308 CAMERON AND PRITCHARD [CHAP. 15 



A . The Highly Stratified Estuary 



a. The frictionless model 



We shall first consider an idealized coastal-plain estuary consisting of an 

 elongated embayment in the coastline, into the upper end of which a single 

 river source of fresh water empties. There is free connection with the sea at 

 the lower end. 



We shall assume the absence of tidal movements and that no significant 

 mixing or friction is present. Under these conditions the sea-water would 

 extend into the embayment along the river bottom to a position where the river 

 surface is approximately at sea-level. The river water would flow out over the 

 salt-water layer unimpeded by any frictional drag along its lower boundary. 

 After entering the estuary, the fresh water would tend to hug the right shore 

 of the embayment (looking toward the sea, in the Northern Hemisphere) under 

 the influence of the Coriolis force. The interface between the two layers would be 

 horizontal in the direction of flow but inclined downward to the right per- 

 pendicular to the flow, providing a transverse pressure force in the fresh upper 

 layer and level pressure surfaces in the salt-water layer. The slope of the 

 upper surface of the fresh water would be that governed by its geostrophic 

 flow. 



The distribution of salt and fresh water in such an idealized estuary would 

 thus be compatible with the simplified dynamic conditions we have imposed. 

 The circulation would consist merely of a static salt-water body and a seaward- 

 moving stream of fresh water whose volume transport would be equivalent to 

 that of the river discharge. 



But such a situation does not occur in natural estuaries. Some mixing of the 

 two types of water takes place. It is due to the variation in mixing agencies 

 and of their vigor that variations in estuarine circulations are established. 



b. The non-tidal model 



We shall next consider an estuary in which no tidal movement occurs, but 

 where frictional forces are no longer negligible. This situation can be visualized 

 by assuming an increased rate of river flow such that interfacial waves between 

 the fresh surface water and the underlying salt water become unstable and 

 break. This is essentially a one-way process which leads to an entrapment of 

 some of the salt water in the upper fresh-water layer. The salt content of the 

 upper water is increased as it moves toward the sea and the seaward volume 

 transport is increased by the amount of sea-water entrained. There must be a 

 slow compensating flow of sea-water toward the river to maintain the continuity 

 of volume within the estuary. 



It is to be expected that owing to the entrapment of salt water from the 

 underlying layer into the surface water, the upstream margin of the salt water 

 would retreat seaward due to this process alone. However, the retreat is also 

 associated with the disturbed force distribution which results from the process 



