The Santee - El Monte Groundwater Basin, located in eastern San Diego County near the city of Santee, is an important source of groundwater to local residents and water agencies within the basin. Groundwater is encountered in the interstices of Eocene sedimentary rocks and Quaternary alluvial deposits as well as within fractures of the metamorphic and plutonic rocks of the Mesozoic basement complex. Groundwater is most abundant and most easily obtained from the alluvial aquifer which occupies the Upper San Diego River Valley within the basin. The alluvial aquifer consists of unconsolidated deposits of sand and silt with lesser amounts of gravel and clay that range in thickness from approximately 10 feet to over 230 feet. Modeling results suggest that the hydraulic conductivity and specific yield of the alluvial aquifer ranges from approximately 1 foot per day to 50 feet per day and from 0.08 to 0.18, respectively. Recharge to the alluvial aquifer is greatest in the eastern portion of the basin and is primarily a result of infiltration of precipitation and streamflow. Total dissolved solids in groundwater ranges from less than 1,000 parts per million in the eastern portion of the alluvial aquifer increasing to approximately 3,000 parts per million to the west. The maximum groundwater storage of the alluvial aquifer is estimated to be approximately 57,000 acre-feet, with a maximum yield of approximately 24,000 acre-feet or approximately 40% of the total storage. A two-dimensional areal finite-difference model was constructed to simulate groundwater flow in the alluvial aquifer to aide in future groundwater use decisions by the San Diego County Water Authority and its member agencies within the basin. The model was developed and calibrated in accordance with available data for the basin. The model was calibrated to historical and recent water level data for the alluvial aquifer with absolute error of less than six percent at all locations. The level of model calibration is tentative given the general lack of data and is, therefore, considered effectively uncalibrated in the majority of the model domain. Model simulations were performed to assess various groundwater management scenarios. In general the modeling results suggest that the greatest potential for groundwater storage and development projects is within the eastern portion of the basin where the alluvium is thickest and most permeable, total dissolved solids are the lowest, and recharge is the greatest. Groundwater development in the western portion of the basin is limited by the low aquifer transmissivity and poor water quality.