PHYSICAL OCEANOGRAPHY OP THE GULP OF MAINE 
811 
this reduced the surface salinity by at least 3.2 per mille between April 9 and May 4 
(29.1 per mille) in 1920, the lowest value recorded at the mouth of the bay in 1925 
was 31.3 per mille on April 23 and again on May 22, though it is possible, of course, 
that the “peak” fell between these two dates, as already remarked (p. 741). 
A considerably higher surface value at this locality on May 4, 1915 (station 
10266, 32.3 per mille), is reconcilable on the assumption (discussed above) that the 
effects of vernal freshening were more closely confined to the immediate vicinity of 
the land in that spring. However, this record is averaged on the graph (fig. 166). 
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Fig. 166.— Seasonal progression of salinity at the surface at the mouth of Massachusetts Bay, 12 miles off Gloucester, 
based on monthly averages of the records in the various years. The data for July, 1916, are omitted for the reason 
given on p. 810 
Taking one year with another, the lowest surface salinity of the year is to be 
expected at this general locality between the last week of April and last week of May. 
Surface values lower than 31 per mille (sometimes as low as 29 per mille) are to be 
expected there at some time during this period — a decrease of more than 2 per mille 
from the maximum salinity at the end of winter. 
The vernal freshening at 7 this particular region results chiefly from the discharges 
from the large rivers to the north (nearest of these is the Merrimac), for no large 
streams empty in the immediate vicinity. Consequently, any fluctuations in the 
volume and direction of the drift past Cape Ann will be mirrored by corresponding 
