a stationary state is ever observed in a single survey. In general it has 

 been found most satisfactory to seek the relation between the oceano- 

 graphic state and the primary factors of tide, run-off, and wind by direct 

 observation. 



Salinity Structure 



In coastal regions the density structure is to a large extent a func- 

 tion of the salinity structure alone, the variations due to temperature 

 differences being relatively small compared to the range of salinity in- 

 volved. Then the fresh water may be regarded as a tracer solution,, 

 which may be detected by the degree of dilution with sea-water. This 

 approach offers considerable observational advantage since the only data, 

 required are geography, tide, run-off, wind, and observations of the 

 salinity at suitable positions and times, in relation to these factors. All 

 other attributes such as temperature, dissolved oxygen, &c., may be 

 regarded as properties of the mixing waters, or effects of extraneous 

 functions, such as insolation. 



Examination of a large number of vertical salinity gradients indicated 

 that they were logarithmic (or inverse square) functions of depth (1,2, 3,. 

 4), each zone being marked by a continuous relation and separated from 

 the next by a discontinuity in the curve. Heretofore this relation has 

 been open to question because observations were always made at discreet 

 intervals of depth with water-sampling bottles. Recently it has been 

 possible to examine the phenomenon with a salinity-temperature-depth 

 recorder(5); which provides a continuous record. Typical data(3) are 

 shown on natural and logarithmic depth scales in Fig. 2. 



The reasons for this mass distribution have not been satisfactorily 

 analysed, but it appears that if the rate of mixing were constant in each 

 zone, this form of curve would result. This would imply that all mixing 

 functions combine within certain depth limitations to provide uniform 

 vertical transfer. Regardless of the explanation, the universal nature 

 of the phenomenon argues for the reliability of interpreting the salinity- 

 depth curves as shown in the figures. 



It has been observed(l, 2, 3) that the structure at any position in the 

 zones of fresh-water influence is a function of tide, wind, and river dis- 

 charge — that is, the structure recurs under like conditions and is not in 

 general a function of time. It is well to remark that, in view of the cyclic 

 nature of the tide, the unit of time is the tidal day. 



The Simple Outflow 



In a simple channel (Fig. 1) the flow is essentially two-dimensional. 

 The transport is seaward in the upper zone, and below this sea- water may 

 enter and leave on the tidal cycle imparting a reversing component to the 

 system. The direction of flow is defined by' the geography of the channel. 



Evidently three mechanisms are involved in such a system — the sea- 

 ward movement of fresh water in the upper zone due to its displacement 

 head, the reciprocal movement in the upper and lower zones due to the 

 hydraulic head of the tide, and the mixing of the two waters. 



Tide 



During the ebbing tide sea-level falls and the tendency for water to 

 leave the channel because of this hydraulic head is the same at all levels^ 

 but is accelerated in the upper zone by the displacement flow. 



268 



