SECT. 3] ESTUARIES 309 



of entrapment. In effect, not only is there a flux of volume into the fresh-water 

 layer but also a flux of momentum, which acts on the upper layer as a force 

 directed upstream. If a steady state is to be maintained this force must be 

 balanced by an equal pressure force directed downstream. Such a force is 

 established by a redistribution of density in the vicinity of the interface. This 

 is facilitated by a tilt of the interface and an associated slope of the upper 

 surface of the fresh water seaward. As the flow of the river increases, so does the 

 frictional drag at the interface, with a consequently greater tilt to both the 

 upper surface and the interfacial boundary, and a retreat of the upstream 

 margin of the salt water further seaward. Keulegan (1940) has described such a 

 process in his study of stratified flows in flumes. 



It should be noted that the movement of sea-water toward the land is 

 assumed to be relatively slow with a negligible frictional drag along the bottom 

 of the estuary. Under this assumption the vertical gradient of the slopes of the 

 pressure surfaces within the salt-water layer would be extremely small. Very 

 little variation of density distribution within the salt-water wedge would occur. 



In his original discussion of such a system, Pritchard (1955) pointed out that 

 the two dominant processes in maintaining the salt balance are the horizontal 

 and vertical advections of salt. We have emphasized the influence of the flux 

 of momentum and the consequent compensating adjustment of the density 

 field. It is clear that the maintenance of a steady state both of motion and of 

 salt demands a nice adjustment of both these processes. However, it would 

 seem that the character of the salt-wedge estuary is determined largely by 

 the type of salt flux that prevails ; that is, flux resulting primarily from advec- 

 tion across the interface boundary. 



The salt-wedge estuary occurs when the ratio of river flow to tidal flow is 

 relatively large, and when the ratio of width to depth is relatively small. The 

 mouth of the Mississippi River shows the essential features of such an estuary. 



B. The Moderately Stratified Estuary 



The next type of estuary occurs when the prime source of mixing is no longer 

 the velocity shear along the interface, but rather the turbulence resulting from 

 tidal motion. 



If we assume a tide of moderate amplitude impressed on the estuary des- 

 cribed above, the tidal velocities will give rise to random movements of water 

 throughout the whole water column. Instead of the mixing being primarily 

 due to an upward advection of salt water across the interface, the turbulent 

 eddies tend not only to mix the salt water upward but also to mix fresh water 

 from the upper layer down into the salt water below. This results in the content 

 of salt increasing toward the sea in both the surface layers and the deeper 

 layers. However, a vertical salinity gradient remains, with the deeper layers 

 having a higher salt concentration than the surface layers. 



This type of mixing adds a greater volume of salt water to the upper layer 

 than in the case of a salt-wedge estuary. Consequently, flow in the upper layer 



