Ground water in the Hudson River basin occurs in unconsolidated deposits and consolidated rock. Sand and gravel units of the unconsolidated deposits, which occur principally in valley bottoms, form the best aquifers and commonly provide well yields of several hundred gallons per minute. Carbonate aquifers are the most productive consolidated rock units. Ground water in the Hudson River basin is generally hard and may contain appreciable amounts of iron, salts in solution, or sulfur locally. Basic data on the availability of ground water in the Hudson River drainage area are compiled in (1) a hydrogeologic map of the drainage basin; (2) a table of well depths, yields, concentrations of selected chemical constituents, and hardness of ground water, listed by county and aquifer type; (3) a short text describing the occurrence of ground water in the basin; and (4) a bibliography of ground-water reports pertinent to the area studied. (Woodard-USGS)