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Salinity.
Method of Determination. The total chlorides and chloride equivalents (bromides, etc.) were determined by titration of a known volume of sample with standard silver nitrate using chloride-free potassium chromate as an indicator. The density of the sample was determined with an immersion hydrometer and the salinity calculated on the assumption that 55.5% of dissolved salts were chlorides. The salinity is expressed in grams per 1000 grams of water.
Fig. 5.
The salinity is a direct result of the diurnal tidal movements and varies from a value which is that of the open sea to nearly zero. The values of the salinity in the Avon and Heathcote channels are summarised in table. It is seen that the greatest rate of change of salinity with distance occurs near the sea, and the inlets of the two rivers, while in the centre of the estuary it remains fairly constant. The horizontal distance between the high and low water graphs gives a rough indication of the distance upstream that the body of the water
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will move on the incoming tide. Thus suspended or floating matter in the centre of the estuary at low water would, under favourable conditions, be expected to be carried about one and a-quarter miles up the Avon channel or about one and a-half miles up the Heathcote. The distance that it would be carried downstream on the flow tide would of course be greater.
Layering. Evidence pointed to a fairly extensive occurrence of this interesting phenomenon, but time did not permit of any quantative estimation and a boat, which would have been necessary, was not available. On the incoming tide, the denser salt water tended to displace the lesser dense fresh water from the channels, with the result that the salinity on the edges of the mud flats was considerably lower than that of the water in the channels at the same place. This also explains the unexpectedly sharp rise of the salinity of the water in the channels with the incoming tide which occurred in many places.
Macormacks Bay. The salinity of the permanently filled portion of Macormacks Bay showed an appreciable rise owing to evaporation during the summer months.