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<br /> <br />'" <br /> <br />During Quaternary times, several episodes of mountain glaciation took <br />place, separated by fairly long interglacial periods. The upper <br />reaches of the American River were altered by glacial ice tongues that <br />pushed down canyon - giving a U-shape to these upper reaches _ and <br />retreated, leaving behind levels of Jll)rainal deposits of sandy to <br />bouldery material (Lindgren [2], Bateman and Wahrhaftig [3]). <br /> <br />The complex surface rock pattern of the current watershed reflects <br />this history - a history that helps determine the hydrOlogy and soil <br />patterns of the present watershed. <br /> <br />CLIMATE <br /> <br />.. <br /> <br />The climate of the American River Basin snow zone (sea level to above <br /> <br />1500 m (5000 ft)) is conditioned by the general maritime climate of <br /> <br />California. Summer and winter temperatures are mild, and'st.DnDlers are <br /> <br />dry with no precipitation except for occasional, small thunderstorms. <br /> <br />For all practical purposes; vegetation must exist on winter deposited <br /> <br />precipitation. Amual precipitation in the basin ranges from about <br /> <br />400 mm (16 in) at Sacramento, increasing with altitude to about <br /> <br />1270 DUD (SO in) at the snow zone line. From here to the crest, pre- <br /> <br />cipi tation ranges from about 1010 to 2030 DUD (40 to 80 in) normally <br /> <br />increasing with altitude. However, precipitation within the inner <br /> <br />7 <br />