SFPUC 2001 Alameda Watershed Management Plan

III. ENVIRONMENTAL SETTING AND IMPACTSD. HYDROLOGY AND WATER QUALITYemergency water, and Calaveras Reservoir surplus water. It has also been used historically tostore groundwater pumped from the Sunol Infiltration Galleries (discussed in Section 1.5,below). The average annual rainfall in the vicinity of the San Antonio Reservoir is 19.8 inchesper year.SFPUC-owned lands include nearly all the drainage area north and northeast of the San AntonioReservoir, and these lands extend eastward to include the downstream portions of each of themajor contributing creeks. However, the upstream portions of the tributaries are outside ofSFPUC ownership, including large areas of eroded and erosion-sensitive soils. Abandoned oractive commercial gravel mines or borrow pits are not located within the San Antonio Reservoirdrainage basin.1.3 ALAMEDA CREEK DRAINAGE BASIN BELOW THE DIVERSION DAMAlameda Creek, below the diversion dam (sometimes referred to as lower Alameda Creek),conveys flows and runoff through the Sunol Valley from tributary drainages in the Diablo Rangeand Livermore Valley. Through the Sunol Valley, Alameda Creek below the diversion damreceives limited surface flows from Calaveras Creek and Arroyo Hondo Creek downstream ofCalaveras Reservoir and from San Antonio Creek downstream of San Antonio Reservoir.Further downstream, north of I-680, Alameda Creek receives flows from Arroyo de la Lagunaand Vallecitos Creek. SFPUC-owned lands primarily include areas adjacent to Alameda Creekin the Sunol Valley and Niles Canyon, and within the downstream end of the southeasterntributaries; these lands are considered part of the secondary Watershed. The major portion of theAlameda Creek drainage area below the diversion dam is outside of the SFPUC-owned lands.Within the vicinity of, and on SFPUC-owned lands, there are numerous public and private roadsthat cross the Alameda Creek drainage basin and affect the natural flow of runoff. These roadsinclude the I-680 crossing through Arroyo de la Laguna and the Sunol Valley, the SR 84 crossingthrough Niles Canyon and Vallecitos Valley, county and private access roads, and a number ofinternal gravel/dirt roads used principally by SFPUC personnel and gravel mining operators.Commercial gravel mining currently takes place within this drainage basin, which hashistorically rerouted surface and groundwater flows in the immediate vicinity of the mining pits.1.4 WATER QUALITYWater quality within the three drainage basins of the Alameda Watershed system is influenced bythe source, storage, and treatment of water. Calaveras Reservoir stores local runoff only, andaeration facilities at this reservoir are used in late summer and fall to increase dissolved oxygenin stagnant zones in the deeper parts of the reservoir. This process reduces the concentrations ofdissolved iron, manganese, and hydrogen sulfide in the raw water. San Antonio Reservoirreceives imported water in addition to local drainage; therefore, it has more variable waterquality than does Calaveras Reservoir. The imported water stored in San Antonio Reservoirincludes water from the Hetch Hetchy system, surplus water from the Calaveras Reservoir, and,in the event of a drought, water from the South Bay Aqueduct (water from the State WaterNOP 96.223E: Alameda Watershed Management Plan III.D-4 ESA / 930385January 2001