PHYSICAL OCEANOGRAPHY OP THE GULF OP MAINE 855 



At most times there is no dominant drift into the gulf across Georges Bank, 

 but on rare occasions overflows of tropic water take place at the surface, probably 

 via that route. 



The discharges of various rivers, added to rainfall, contribute annually to the 

 gulf sufficient fresh water to form a layer half a fathom thick over its inner parts 

 out to Georges Bank. The gulf also receives annually a blanket of rain water about 

 a foot in thickness, in excess of the amount withdrawn by evaporation. 



The gulf discharges water as a surface current around Nantucket Shoals to the 

 westward; to some extent around the eastern end of Georges Bank, ^^ and so out 

 through the Eastern Channel. 



It is not likely that the gulf ever receives water from the oceanic abyss, by 

 up welling, or directly from the Labrador current. 



CIRCULATION IN THE GULF OF MAINE 



Study of the circulation that dominates any part of the sea can be attacked in 

 two different ways: (1) Directly, by observation with current meters or drift bottles, 

 by ships' log books, and by interpreting the distribution of salinity and temperature, 

 or (2) indirectly, by calculation of the hydrostatic forces tending to set the water in 

 motion. The second method has greatly concerned oceanographers of late, and its 

 value can hardly be overestimated in the study of ocean currents in the open sea; but 

 its application to the Gulf of Maine is complicated by the disturbing factors intro- 

 duced by the irregular contour of the bottom, the limiting coast line, and the strong 

 tides, which not only produce currents of considerable velocity, but are constantly 

 altering the slope of the surface. It is fortunate, therefore, that the following account 

 can be based on the more direct methods of observation, supported by consideration 

 of hydrodynamic forces as causative agents (p. 930). 



TIDAL CURRENTS 



No one can sail the Gulf of Maine without soon learning that its tidal currents 

 run so strong that they must always be taken into account in coastwise navigation. 

 Their velocities are so great, in fact, in most parts of the gulf, at the strength of ebb 

 and flood, that for the ordinary observer they entirely obscure any dominant or 

 nontidal drift that may be in progress. 



No attempt has been made to add to the knowledge of the tides during our 

 survey; but the following brief statement, condensed from the Coast Pilot, the 

 tide tables and current tables of the Atlantic coast published by the United States 

 Coast and Geodetic Survey (1923 and 1926), from the investigations of the Tidal 

 Survey of Canada (Dawson, 1905 and 1908), and from other scattered sources, may 

 be of interest.^' 



The flood, at its strength, runs northerly, the ebb southerly, along the whole line 

 between Nantucket Shoals and the Northern Channel and likewise in the basin to 



" For discussion of the discharge from the gulf see p. 974. 



" In 1912 the Grampus recorded the velocity of the current near the mid-period of flood or ebb, hoping to learn the approii- 

 mation of the direction and velocity at its strength. The value of these measurements is discussed in an earlier report (Bigelow, 

 1914, p. 83). 



