REPORT ON OCEANOGRAPHY 35 



water in them is appreciably diluted with fresh water in the upper lay- 

 ers. This fresh water comes from the rivers, mostly glacial in origin, 

 which empty into the heads of almost all the inlets, and it is the lack of 

 such a river which distinguishes the upper 10 miles of Belize Inlet from 

 the remainder. This is the only region in which there was any evidence, 

 in the form of oxygen depletion, of stagnation. 



The depth of the mixing layer between fresher surface water and 

 deeper saline water varies from 3 to 10 meters in different inlets and 

 in some cases varies somewhat in depth along individual inlets. There 

 is ample evidence in the drift of the vessel, and from current measure- 

 ments, of the outward flow of the surface water. In many cases there is 

 a very marked velocity shear in the mixing layer, the major part of the 

 change in velocity taking place in as little as 1/2 to 1 meter. At the 

 heads of the inlets the fresh water forms a sharply defined layer over the 

 saline water but the mixing which occurs along the length of the inlet 

 results in a more gradual change of salinity with depth toward the 

 mouth of the inlets. It appears that the change from the two-layer 

 system to the continuous distribution takes place to the same extent 

 irrespective of the length of the inlet— the significance of this fact is not 

 yet fully understood. 



The greater part of the fresh water resides in the top 20 m. of the 

 water and below about 30-50 m. Each inlet shows only a slight change 

 with depth or with longitudinal character. 



In the summer the temperature of the upper water is increased 

 above that of the deeper water by solar radiation but rarely rises above 

 17 °C, while the deeper water temperature lies between 5.5° and 7.5 °C. 



The deep water in the southern inlets (south of Knight Inlet) aver- 

 ages about 30% salinity and 7.5 °C temperature while the corresponding 

 values for the northern inlets are 32.5% and 6.5°C. 



A feature of the vertical temperature distribution, not previously 

 reported, is a temperature minimum, at intermediate depths, which va- 

 ries in character during the year. In several of the inlets the temperature 

 passes through a minimum value at 50-60 m. depth, increases to a maxi- 

 mum at 60 to 100 m. and then decreases slowly to the bottom. The 

 water in the minimum temperature layer is colder but slightly less saline 

 than the deeper water. In Bute Inlet, where this feature has been 

 studied in most detail, it is found that this low temperature layer comes 

 into existence suddenly about February and gradually dissipates until 

 it becomes indistinguishable by the end of the year. It is at present 

 thought that this layer results from the cooling of surface water during 

 the period of lowest air temperature when the salinity is at its highest 



