It was found that the points were considerably more scattered from 

 a simple relation than would be expected from variations in tidal height. 

 The deviations in one direction were associated with increasing run-off 

 and were designated as freshet deviation. Deviations in the other 

 direction were associated with strong winds opposing the seaward 

 direction of surface flow, and were referred to as wind deviation. The 

 biest line drawn through points representing steady run-off, or falling 

 river-level, and light winds was taken to represent the normal state. 



The form of the curves are remarkable. Evidently from low to 

 intermediate discharge level, the upper zone becomes shallower and 

 fresher, while at greater discharge it becomes deeper and more saline, 

 although the displacement increases almost linearly. These phenomena 

 may be interpreted with the aid of salinity gradients shown in Fig. 6 {a) 

 observed during the low, intermediate, and high river discharge at a 

 position near the head of the inlet. When land drainage is small the, 

 displacement is necessarily small, and the upper-zone water remains 

 in the region for a considerable time so that a deep, saline upper zone 

 is formed, as shown in the first gradient of Fig. 6 (a). As the run-off 

 increases displacement increases, and the time during which the upper 

 zone is exposed to tidal mixing in the region decreases. In consequence 

 it becomes shallower and fresher, as indicated in the second gradient. 

 Further increase of river discharge implies that the front between the 

 fresh water impounded in the channel and the intruding sea-water must 

 move further seaward. This requires increased isostatic forces, 

 wherefore the upper zone must remain fresh and become deeper, as shown 

 in the third gradient. This case implies considerable transport velocity, 

 with consequent turbulence, so that the boundary gradient is thicker 

 than in the previous case, and since it is included in the assessment 

 of the upper zone, the salinity appears to increase. 



Wind Effects 



When the surface seaward flow is opposed by strong winds it would 

 be expected that the depth of the upper zone would become greater than 

 normal, because of the increased mixing forces, and fresh water would 

 accumulate until the isostatic head was great enough to provide normal 

 displacement against the restriction of the wind. This is illustrated 

 in the sequence of group (&) gradients in Fig. 6. It is shown in Fig. 5 

 that the effect of the wind is greater when the depth of the upper zone 

 is the least. The accumulation of fresh water in the region is shown to 

 increase with the run-off. The effect on displacement increases to a 

 limiting value as the boundary approaches a minimum depth, with 

 intermediate discharge, and the surface waters are freshest. The limit 

 is reached when the depth of the upper zone becomes great enough 

 to permit displacement in the upper zone, below the level of wind 

 influence. 



Freshet Effects 

 It is apparent that the displacement must increase with run-off, 

 but the evidence that the depth of the upper zone decreases is enlight- 

 -ening. This is illustrated by the sequence of gradients shown in Fig. 6 (c). 



278 



