SFPUC 2001 Alameda Watershed Management Plan

III. ENVIRONMENTAL SETTING AND IMPACTSC. GEOLOGY AND SOILS1.0 SETTING1.1 TOPOGRAPHY AND GEOLOGYThe Alameda Watershed is located along the western flank of the northern Diablo Range, withinthe natural region of California referred to as the Coast Ranges Geomorphic Province. Thisprovince is geologically complex and seismically active and is characterized by northwesttrendingfaults, mountain ranges, and valleys. The Diablo Range forms the eastern boundary ofthe Coast Ranges and separates the geologic structural depressions of San Francisco Bay andSanta Clara Valley to the west and the San Joaquin Valley to the east.Northwest-trending ridges and valleys control the relief of the Watershed. The major valleysinclude Sunol, Calaveras, Alameda Creek, and Arroyo Honda Creek. The east-west trending LaCosta Valley includes San Antonio Reservoir and constitutes a major portion of the Watershed inthe northern section. The rugged upland terrain of Oak Ridge, Poverty Ridge, and AppersonRidge surrounds the major valleys.Elevations in the Watershed range from about 230 feet above mean sea level (msl) at the northernend of Sunol Valley to about 3,300 feet above msl in the southeastern corner of the Watershed onPoverty Ridge. Slopes in the upland areas are steep, with average gradients ranging from about3:1 (horizontal to vertical ratio) to 1:1. The major valleys have nearly level floors. Most of thetributary stream valleys are very narrow, with V-shaped cross sections.The Watershed is composed of two distinct stratigraphic rock sequences separated by thenorthern section of the Calaveras Fault Zone. The fault zone separates the Hayward Hills (to thewest) from the Diablo Range (to the east). The Hayward Hills consist of Cretaceous (about 65 to135 million years old) sedimentary rocks of the Great Valley sequence. The Diablo Range iscomposed primarily of sedimentary and metamorphic rocks of the Franciscan complex ranging inage from Cretaceous to Jurassic (135 to 190 million years old). Within the Watershed, Tertiary(about 2.5 to 65 million years old) marine deposits overlie both of these units, along withQuaternary (less than 2 million years old) surficial alluvium (deposited by streams).Surficial deposits include Quaternary alluvium and landslide deposits. The alluvial depositsinclude older stream terrace and active stream channel deposits that are most extensive in theSunol, La Costa, and Amador Valleys. They are an important source of aggregate mineralresources and include large areas of prime farmland soils.1.2 SOILS AND EROSIONThe soils in the Watershed generally reflect the underlying geology, with variations related toslope position and stability. In areas underlain by sedimentary rocks, the soils generally consistof the Millsholm-Los Gatos-Los Osos association; in areas underlain by rocks of the Franciscancomplex, soils generally consist of the Vallecitos-Parish association (Environmental ScienceNOP 96.223E: Alameda Watershed Management Plan III.C-1 ESA / 930385January 2001