The situation is dynamically stable while the front is being 

 continually renewed by a supply of water from both regions, and the 

 mixed waters are being continually dispersed at an equal rate. Its 

 position represents the balance where the acceleration in the tidal and 

 isostatic flows are equal. Such regions of convergence vary from 

 " tide lines " marked by a collection of flotsam, and/or a difference 

 in colour of the water, to violent tide rips which may endanger small 

 ships, depending on the velocities and magnitude of the opposing upper 

 zones. 



The synoptic charts of the concentration of fresh water shown in 

 Fig. 7 were compiled from data observed during four days in Chatham 

 Sound, on the Pacific Coast of Canada. The position of the tide rips 

 in relation to the water masses indicates that they represent zones of 

 convergence between flow across the isohalines and opposing movements 

 in the more saline water from the adjacent sea. The salinity gradients 

 associated with this structure were observed to be complex in almost 

 every case, as indicated in those illustrated. 



The Intermediate Zone 

 It is anticipated, and observations appear to show(3), that the inter- 

 mediate zone associated with a front (Fig. 1 (c) ) is usually of moderate 

 dimensions and restricted to the vicinity of the front, because it is essen- 

 tially a transitory state in the formation of a new boundary zone. The 

 intermediate zone is evident because the mixing process at the front is 

 proceeding faster than in the adjacent parts of the upper and lower 

 zone, and mixed water accumulates in the boundary. This process has 

 been illustrated in Fig. 3 {a) and associated discussions. As the inter- 

 mediate zone is removed from the front it tends to contribute to the 

 upper zone, and forms a simple boundary, usually at a new depth. It 

 is implied that this metamorphosis is associated with considerable 

 movement which is continuous with the convergence at the surface 

 front, and has a large component parallel to it. This situation 

 corresponds to the high tide gradients shown in Fig. 7. 



The intermediate zone may also be generated when the river outflow 

 overruns a previous discharge, which meanwhile has been considerably 

 diluted with sea-water. This is the situation illustrated in the low-tide 

 gradients of Fig. 7. It has been remarked that this is particular!}' 

 noticeable during a freshet. It also occurs in the vicinity of a river- 

 mouth, because of the oscillatory nature of the tide. By a combination 

 of these functions as many as three intermediate zones have been noted(2), 

 but it is remarked that they tend to approximate the structure of a 

 simple boundary in which the salinity gradient is a logarithmic function 

 of depth. These multiple jintermediate zones usually occur in pro- 

 tected sounds or bays where the waters may be confined for some time 

 before reaching the open sea. The data illustrated represents this 

 combination of circumstances. 



Evidently the fate of the intermediate zone in the vicinity of the 

 outflow is to become identified with a simple boundary structure. 



282 



