S%« 26 



(a) 



Fig. 



(b) (c) 



-Idealized vertical salinity gradients showing type boundaries 



The upper and boundary zones become progressively more saline to 

 seaward as they degenerate to obscurity. Evidently sea-water is 

 being transferred upwards and retained in the upper zone, and no fresh 

 water is being transferred below the boundary. This identifies the 

 boundary as the zone into which fresh water is transferred in the mixing 

 process, and from which it is preferentially returned to the upper zone, 

 along with the associated sea- water supplied from the lower zone. 



From this it may be concluded that all fresh water leaves the region 

 by way of the upper zone, and that the fresh-water influence is finally 

 lost by infinite dilution with sea-water, when the upper zone becomes 

 indistinguishable from the lower zone. 



Evidently the upper zone is subject to two accelerations — the 

 displacement of fresh water, which is constant through any complete 

 cross section, and the transfer of sea-water from the lower zone. The 



274 



