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resulted from erosion by rivers and glaciers, and from local faulting which produced features such as the Tahoe Basin, in which the Pyramid Project was centered. To the east of the Sierra lies the Basin and Rance Province, consisting of some 800,000 km2 (300,000 sq mil of bloc~-faulted mountains and intervening desert basins. During the last pluvial period most of these basins contained lakes. Pyramid Lake is a remnant of one of the largest of these, Lake Lahontan, which Govered some 20,000 km2 (8,000 sq mil at its maximum.- Virtually all rivers rising in or draining into the Basin and Range end there in desert lakes or dry sinks (Fig. 2-2). In winter and early spring, northern California and Nevada lie in a zone of interaction between unlike air masses which con- tains a series of eastward-moving cyclonic storms. Most of the region's precipitation falls between October and May when mid- latitude cyclones carry relatively warm and moist maritime air over the land. During the summer when the north Pacific sub- tropical high pressure is most powerfully developed, little precipitation falls. The general conformance of various climatic elements with the topographic zones described abov~ is illustrated in Figure 2-3. The Coast Ranges and the Great Valley have a Mediterranean climate (Koppen-Geiger Csa, Csb) with a wet, cool winter and warm or hot, dry summer. A gentle, but distinct, north-south gradie~t or precipitation exists within the Great Valley (Figs. 2-4 and 2-5). A linear re~ression on precipitation at valley stations as a function of distance south of Redding yielded a correlation coefficient of 0.91 (0.45 for precipitation versus station elevation). As is typical of major highlands throughout the world, the distribution of the various elements of the climate of the Sier~a ~evada is very elevation-dependent. Higher elevations are generally cooler and moister (Fig. 2-3). As s~ggested by Figure 2-3, the higher elevations receive a larger percentage of 2-4