SECT. 3] ESTUARIES 313 



seaward at all depths. This advective process tends to drive salt out of the 

 estuary. The salt balance is maintained by a compensating non-advective 

 longitudinal flux of salt from the sea toward the head. 



Again, in contrast to the lack of vertical and lateral variation in salinity, 

 there exists a significant variation in the vertical gradient of the horizontal 

 pressure field. The longitudinal variation in salinity provides for a decreasing 

 seaward slope of the pressure surfaces with increasing depth. We are unable at 

 this stage to suggest what opposite force similarly varies with depth. It would 

 appear to be in some way associated with a vertical variation in the asymmetry 

 of the tidal flood and ebb. A better understanding of this aspect of estuarine 

 dynamics may follow from more precise tidal velocity measurements and a 

 more critical theoretical examination of the non-linear term in the tidal 

 equation. 



It is, in fact, quite possible that the vertically homogeneous estuary does not 

 exist. Our observational methods may not be sufficiently sophisticated to show 

 the slight degree of vertical stratification which might, on the average, exist in 

 such systems. Only a small vertical stratification would be required to remove 

 some of the anomalous factors mentioned above which are associated with this 

 class of estuary. 



D. The Fjord Type of Estuary 



The foregoing discussion has been concerned with estuarine types associated 

 with relatively shallow basins. The long and deep inlets of the Canadian Pacific 

 coast present conditions which facilitate the study of certain aspects of estua- 

 rine circulation, but which, on the other hand, make observation and study 

 more difficult. 



Pickard's (1956) comprehensive examination of the geomorphology of 

 British Columbia inlets has demonstrated the wide variation in depths, widths 

 and lengths, but confirms that the important difference between these bodies of 

 water and the estuaries of the Atlantic coast lies in the greater relative depth 

 of the former. 



In many inlets the fresh-water discharge, generally concentrated at the head, 

 is sufficient to establish and maintain a shallow layer of brackish water over- 

 lying a deep reservoir of denser salt water. The horizontal variation of salinity 

 in the deep water is relatively small when compared to the horizontal gradient 

 in the upper layer. Tully (1949) has indicated that the essential characteristic 

 of the circulation in such an inlet is the entrainment of sea-water from below 

 into the less saline seaward-flowing layer above. 



Cameron (1951a) was able to demonstrate that the distribution of salinity in 

 such an inlet is sufficient to provide for a seaward-accelerating flow of water in 

 the surface layer in the presence of reasonable values of eddy viscosity associated 

 with the simple vertical velocity profile he assumed. He also suggested that the 

 field accelerations necessary to maintain a steady state demand an increasing 

 proportion of the total pressure-gradient force toward the mouth of the inlet, 

 leading to an upper limit of the permissible seaward velocity, analogous to the 



