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_ The San Joaquin River data in each model cell intersected by the river includes length and width of the river channel, the normal and flood elevation of the river, the thickness of the river bed sediments, the bed's hydraulic conductivity, and the river bed r elevation. In the analysis, the thickness of the river bed sediment is assumed to be 1.0 foot with a hydraulic conductivity of 3.3x10-8 ft/sec (2.9x10-3 ft/day). The length of river channel in each model cell was defined depending upon river location in the model grid. The width of the river channel is assumed 400 feet throughout the river reach. In the modeling approach, the river stage over the 1983 flood year was assumed to be the extreme river condition, rather than the design flood plane elevation which occurs over a relatively short length of time and, therefore, would have less impact on pond performance. The 1983 flood year was selected based upon the total amount of runoff in the San Joaquin River. Runoff of the San Joaquin River as measured at Vernales was the maximum of record in 1983, passing 15,410,000 acre feet, 4.5 times the average annual discharge. River stages used in the model were as measured by the California Department of Water Resources at B95400 station (Old River Head). The river elevation data showed an approximate average river elevation of 15.0 feet during the peak runoff period in the 1983 flood year and 4.0 feet during 1992. During the _ peak runoff period in 1983, the river maintained an average river elevation of 15.0 feet for about six months. The river bed elevation is selected -9.0 feet per the levee profiles of the San Joaquin River and is kept constant throughout the river reach (Corps of Engineers, 1955). The hydrologic parameters of subsurface soils, e.g., hydraulic conductivity, specific storage, and approximate thickness of each horizon were estimated based upon the geologic information given in the Vector's June 1993 report and the log of monitoring well No. 5 on the site. In the simulation, the horizons of various soils are assumed to be continuous in the horizontal direction. The estimated horizontal hydraulic conductivity for the site is approximately 9.6x103 cm/sec (27.1 ft/day) and is calculated based upon the approximate thickness of each horizon and published typical hydraulic conductivity values for various soils (Freeze and Cherry, 1979). M APRIL 1994 CCIP/SIMULATION OF PERCOLATION POND PERFORMANCE PAGE 4