352 BosTwiCK H. Ketchum and D. Jean Keen 



In the summer period the fresh water content is at a maximum for all fo the depth 

 ranges, decreasing from 7-86% for the innermost band to 0-41 % for the waters over 

 the continental slope. Within the 30 fathom contour, there is an increase of fresh 

 water content from the Martha's Vineyard section to the offing of the Hudson 

 River, but to the southward there is a gradual decrease. Beyond the 40 fathom 

 contour the fraction of fresh water shows little variation from north to south for the 

 various areas. 



The conditions observed in the October-March period have marked similarities 

 to the spring distribution. The inshore band is least fresh (6-75 %) at this time of 

 year. There is, however, a considerable accumulation of fresh water from the middle 

 of the shelf to the 100 fathom contour over the southern part of the area. All of the 

 areas south of New York between the 30 and the 100 fathom contour contain greater 

 proportions of fresh water than are found in these areas in the spring, though they 

 are consistently less than the concentration found in these areas in the summer. 



The decrease in the concentration of fresh water over the inner part of the shelf to 

 the south is remarkable. As mentioned above, there is a consistent southward drift 

 of the inshore waters, at least below New York, and there are substantial increments 

 of fresh water from the Hudson, Delaware and Chesapeake Bay systems. Thus in 

 each succeeding area some of the river water entering from the adjacent area, and the 

 water locally added, must be dispersed across the shelf, and mixed locally with higher 

 salinity water to give the observed decreases in average concentration. Although the 

 20 and 30 fathom contours are further off shore in the south, the volume available 

 for dilution (Table I) is not enough greater to give the observed decrease in the con- 

 centration of fresh water. Qualitatively, such a distribution can be maintained only 

 when the transport of fresh water normal to the coastUne exceeds the transport 

 parallel to the coast. This will be discussed further in the section on diffusion co- 

 efficients. 



THE FLUSHING TIME 



The ratio between the volume of river water accumulated and the rate of river 

 flow gives the flushing time for the region. This has been calculated for the various 

 depth ranges at three times of year with the result shown in Table III. The accumu- 

 lation of river water during the spring (April-June) agrees closely with the accumula- 

 tion found in the winter period (Oct.-March) and corresponds to the contribution 

 from the rivers during a period of about one and a half years (1-61-1'65). The accumu- 

 lation during the summer period (July-September) is about 25 % greater than at the 

 other times. Using the annual average river flow, this corresponds to nearly half 

 a year of excess. This excess does not necessarily indicate a slower circulation at this 

 time, but may reflect the excess river flow during the period of maximum discharge. 

 The lag of 3-4 months before this excess discharge is recognizable over the shelf is 

 not unreasonable for the time required for the exchanges through the estuaries and 

 sounds. With a three-month lag period, this 25% excess accumulation during the 

 summer corresponds to expectation since " fifty per cent of the total annual discharge 

 of river water is concentrated in March, April and May " (Bigelow and Sears, 1935, 

 p. 88) and since the excess spring flows of two successive years are represented by 

 the accumulation during the summer. The surface inshore waters reflect this increased 

 flow within a month after maximum river discharge, but on the average the maximum 



