RIVERS OP NEW SOUTH WALES. 387 



positions of the sources of these creeks were permanently altered 

 — or rather, altered for such time as their catchment areas remain 

 in tlie condition to which they were brought by the ring-barking. 

 When the origin of a river is a spring, it is to be remembered that 

 the spring is the apparent and not the real source ; for springs are 

 merely the result of underground percolation, and represent the 

 outflow of subterranean rivulets whose sources are at higher 

 levels. The conclusion which these facts involve, namely that the 

 position of the source of a river is constantly changing, may 

 appear obvious ; yet it is seldom realized, and scarcely ever taken 

 into account. 



The same causes which operate in producing constant clianges 

 in the source of a river, have an important effect in reudering the 

 destination of its waters also uncertain. When the course of a 

 river, after leaving the higher and more eft'ective portion of its 

 drainage area, lies through a great extent of comparatively level 

 country in which the rainfall is light, the discharge of the river 

 diminishes as the distance from the mountainous portion of the 

 catchment area increases. The question as to whether any portion 

 of the waters will reach the ocean, or, if not, where they will 

 cease to flow, is determined by the rates of evaporation, percola- 

 tion, and absorption. 



In addition to the recurring changes produced by the causes 

 already referred to, the conditions of every river are being steadily 

 altered by the action of its own current. If the coui'se of a rivulet 

 be followed down, it is observed that in addition to carrying silt 

 in suspension, the stream constantly rolls forward the sand and 

 other detritus in its bed, thereby wearing down rocks and boulders, 

 precisely as the smdblast cuts into and carves the surface of glass 

 or of polished granite. The rivulet thus by its erosive action and 

 moving force carries on incessantly the manufacture and transport 

 of material to raise the level of its bed and banks in the lower 

 lands, or to contribute to the formation of a delta. But the 

 erosion done by the direct action of the rivulet is generally trifling 

 compared to that done by the innumerable rills which are formed 

 during periods of rainfall. These acting on the softened soil con 

 tribute large quantities of silt, which the increased velocity of 

 the swollen rivulet enables it to carry. 



In addition to the constant erosion of the river bed and the 

 general denudation of the catchment ai-ea during rains, erosion of 

 the river banks is caused by bends and other obstructions which 

 divert the current. In such cases, however, the transport of silt 

 is, as a rule, non-continuous ; the products of erosion at the 

 concave bends being deposited at the convex bends. 



In connection with the subject of river action, it is important to 

 note that the capability of a current to roll forward the loose 

 material in its bed is proportional to the sixth power of the 

 velocity. Hence when the velocity of a stream is doubled its 



