The Ramapo River, a major stream in the Passaic River basin, drains an area of 161 square miles, 70 percent of which is in Orange and Rockland Counties, N.Y., and 30 percent is in Bergen and Passaic Counties, N.J. This report describes the hydrology of the New Jersey part of the basin and evaluates the feasibility of developing large ground-water supplies from the stratified drift in the Ramapo River valley by inducing recharge to the aquifer from the river. The ground water and surface water of the basin are considered as a single resource because the development of either ground water or surface water affects the availability of the other. Precambrian gneiss, sparsely mantled with Pleistocene glacial drift, underlies the basin west of the Ramapo River in New Jersey. To the east, bedrock consists of the Watchung Basalt and of shale, sandstone, and conglomerate of the Brunswick Formation of Triassic age. Glacial drift occurs nearly everywhere in the eastern part of the basin, and deposits of stratified drift more than 100 feet thick occur in the Ramapo valley. Average annual runoff at Pompton Lakes accounts for 25 inches of the 45 inches of annual precipitation in the New Jersey part of the basin, and the remaining 20 inches is accounted for by evapotranspiration. Streamflow is highly variable--particularly in the area underlain by gneissic rocks-because of the low storage capacity of the rocks and the rough topography. Many of the small tributaries go dry during extended periods of no precipitation. Small domestic supplies of ground water can be obtained nearly everywhere, but the Brunswick Formation is the only consolidated-rock aquifer in the basin that can be depended upon to yield 100-200 gallons per minute to wells. Supplies of more than 1,000 gallons per minute are available from wells tapping the stratified drift in the Ramapo valley. The drift supplies 75 percent of the ground water pumped for public supply in the basin. Sustained ground-water yield in upland areas, based on stream base-flow recession, is estimated to be 200,000-300,000 gallons per day per square mile for the drift-covered Brunswick Formation and about 100,000-200,000 gallons per day per square mile for the gneiss and basalt. Potential sustained yield of the stratified drift in the valley depends on the availability of the streamflow and on the induced rate of infiltration. Pumping from the stratified drift results in a reduction in streamflow, which may be undesirable, mainly because of prior downstream water rights. On the basis of the storage available in the stratified drift and an analysis of daily flow during the drought period of October 1964 to September 1967 at Pompton Lakes, 20-25 million gallons per day of Ramapo River water are available for development after existing downstream water requirements are supplied. However, some low-flow augmentation will be. necessary to insure downstream rights. Rates of infiltration computed from seepage losses observed near Mahwah indicate that at least 11 million gallons per day, on an average basis, can be infiltrated from the river by the pumping of wells tapping the stratified drift. The use of recharge pits and spreading areas would increase the rate of infiltration. Losses from the Ramapo River could be minimized by returning treated sewage effluent directly to the river or, preferably, by recharging the stratified-drift aquifer with the treated effluent. Ground-water quality and surface-water quality at times of low-flow vary according to the type of rock from which the water is obtained. Water from the gneiss is low in dissolved solids--less than 127 mg/l (milligrams per liter)--and soft to moderately hard--less than 94 rag/l. Water from the Brunswick Formation is more mineralized--total dissolved-solids content is as much as 278 mg/1 and hardness as much as 188 mg/1. Water from the stratified drift is generally intermediate in quality--that is, total dissolved-solids content is as