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5.ENVIRONMENTAL CHECKLIST MOUNTAIN HOUSE NEIGHBORHOODS K AND LINITIAL STUDY 8.HYDROLOGY AND WATER QUALITY Setting Climate and Topography Mountain House is located on the western edge of the San Joaquin River Valley. Average annual rainfall in the vicinity of Mountain House is approximately 12 inches. Average temperatures generally range from 38 degrees Fahrenheit in winter months to 93 degrees Fahrenheit in summer months(WRCC, 2002). The Neighborhoods K and L project site is generally level and slopes gently(less than 1 percent) northeast toward Old River. Ground surface elevations range from 0 feet above mean sea level (msl) in the northern portion of the site along Old River to about 25 feet above msl in the southern portion of the site(USGS, 1978). Surface Water Hydrology Regionally, Mountain House is located within the Sacramento-San Joaquin River Delta(Delta).California's two largest rivers,the southward-flowing Sacramento River and the northward-flowing San Joaquin River, converge in the Delta.The Delta is a flat, low-lying network of 57 islands, interconnected by approximately 700 miles of waterways.The Delta, comprised of over 1,100 square miles, is the largest estuary on the West Coast and drains over 40 percent of the water in California.The Delta and its watersheds provide approximately two-thirds of California's water supplies(PACE, 2005). Mountain House is located within the Mountain House Creek and Dry Creek watersheds. Mountain House Creek drains an area of approximately 6.5 square miles between the Altamont Hills and Old River. Mountain House Creek runs in a northeasterly direction between Neighborhoods K and L and discharges to Old River at the northern project site boundary. Dry Creek flows in a northeasterly direction parallel to and approximately 1.5 miles northwest of Mountain House Creek, Dry Creek has a drainage area of approximately 6.8 square miles and discharges to Old River at the western corner of Neighborhood K. Old River, which flows east to west along the northern project site boundary, is bordered by levees and is part of the Delta conveyance system. Currently, runoff on the Neighborhoods K and L project site generally flows in a northeasterly direction to Old River. Existing drainage infrastructure and improvements on the project site are limited to agricultural drainage ditches and some drainage pipes at street crossings, a 13-acre water quality basin at the southern corner of Neighborhood L, and restoration improvements along the Mountain House Creek corridor. Historically,agricultural drainage ditches were used to convey irrigation water from Old River to farmland in Mountain House, including Neighborhoods K and L.These agricultural ditches are gradually being replaced with new drainage infrastructure as Mountain House builds out(Condor, 2010). Drainage improvements proposed for the Neighborhoods K and L project site are discussed later in this section. I97ni� 5-104