The San Luis Rey River, located in the northern region of San Diego County, has its head waters at Lake Henshaw and ends in the city of Oceanside, just before reaching the Pacific Ocean. The area of study is the alluvial basin, up to one and one-half miles south east of the town of Bonsall. This hydrologic study concentrates on the time period from December 1965 through April 1966. The basin is comprised of Holocene deposited, unconsolidated alluvium surrounded by Pleistocene and Miocene consolidated sedimentary rocks and Cretaceous crystalline rocks. The climate is semiarid receiving only 13.45 inches of rain from July 1965 to June 1966. The San Luis Rey River flows continuously December through January in the eastern, or upstream portion of the basin and December through April in the western, or downstream portion of the basin. Water contours show groundwater movement southwesterly in the basin except just east of San Luis Rey Mission were there is a cone of depression. Water table elevations range from 138.5 feet in the east to 9.8 feet in the west and as low as 2.5 feet in the cone of depression. Hydraulic gradients ranged from 0.004 to 0.010. Hydraulic conductivities were not estimated for the alluvium in this basin, but adjacent basins had hydraulic conductivities ranging from 43.2 feet per day to 668 feet per day. Assuming a hydraulic conductivity of 200 feet per day and a porosity of 0.30, pore water velocities will range from 2.67 to 6.67 feet per day. Overall the water table in the basin is higher in December 1965 through April 1966 than shown in a previous report for the year 1957, which had a cone of depression as low as 50 feet below mean sea level.