As the displacement increases the freshet overruns the surface, 

 repeating within the upper zone the phenomenon of the upper zone 

 in the sea. This is shown by the transition from the first to the second 

 gradient. If the rise is a true freshet, of short duration, the process 

 einds there, and the original state is slowly re-established. However, 

 if the higher discharge continues at a constant level fresh water accumu- 

 lates until a new equilibrium is established, as indicated in the third 

 gradient, and the boundary finds a new level. 



It is concluded that a freshet has the effect of flushing the channel, 

 and produces anomalous values of depth and salinity, since it does not 

 represent stationary conditions. 



Dynamic Equilibrium 



It may be concluded from these discussions that the salinity 

 structure is recurrent within the limits of variation of wind and run-off 

 at corresponding states of the tide-cycle. It might better be regarded 

 as a probability, whose deviation from the normal or median state 

 depends on the wind and run-off history immediately before the 

 observation. 



The fresh-water content of the upper zone represents the accumulation 

 of fresh water required to provide the isostatic head necessar\^ for the 

 continuous displacement of fresh water at a rate equal to its outflow 

 from the river. Evidently this can be accomplished in a shallow, very 

 fresh zone, or a deeper, more saline zone, depending on the magnitude 

 and time of action of the extraneous mixing forces of the tide, and the 

 wind. 



The Complex Outflow 



The coastal seas are subject to tidal currents (7) which are more or 

 less rotary, so that there is no period of slack water, and there is a 

 shoreward or seaward component of flow as the tide rises or falls. 



As the outflow moves seaward, whether it be radially off-shore or 

 coastwise it will encounter an opposing tidal current on some margin, 

 and a front will be formed wherever the movements converge. 



This phenomenon is illustrated diagrammatically in Fig. 7. The 

 fresher outflow overruns the adjacent sea- water, and forms a frontal 

 surface which intercepts the sea surface. This frontal surface is 

 maintained by convergence towards, and a compensatory flow along, it. 

 At the " front " (the intercept of the boundary with the sea surface) 

 the horizontal convergence is balanced by a compensating vertical 

 flow along the boundary surface. This flow consists of water contributed 

 by both masses in proportion to the horizontal velocities normal to the 

 front. The upper zone of the outflow may move faster than the adjacent 

 sea-water at the surface, and so contribute more water at that level, 

 but generally the normal velocities decrease downwards in the fresh- 

 water mass more rapidly than in the sea-water mass, and the relative 

 contributions to the boundary may change with depth. The waters 

 contributed by the fresh and sea-water regions are sheared off the 

 front, mixed, and submerge to a position of isostatic stability below 

 the upper zone of the outflow, forming a boundary zone or, in more 

 extreme cases, an intermediate zone between the upper and lower zones. 



280 



