Streamflow patterns on Long Island, N.Y., have been changing over the last three decades in response to eastward population growth and urbanization. Flow-duration curves for nine streams in Nassau and Suffolk Counties indicate that, during this time, streams in urbanized areas have had increased peak flows and decreased base flows, whereas streams in sparsely populated areas have shown little or no change. The increase in peak flows reflects the greater volume of storm runoff from impermeable surfaces such as roads, parking lots, and roofs. The decrease in base flows results from the lowering of groundwater levels, principally by sanitary sewerage and extensive pumping for public water supply. At East Meadow Brook, in a sewered, densely populated area of Nassau County, peak flows (1-percent duration) increased by 41% during 1946-74, and base flows (98-percent duration) decreased by 99%. In contrast, peak and base flows of Carmans River, in a largely undisturbed, rural area of Suffolk County, showed fluctuations no greater than 19% during the same period. (USGS)