ESTUARY HARBOURS, ETC. i$i 



Waves play an important part in the formation of bars, not 

 only by checking the outgoing current, but by throwing up 

 material, and endeavouring to establish and maintain the line 

 of beach past the river-entrance in spite of the ingoing and out- 

 going currents. 



When waves impinge on a beach at an acute angle, they 

 cause the material of which it is composed to travel along the 

 coast, and their tendency is to drive it into river channels and 

 estuaries; whereas the action of the ebb current is to drive it 

 out and keep the channel clear. 



A battle between these contending forces is, therefore, con- 

 tinually being waged ; but the wave power is, as a rule, more 

 than the river current can cope with, the result being that the 

 river-channel is deflected, and a ridge, forming its submerged sea- 

 ward bank, is heaped up by the waves aided, possibly, by littoral 

 currents and by them pushed more and more landward till at 

 length it runs parallel, or nearly so, to the coast-line. 



In extreme cases, especially in countries where winds blow 

 steadily in one direction for long periods, and where long droughts 

 occur, river-mouths are often completely closed, the water thus 

 impounded spreading out and forming lagoons of greater or less 

 extent, until, on the occurrence of a freshet, the river bursts 

 through its barrier, usually at a point nearly in the direct normal 

 line of the channel. 



The following woodcut, indicating the deflection of a river 

 channel from a to c, may be of service in illustrating the 

 manner in which the contending forces act : 



i / f\\ 



i B ,'A R 



. /\. 



. 



FIG. 21. Feathered arrows indicate assumed direction of waves. 



