312 CAMERON AND PRITCHARD [CHAP. 15 



by both horizontal and vertical advection of salt and by the non-advective 

 vertical flux. 



C. The Vertically Homogeneous Estuary 

 When mixing due to tidal motion becomes so vigorous that vertical homo- 

 geneity results, the dynamic and kinematic processes are associated with the 

 horizontal variation in the distribution of salinity. The degree of lateral homo- 

 geneity has an important bearing on the type of circulation which occurs. 



a. With lateral variation 



Estuaries which are vertically homogeneous may show not only a variation 

 in salinity in a longitudinal direction, but also a variation in salinity across the 

 axis. In this type of estuary the salinity on the right of an observer looking 

 seaward is lower than on his left. The main stream of seaward-flowing water is 

 concentrated to the right of center ; the compensating flow of salt water, instead 

 of being found below the outflowing current, is concentrated on the left side of 

 the estuary. The existence of a cyclonic circulation within the estuary becomes 

 more evident. 



The absence of vertical gradients of salinity suggests that the salt distribu- 

 tion in such an estuary is not governed by vertical processes, but rather by 

 lateral and longitudinal transfers by advective or non-advective means. On 

 the other hand, it is recognized that vertical mixing of water elements is 

 responsible for the homogeneity in a water column. The apparent inconsistency 

 of these two factors arises from an as yet unsatisfactory representation or 

 description of the non-advective flux process. More precise and sophisticated 

 monitoring and interpretation of the short-term variabilit}' of salinity in the 

 vertical are desirable before a satisfactory understanding of the role of vertical 

 mixing processes is achieved. 



In contrast to the apparently monotonous vertical profiles of salinity, there 

 exist significant variations in the horizontal pressure gradient with depth. 

 Vertical shears of velocity occur and important vertical fluxes of momentum 

 are present. The magnitudes of these fluxes of momentum must be balanced 

 by an appropriate pressure-gradient field, which is established and maintained 

 by the free-surface slope and the horizontal variation in salinity. This again 

 underlines the intimate relationship that must exist between the salinity 

 distribution and the dynamic influence of the eddy frictional forces deriving 

 from the intense tidal mixing. 



b. With lateral homogeneity 



In certain estuaries, generally those with a relatively small width-to-depth 

 ratio, the cyclonic circulation described above is not observed. The movement 

 of water is essentially symmetrical about the main axis of the estuary. Varia- 

 tions in velocity are associated with the flood and ebb of the tide, and when 

 the velocities are averaged over several tidal cycles the net flow is apparently 



