Altitude of the potentiometric surface ranged from more than 20 feet below mean sea level in Queei1S County to more than 40 feet above mean sea level in Suffolk County.
Hydrograph-separation techniques were used to quantify the 1976-85 base flows of 10 continuously gaged streams on the south shore of Long Island. Base flow is the water that enters a stream channel as discharge from the groundwater reservoir the "fairweather" flow of the stream. Base flow during 1948-52, the last 5 years before extensive urban development, constituted about 95 percent of the total annual stream discharge, but in 1976-85, it averaged 14 percent in streams in a highly urbanized, sewered area; 79 percent in streams in a less urbanized, more recently sewered area; 88 percent at streams in a suburban area in which sanitary sewerage is nearly complete; and 96 percent at streams in an unsewered area where development is minimal. A major cause of base-flow decreases on Long Island has been a lowering of the water table as a result of urbanization. The principal factors that cause this lowering include a decrease in the amount of permeable (unpaved) area, the routing of storm runoff directly to streams through storm sewers, and sanitary sewers, all of which intercept recharge and prevent it from entering the groundwater system. Water-level declines and the attendant losses of base flow are minimized in areas where stormwater is routed to recharge basins. Flow-duration analysis shows that urbanization also causes an increase in the magnitude and frequency of high flows and in the flow variability of each stream. These effects currently are seen as far east as Carlls River in southwestern Suffolk County. Doublemass-curve analysis shows that a new base-flow equilibrium has been reached at the three westernmost streams studied.
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