Papers in Marine Biology and Oceanography, Suppl. to vol. 3 of Deep-Sea Research, pp. 346-357. 



The accumulation of river water over the continental shelf 

 between Cape Cod and Chesapeake Bay * 



By BosTwiCK H. Ketchum and D. Jean Keen-]- 

 Woods Hole Oceanographic Institution 



Summary — The depth mean salinities for the waters of the continental shelf between Cape Cod 

 and Chesapeake Bay show a seasonal variation in the concentration of river water. The spring and 

 the winter accumulations are about the same, but about 25% more river water is present in the 

 summer. The total volume of fresh water in spring and winter is equivalent to that produced by the 

 rivers in about one and a half years. The extra accumulation in summer is equal to half a year's 

 flow, and reflects, in part, the fact that the high spring flows of two successive years are present on the 

 shelf at this time. 



There is a decrease in the average content of river water in the direction of the flow of the coastal 

 current, in spite of the addition of river water along its course. It is concluded that considerable 

 transport of river water and of salt normal to the coast is necessary. The horizontal mixing coeffi- 

 cients normal to the coast are computed from the seasonal changes in salinity. They range from 0-58 

 to 4-96 X 10* cm^/sec, with the values for the decrease in salinity from spring to summer being 

 smaller than those for the increase from summer to winter conditions. At both times, the values 

 decrease with increasing depth and distance from shore. 



Professor H. B. Bigelow initiated extensive studies of the distribution of tempera- 

 ture and salinity over the continental shelf between Cape Cod and Chesapeake Bay 

 in 1913. In a series of papers, he and his associates have described the seasonal 

 variations and correlated these with vernal warming, river runoff and the water 

 circulation of the area. 



The sea water of this section of the continental shelf is greatly diluted by the fresh 

 water continuously supphed by several major rivers. Both regional and seasonal 

 variations in salinity are great, and the area is one of comphcated dynamic structure 

 and circulation. The accumulation of river water affords a direct means of evaluating 

 the rate of the circulation and mixing when compared to the rate of discharge of the 

 rivers. This approach has been used extensively in estuaries in recent years. While 

 it may be unorthodox to treat this open coastal area as an estuary, it was felt that 

 some of the methods of study should be informative, and provide a time scale for 

 the circulation in various parts of the region. 



The area included in this study (Fig. 1) extends across the continental shelf and 

 slope from the coastUne to the 1000 fathom depth contour. It is bounded to the 

 north-east by a section extending SSE from the western tip of Martha's Vineyard, 

 and to the south by a section extending ESE from the mouth of Chesapeake Bay. 

 This region has a surface area of 104 x 10'" ft^ or 29,000 square nautical miles, and 

 contains a volume of water equal to 732 x 10'^ ft' or 3400 cubic nautical miles 

 (Table I). 



* Contribution No. 775 from the Woods Hole Oceanographic Institution. 

 t Present address: U.S. Navy Hydrographic Office, Washington, D.C. 



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