REPORT ON OCEANOGRAPHY 31 



changes. Surveys are made at weekly or monthly intervals throughout a 

 year or so. The oceanographic state is related to the weather, the run- 

 off, and to daily seawater observations. Thereafter, these readily ob- 

 servable characters are used as indices to recognize the oceanographic 

 states, and in some cases to predict the conditions. The tedious and 

 costly surveys are only carried on long enough to establish the relations 

 with the daily observations. This is a very simple and effective plan 

 which allows for the accumulation and use of oceanographic knowledge. 



Juan de Fuca Strait is a broad (11 miles) deep (100 fathoms) chan- 

 nel connecting the ocean with the inland sea of Georgia Strait. It con- 

 tains most of the shipping and the principal migration route of the sal- 

 mon and herring. Here the tidal currents are reversing at all depths 

 and are linear functions of the difference of tidal height between the 

 ocean and Georgia Strait. The tidal wave progresses along the strait 

 at about 30 knots, less than a fifth of the theoretical speed. The reasons 

 for these effects have not been solved as yet. However, a system of pre- 

 diction of the tidal currents at all depths has been worked out, and is 

 as accurate as the normal weather variations will allow. 



At the northern end of Georgia Strait the tidal flow through the 

 narrow passages is purely hydraulic. At Seymour Narrows the currents 

 attain thirteen knots, the fastest in the world, and are in effect two-way 

 falls. Because of the great velocities there is a half-foot depression of 

 mean sea level. Evidently mean sea level determinations should not be 

 made in the vicinity of rapid currents. 



Georgia Strait, Chatham Sound, and a number of the coastal inlets 

 have been the subject of intense studies. These regions are dominated 

 by the outflow of the major rivers. The fresh water flows out over the 

 surface, mixing with the sea water as it goes. It forms an upper zone 

 of brackish water which oscillates with the tide, but moves persistently 

 seaward. The brackish water occurs as clouds which represent the river 

 discharge during the falling tide. During the rising tide the sea water 

 intrudes the estuary and stops the outflow. As the clouds move about 

 in the vicinity of the river they become more saline and override earlier 

 clouds, and so form multiple layers. 



In all these systems the water tends to form layers in which the 

 salinity varies as the logarithm of depth. The reason for this has not 

 been determined. However, its occurrence has been established, and 

 has been used to considerable advantage in analyzing the structure and 

 origin of the waters. 



Some progress has been made in the use of hydraulic models for 

 the study of harbours, bays, inlets, and narrows. These are systems 

 where the boundary conditions are known. We have just completed a 

 study of Alberni Harbour. Tides are generated by the inflow of out- 



