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III. Environmental Setting, Impacts, and Mitigations K Hydrology, Geohydrology,and Water Quality Johns Creek and Lone Tree Creek converge and enter French Camp Slough(U.S.G.S, 1987a and 1987b). French Camp Slough discharges to the San Joaquin River on the southwest side of the City of Stockton, and the waters enter the Sacramento-San Joaquin Delta. French Camp Slough drains a total area of approximately 250 square miles, with a maximum watershed elevation of 2,700 feet MSL(City of Stockton, 1990). French Camp Slough is the tidewater terminous of the Little Johns Creek system. Due to historical modifications for various purposes,former connections with Duck Creek have been eliminated,except under flood flow conditions(City of Stockton, 1990). The Department of Water Resources has collected streamflow data since 1950 on French Camp Slough at Airport Way(DWR, 1985). Site Hydrology Topographic modifications of the landscape associated with the current landfill,and historical agricultural practices have influenced site hydrology. The North Branch of the South Fork Little Johns Creek flows through the project site,along the eastern and southern margin of the existing landfill area(Figure III.K.1). The channel meanders for a total channel distance of about 6,500 feet on the site,with a bed gradient of 0.0411 percent (R.W. Beck and Associates, 1992b). The modified natural channel is of fairly uniform width and depth throughout the site,typically 25 to 35 feet wide, and 10 to 12 feet deep. Channel banks are earthen and steep,and a strongly cemented soil hardpan forms the channel bed and lower bank for most of the reach. Bank erosion and down cutting of gullies through the banks has occurred in several locations along the channel,particularly on the outer bank of stream bends. Surface runoff draining from the landfill surfaces has contributed to the formation of gullies perpendicular to the banks. Riprap has been placed in strategic positions to absorb stream energy on channel bends. A small agricultural drainage ditch empties into Little Johns Creek on the eastern edge of the project, via a piped outfall near the existing landfill entrance. Drainage ditches that parallel the western property boundary also have piped outfalls that discharge to the creek on site (Figure III.K.1). The stormwater drainage system for the CYA facility discharges through an engineered open channel along the northern boundary of the proposed project to a detention basin (see Figure III.K1). The basin outfall to Little Johns Creek is located on the upstream side of the landfill entrance. III.K2