The site is underlain by alluvial sediments which can be divided into three hydrostratigraphic units. These units consist of the upper alluvial unit (UAU), the middle alluvial unit (MAU), and the lower alluvial unit (LAU).

The UAU varies in thickness; however, in the vicinity of the study area, the thickness of the UAU is approximately 120 to 180 feet. The UAU consists primarily of unconsolidated sand, coarse gravel, cobbles, and boulders in this area. Groundwater occurs at depths ranging from approximately 55 feet to approximately 100 feet, with up to 90 feet of saturated thickness. The saturated thickness of the unit changes with the time of year, but generally decreases to the north. Groundwater in the UAU appears to be flowing in a west-southwest direction.

The MAU primarily consists of unconsolidated silt, clay, and interbedded fine sands. Relatively thin layers of coarser deposits are scattered throughout the unit. The thickness of the MAU ranges from non-existent to approximately 660 feet, increasing in thickness to the east from the western basin margin. Groundwater flow in the MAU appears to be toward the north-northwest in the study area. Water levels in wells perforated in the MAU occur at depths of 85 to 145 feet.

The LAUconsists of moderately to well-cemented sands and gravel. The depth of the unit is not well defined; however, it is known that the LAU is underlain by the red unit which consists primarily of fanglomerate, conglomerate, and sandstone. The direction of ground water flow in the LAU is northerly toward the regional pumping center.

Water level data indicate that there is a downward-directed, vertical hydraulic gradient between the UAU and the MAU, and between the MAU and the LAU.