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BUlletin 160-98 Public Review Draft Chapter 1. Introduction The Klamath Mountains are a rugged mountain range located on the California-Oregon border, through which the Klamath River has cut a deep canyon. To the east, the Cascade Range is a chain of volcanic cones that stretches from California into Washington. Until the 1980 eruption of Mount St. Helens in Washington, Mount Lassen, the southernmost ofthe Cascade volcanos, was the most recently active volcano in the contiguous UB, The Modoc Plateau lies to the east of the Cascade Range, and is the southernmost part of a broad area of lava flows and small volcanic cones that covers much of eastern Oregon and southeastern Washington, The Pit River, a major Sacramento River tributary, winds through the Modoc Plateau and cuts a deep canyon through the Cascade Range, crossing that range between two of its large volcanos .- Shasta and Lassen, IlWPhoto: Mount Shasta The Central Valley is an alluvial basin about 50 miles wide by 200 miles long, bounded by the Coast Range on the west and the Sierra Nevada on the east. Except for the Tulare Lake drainage at the southern end of the valley (a closed drainage basin), rivers draining the Sierra Nevada flow onto the valley floor, join with the Sacramento or San Joaquin rivers, and flow through a gap in the Coast Range to San Francisco Bay. The.Central Valley is California's most productive agricultural area, constituting about 80 percent ofthe State's total production. The Sierra Nevada, California's dominant mountain range, is a fault block structure whose western slopes are marked by deep river.cut canyons. Sierran rivers furnish much of California's developed surface water supplies. IlW Photo: San Andreas Fault The Coast Ranges are bounded on the north by the Klamath Mountains and on the south by the Transverse Ranges, The San Andreas Fault is a prominent geologic feature of the Coast Ranges; its path can readily be traced in areas where faulting has controlled the direction of watercourses such as the Gualala River. The San Andreas Fault extends into the San Bernardino Mountains of the Transverse Ranges geomorphic province (so called because these mountain ranges trend east-west), The Peninsular Ranges (which trend north-south) are a cluster ofranges separated by long valleys dividing, for example, the Riverside area from the Los Angeles coastal plain, The western edge of the Mojave Desert is delineated by the Garlock Fault and by a 1-8 DRAFT