A series of five east-west and two north-south hydrostratigraphic cross sections were drawn from drillers' logs of water wells within the southern half of Cache Valley, Utah. These cross-sections demonstrate that ground water flow to streams is restricted by a continuous low- II permeability layer, nearly 100-feet thick. This layer was correlated to the lake-bottom deposits of the Bonneville (30,000 -13,000 years ago) and Little Valley (140,000 - 90,000 years ago) cycles of the ancient Lake Bonneville. The most productive aquifers in the valley, collectively termed the principal aquifer , are in the southeast corner , approximately between Smithfield and Hyrum, and between the eastern valley margin and the valley center. Sands and gravels of the principal aquifer were deposited as alluvial fans and deltas by streams draining the Bear River Range. Ground water chemistry in the principal aquifer system is of the calcium-magnesium bicarbonate type with total dissolved solids (TDS) averaging about 300 ± 100 mg/L. TDS and the relative proportions of sodium, potassium, and chloride increase down flowpath, from recharge areas in the east to discharge areas in the west. Oxygen-18 (18O) and deuterium (D) analyses were performed on precipitation samples at three locations on the east valley benches, four surface water samples from streams entering the valley, and fourteen ground water samples from either wells or springs. Precipitation and surface water values generally plotted along the Global Meteoric Water Line (GMWL), although the precipitation values plotted significantly lower on the GMWL than the surface water values. Of the ground water samples, twelve from the principal aquifer generally clustered near the surface water data points, suggesting that water from streams, rather than infiltrating precipitation, recharges the principal aquifer. Twelve ground water samples were analyzed for tritium. The tritium values of eight samples from wells or springs in the principal aquifer suggest recharge after 1952. Two samples with tritium values dating prior to 1952 are from wells in the principal aquifer, and two are from wells west of the principal aquifer. Four samples were analyzed for 14C. Two of these wells were completed in the principal aquifer and two west of it. Correcting for partial carbon dilution, the age difference between the different areas is on the order of tens of thousands of years.