732 



[CHAP. 22 



different density (Fig. 2). Clouds of fresh water created a tidal front, or zone of 

 convergence and divergence, in which internal waves developed. Internal 

 waves coincident with tidal periods, whether semi-diurnal or diurnal, were 

 commonly observed ; thus, it was concluded, tidal forces must be instrumental 

 in generating them (LaFond and Rao, 1954; Munk, 1941). 



Fig. 2. Sea-surface appearance showing the probable formation of internal waves by tidal 

 action between low- density discharge water and the Georgia Strait water. (British 

 Columbia Government Air Photo.) 



Certain internal waves may be created by two adjacent flows or by a flow 

 impinging on a continental shelf or other obstruction. Zeilon (1934) conducted 

 experiments that showed internal waves to occur when a tidal stream flowed 

 against a coastal bank. He further proved that obstacles in the path of an 

 advancing wave give rise to internal waves (Zeilon, 1913). 



Among possibly many more causative factors could be variations in atmos- 

 pheric pressure and strong winds. 



2. Measurements 



A. Equipment and Techniques 



Internal waves can be present only in water where a vertical density gradient 

 exists. The vertical gradient may be caused either by temperature or salinity, 

 or both. In freshwater lakes, measurements of temperature alone are sufficient 



