The basin is covered by sedimentary rocks overlain by volcanic flows in the 1220- to 1830-m <br />(4000- to 6ooo-ft) elevation range. The terrain is deeply incised with numerous steep-sided can- <br />yons. Soil development is good in some places but soils are generally very porous with slight water <br />storage capacity. On the average, this zone receives 114 to 140 cm (45 to 55 in) of snow and rain <br />annually, mostly during the winter months. The snowpack is usually thinner here than higher in <br />the basin except where local orographic influences prevail, causing heavy snowfalls. Since storage <br />capacity is slight in this zone, as well as in the one above, streamflow is high during snowmelt and <br />during rainstorms but tapers off rapidly when the primary source disappears. Because snowpack <br />melts relatively early in this zone, stream flows in late spring originate largely from upstream <br />areas. <br />Below 1220 m (4OOP ft), the terrain is more gently sloping and less deeply incised. Soils tend to <br />be deeper and better developed with greater water storage capacity. Precipitation in this zone is <br />almost exclusively rain or swiftly melting snow and averages 56 to 114 cm (22 to 45 in) a year. No <br />snowpack develops. <br />Flow-duration analysis of discharge data from stations in the basin as shown demonstrates the <br />dependence of streamflow on the snowpack and the lack of substantial base flow when the <br />snowpack is gone. <br />The type of flow-duration curve common to tributaries to the major forks of the American <br />River (fig. 2) indicates persistence of daily flows exceeding 28 m3 Is (1000 ft3 Is) for half the <br />discharge record. In the other half of the record, however, flow drops precipitously to less than <br />0.3 m3/s (10 fP/s) for about 1 percent of the record. Apparently, as long as the snowpack re- <br />mains, streamflow in the range of 28 to 283 m3/s (1000 to 10 000 fP/s) persists, but when the <br /> <br />;. <br /> <br />IV-13 <br />