350 BosTwicK H. Ketchum and D. Jean Keen 



unusual years. The Current Atlas of the North Atlantic Ocean shows southerly or 

 easterly drifts most of the year south of Martha's Vineyard, and, when westerly 

 components are present, adjacent currents suggest an eddy which does not penetrate 

 beyond the tip of Long Island. Some of the drift bottles released by Bigelow (1927) 

 in the Gulf of Maine were recovered on Nantucket and Martha's Vineyard, but rarely 

 entered the area under consideration. Several bottles, however, which were released 

 in July 1922 outside the 50 fathom contour southeast of Cape Cod, were recovered 

 along the coast to the west. A series of drift bottles were released in October 1951 

 from the Caryn between Cape Cod and Nantucket Lightship, but none of these 

 were returned from within the area considered here. It is concluded that, although 

 there may be some influx of fresh water from the northeast into the area, it cannot 

 be evaluated, and is probably small relative to the volumes supplied directly by the 

 rivers. 



The details of the salinity distribution are given by Bigelow^ and Sears (1935). 

 The following brief review of their conclusions will be helpful in understanding the 

 general patterns. " The basic feature in the pattern of salinity of the region, the year 

 around, is that isohaUnes tend to parallel the coastal trend with values increasing 

 continuously from the shore, seaward, along any given profile normal to the coast" 

 (p. 6). The salinity is at a maximum during the winter months when water of salinity 

 lower than 32° /oo is found only in the mouths of Delaware and Chesapeake Bays. 

 About 50% of the total annual discharge of river water is concentrated in March, 

 April and May. This produces local and irregular freshening off" the mouths of the 

 rivers. The salinity off" the Husdon River and Chesapeake Bay falls to 21° I ^o 8-10 

 miles from land, and all of the inshore water is freshened to 32°/oo or less at this time. 

 This vernal freshening may not culminate before late summer in exceptional years, 

 though in all years of record the extent of water freshened to less than 32°/oo was 

 greatest during the summer. The vertical stratification was also a maximum at this 

 time, being most pronounced off" Chesapeake Bay, and least over Nantucket Shoals 

 to the northeast, where turbulence keeps the waters nearly homogeneous throughout 

 the suimner. During the autumn the vertical gradients of saUnity decrease and the 

 surface salinities increase, so that by January the distribution has returned to the 

 winter maximum condition. 



THE ACCUMULATION OF RIVER WATER 



The total volume of river water accumulated in the area has been computed from 

 the distribution of salinity. When this is compared to the rate of river flow, the 

 average flushing time can be determined. The calculations have been made for three 

 periods of the year, namely (1) April, May and June; (2) July, August and Septem- 

 ber; (3) October through March inclusive. 



o — s 



The fraction of fresh water (/) in any sample is given by : / = 



a 



in which s is the saUnity of the sample considered and o- is the saUnity of the undiluted 

 sea water. For this value we have taken 35°/oo, since water of this salinity is found at 

 the bottom along the continental slope, usually in a continuous band throughout 

 this region. Data from 856 hydrographic stations were utihzed. These included 

 stations reported by Bigelow and his associates, stations along the continental slope 

 (IsELiN, 1936), and in the New York Bight (Ketchum, Redfield and Ayers, 1951), 



