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Sustained flows of fresh water greater than 3,500, 4,400, and 5,300 cubic feet per second into the Delaware River estuary at Trenton, N.J., assure lew salinity at League Island, Eddystone, and Marcus Hook, respectively. When the discharge at Trenton is less than these critical values, salinity is very sensitive to change in discharge, so that a relatively small decrease in fresh-water discharge results in a relatively great increase in salinity. Comparison of the dischargesalinity relations observed for the 14-year period August 1949-DeceTiber 1963 with relations proposed by other workers but based on other time periods indicate that such relations change with time and that salinity is affected not only by discharge but also by dredging; construction of breakwaters, dikes and tidal barriers; changing sea level; tidal elevation; tidal range; and wind intensity and direction. INTRODUCTION Anyone interested in water management in the Delaware Eiver estuary longs for some simple formula that will enable him tc estimate the fresh-water discharge required to maintain a particular salinity at, or downstream from, a specified location. No such formula has yet been developed, but the behavior of the river in this respect over a 14year period can be reported concisely. The U.S. Geological Survey, in cooperation with the city of Philadelphia, measured the chemical quality of tidal Delaware Ri^er water at eight locations once each month from August 1949 to th<$ present time. These analyses are summarized in graphic and tabular form in a report prepared by the Geological Survey (Keighton, 1965). In the present report these analyses are examined to determine whether any general statements can be made about the relation between fresh-water discharge and the salinity as measured by the concentrators of dissolved solids or chloride in the river water. Gl G2 HYDROLOGY OF TIDAL STREAMS Only the samples taken at League Island, Eddystone, and Marcus Hook are considered, for it is in this reach that salinity invasion generally takes place. The monthly samples were not all taken at the same tidal stage, but at random times. The analyses were grouped according to the antecedent fresh-water discharge at Trenton, N.J. For example, the analyses for all samples taken at water discharges cf 1,500-2,500 cfs (cubic feet per second) are in one group, those taken at discharges of 2,501-3,500 cfs are in another group, and so on. Because the samples in each group were taken at random tidal stages, the specific conductance varies within any group. The median specific conductance for any group of discharges, however, probably best represents the specific conductance of the specified water discharge ?t mean tide. From specific conductances one can readily estimate dissolved-solids or chloride concentration. Before relating salinity to discharge, "fresh-water discharge" must first be defined. Obviously, the daily discharge on the day the sample was taken cannot be used, for the salinity of the water depends upon the fresh-water discharge for some days preceding the day of sampling. At a flow of 4,000 cfs, it would take 49 days to replace the high-tide volume of water between Trenton and Marcur Hook. Dye experiments with the Corps of Engineers' Vicksburg Delaware River Model (U.S. Army Corps of Engineers, 1962) indicated that the mass transit time of water from Trenton to Marcus Hook, when the flow at Trenton is 4,000 cfs, is 43 days. The salinity is flushed seaward by any flow of fresh witer originating upstream from the salt front that flowing into the estuary at Trenton, that from tributaries below Trenton, and that from the "fresh-water lake" in the river between Trenton and the salt front. In this report, however, "fresh-water discharge" will mean inflow into the estuary at Trenton and will neglect tributary flows. Most of the water samples were taken early in the calendar month. Because the flow prior to sampling is a more significant factor in determining the salinity than the flow on the day of sampling, the flows used in the correlations are the average flows for the calendar month preceding the date of sampling. The results would be little changed if a shorter period than 1 month had been used. The median values of specific conductance were then plotted against average freshwater flow at Trenton for the preceding calendar month, and smooth curves drawn through the plotted points. No doubt the concentration of dissolved solids or of chloride in the river water is a more useful variable than the specific conductance. The dissolved-solids concentration can readily be estimated from the conductance by the relations: DISCHARGE-SALINITY RELATIONS, DELAWARE RIVER G3 £=0.52,5:+16 for K<300 micromhos £=0.60#" for #=300-4,000 micromhos where S= dissolved solids, in parts per million, and K specific conductance, in micromhos at 25°C (Keighton, 1965, figs. 9,10). DISCHARGE-SALINITY RELATION The dissolved-solids concentration is plotted against frerh-water discharge in figure 1. It is evident that the fresh-water flow is most effective in changing the concentration when the flow is less than a certain critical value. The critical flow values for League Island, Eddystone, and Marcus Hook are, respectively, 3,500, 4,400, and 5,300 cfs. For example, at flows of more than 5,300 cfs, the dissolved-solids concentration at Marcus Hook decreases only 16 ppm for each 1,000 cfs

Fresh-water discharge salinity relations in the tidal Delaware River