36 DEPARTMEXT OF THE XATAL SERVICE 



embryos, where a comparatively slight Tariation in temperature may make considerable, 

 difference in the time of hatching, e.g., in the spring of 1916, when the waters were| 

 colder than usual, many larval forms were much later in appearing and some at least 

 were killed during development. A lack of larvae in smaller species is sure to mean a, 

 lack of food supply in larger species and hence the whole fauna of the sea may be 

 more or less affected. 



In the spring and fall the water temperatures and the mean air temperatures prac- 

 tically coincide. In the winter the water does not keep up with the air in its extremes 

 of cold and in the summer with its extremes of heat. This is particularly true if the 

 change to the extreme is sudden and if the extreme is of short duration. 



As to the minor fluctuations in the water temperature that do not necessarily 

 coincide with air fluctuations, the main cause is evidently the variation in the amount 

 of fresh water present. Taking this by itself it may be said in general that an influx 

 of fresh water tends to raise the temperature in the summer and lower it in winter. 

 The raising or lowering may accentuate or may counteract to some extent the fluctua- 

 tions corresponding to those present in the air temperature records. 



The causes for the fluctuations in the speciflc gravity are not so easily placed. A 

 study of the five years' records shows certain of the predominating factors. The specific 

 gravity is high every year in the spring, about April, and in the fall, about October, 

 and drops low in the winter, about January, and in the summer, about June, but vary- 

 ing somewhat in each case from year to year. 



In the previous paper it was indicated that the low salinity in the summer was due, 

 largely to -the waters of the large mainland rivers, the Fraser in particular. This has 

 been fully borne out by the more extended records. Mr. C. C. Worsfold, District, 

 Engineer of the Department of Public Works, New Westminster, has been kind enough 

 to give me a copy of the records of the height of the water in the Fraser river at 

 Mission bridge, about forty miles abo-se the mouth of the river, during the months of, 

 May, June and July for these years. The records are kept for these months only as 

 these cover the flood period of the river, but they are sufficient to show the coincidence 

 in time of the high water and the low specific gravity of the water in Departure bay, 

 the change in Departure bay naturally takilig place a little later than the time when 

 flood appears at the Mission bridge. 



The main driving force to take the fresh water across the strait is the flood from, 

 the river itself. When the river is in flood a large volume of water is being continually 

 emptied into the strait, forced onward by the himdreds of miles of water following it 

 up. The fresh water, particularly while it is going with a strong current, mixes little, 

 with the more saline water below, so that its effect in reducing salinity is felt but a, 

 few fathoms down, scarcely five fathoms, down where the current enters the strait, but, 

 somewhat deeper than that when the strength of the current is reduced. At times the, 

 muddy water of the Fraser appears to be carried right across the strait of Georgia 

 to Gabriola, Yaldez and Galiano islands, without, on the surface, becoming materially 

 mixed with the saline water of the strait, while logs, sticks and other debris, carried 

 with the current, give some idea of its rate of flow. The ebb and the flow of the tide 

 have some effect on the current, swerving it somewhat southward or northward, as the 

 case may be. As the flow spreads out over a wider and wider area, the rate of flow 

 diminishes, and the water brought down mixes to a greater degree with the waters 

 ■of the strait. When the main current strikes the shore of the islands across the 

 strait, is is deflected northward or southward and the effect is gradually extended 

 for a great distance from the mouth of the river. The degree of extent is evidently 

 affected somewhat by weather conditions. More or less still weather is conducive to 

 .extension at the surface, while a strong wind, particularly if it is strong enough to 

 cause the waves to break, disturbs the surface, causing a greater mixing with the deeper 

 water and hence tends to hinder the spread. In many cases when the layer of fresh 

 water is very superficial, much of the fresh water seems to be carried along by the wind 

 in somewhat the same way as a floating object is carried. 



