Delineation of ground-water contributing areas of streams of southwest Suffolk County, New York

The hydrologic system of southwest Suffolk County consists of two major components--a deep, southward-flowing ground-water system bounded on the north by Long Islands ' regional ground-water divide and on the south by the south-shore bays and ocean, and an overlying series of shallow ground-water flow systems that are separated from one another by north-south-trending interstream water-table divides. The shallow flow systems are the source of base flow to all streams on the island. The ground-water-contributing areas of southwest Suffolk County 's 21 streams are delineated on a map. The drainage-divide locations were derived from a 1975 water-table map on the assumption that flow in the shallow flow systems is two dimensional (horizontal). This assumption is valid for the southern part of the area (adjacent to flowing streams), but, because the direction of flow in the northern part, near the regional ground-water divide, is mainly downward, the delineations for the northern part are only inferred. (USGS)