Discontinuous Outflow 



Both the character and volume of the outflow vaxy with the tide. 

 When the tide is falling, the upper zone has free egress from the estuary 

 to the sea, since the flow in the lower zone is also seaward. During this 

 tidal phase the upper zone tends to move rapidly away from the estuary, 

 in a balance of gradient and displacement flow, modified by the tidal 

 currents in the vicinity, as shown by the low tide distribution of fresh 

 water in Fig. 7. 



In the presence of rotary tides the direction of the stream may be 

 seen to alter from time to time during the discharge. Usually it will 

 flow in one direction for a time, and then change direction at the source 

 quite suddenly, leaving a cloud of upper zone water isolated in the 

 seaway. 



When the tide rises sea-water usually invades the estuary, forming a 

 front near the channel mouth, as has been described. Usually the 

 upper-zone water discharged during the ebb has been removed from the 

 vicinity by the local current systems, so that sea-water intrudes the 

 estuary. This movement has the effect of cutting off the upper-zone 

 water extruded during the previous ebb, which then floats away as a 

 cloud. This mechanism is indicated in the high tide synoptic of Fig. 7. 



When separated from their source these clouds of upper-zone water 

 lack the isostatic forces to continue their flow and are pelagic in the 

 surface of the seaway, where it is common to observe considerable 

 variations of current and direction in short distances. In some circum- 

 stances the successive upper zone clouds may follow quite different 

 courses(2). When this phenomenon is observed in the usual manner 

 by water sampling at intervals of distance it may appear that the flow 

 from the estuary is forked, whereas a truer interpretation would be that 

 clouds of water are discharged alternately in the several directions. 



Evidently these clouds will tend to expand radially over the sea 

 surface under the influence of their own isostatic potential. In this 

 case they should tend to rotate, because of the Coriolian force, and 

 should be deepest at the centre. The formation and existence of these 

 clouds of discharged water have been observed (9), but to present know- 

 ledge they have not been studied. 



In the immediate vicinity of the outflow into a large seawaj' or the 

 ocean it has been observed(2, 3, 6) that the upper-zone flow tends to 

 follow the course that would be anticipated from dynamic topography 

 of the sea surface. However, the upper zone appears to contain clouds 

 of fresher water, corresponding to the discharge during successive tidal 

 periods, since the depth and composition of the upper zone fluctuates 

 along the line of flow. In general, the system tends to become more 

 saline and regular with distance from the parent river. 



In the simple system (1) the anticipated variation of depth of the 

 upper zone with the tide was observed. However, in a complex system(2) 

 the tidal variation was completety masked by a secondary variation of 

 equal magnitude, as illustrated in Fig. 8. This apparently random 

 variation could be explained by internal waves or by the discontinuous 

 nature of the outflow, or possibly both. Associated data indicates that 

 the boundary layer did oscillate in short periods, and that a cloud struc- 

 ture was present. This is indicated in the charts of Fig. 7, and probably 

 explains the large variations in depth of the boundary shown in the 

 corresponding salinity gradients. 



284 



