SECT. 3] ESTUARIES 311 



cross-channel inertial force of significance in the lateral equation of mean 

 motion. 



If these effects of the tidal oscillations on the mean pressure distribution in 

 the estuary are independent of depth, they will be equally significant from the 

 top to the bottom and will be detected only when the absolute slopes of the 

 pressure surfaces are examined. However, if there is a large variation in tidal 

 oscillations with depth, the mean effect of the non-linear terms might be 

 expected also to vary with depth. On the average, therefore, a varying fraction 

 of the mean pressure gradients might be required to balance these non-linear 

 effects. Thus it is possible to conceive of a portion at least of the spatial varia- 

 tion of salinity within the estuary being governed by the inertial elements of 

 the tidal motion, in addition to the mixing resulting from the motion of the tide 

 itself. 



Preliminary examination of the possible magnitude of the inertial effect 

 described above suggests that it is probably of secondary importance compared 

 to the effect of the flux of momentum associated with turbulence. 



In our consideration of the moderately mixed estuary, we have emphasized 

 the forces that act mainly in the direction of flow. As in the case of the salt- 

 wedge estuary, the effect of Coriolis force will be evident laterally at right angles 

 to the main direction of flow. There will be a tendency for a thicker and fresher 

 layer of surface water to be present to the right of an observer looking seaward 

 down the estuary. Pritchard (1956) has shown that the distribution of salinity 

 across the James River estuary indicates that the Coriolis force resulting from 

 the net horizontal motion is balanced primarily by the lateral pressure force. 

 A similar balance in a deep inlet was reported by Cameron (1951). 



It has been inferred that the horizontal pressure gradients in the longitudinal 

 direction which are established by a sloping water surface and modified by the 

 internal distribution of salinity are largely balanced by the frictional forces 

 associated with tidal turbulence. Reference should be made to the magnitude 

 of the forces required to accelerate the surface water seaward and decelerate 

 the opposite flow of the deeper water. Pritchard (1956) has evaluated the field 

 accelerations at a typical location in the James River estuary and shows them 

 to be relatively small in comparison with the frictional terms. It would be 

 expected, however, that the field acceleration terms would increase in im- 

 portance as the salinity of the surface water more closely approached that of 

 the sea-water. Stommel and Farmer (1953) have shown, for example, that the 

 interrelation of the inertial term and the degree of salinity stratification in 

 certain estuaries sets an upper limit on the volume of mixed water discharged 

 past a control. They suggest that St. John Harbor (New Brunswick) represents 

 such a condition. 



The Chesapeake Bay and its tributary estuaries exhibit the characteristics 

 of the moderately stratified estuary. Bousfield (1955) has reported the estuary 

 of the Miramichi River to be of a similar type. The Savannah, Charleston and 

 Delaware estuaries might also be classified as moderately stratified. 



In estuaries of this kind the distribution of salinity is kinematically governed 



