396 PROCEEDINGS OF SECTION E. 



Clarence at Copmanhurst in a high flood is seventy-seven feet, 

 while ninemiles further clown it is sixty-two feet, and at Moleville, 

 twenty miles from Copmanhurst, only thirty-four feet. The range 

 of surface level of the Nepean at its junction with the Warra- 

 gamba is about seventy feet ; while ten miles further down the 

 river, near Penrith, it is only forty-two feet. Similar conditions 

 €xist on neai'ly all the other coastal rivers. 



When we consider the effect of such changes of slope of the 

 river surface, the causes of the extent and destructiveness of 

 floods on the coastal rivers are easily understood. Taking the 

 case of the Clai-ence, we find that in a high flood the average slope 

 of the surface from Copmanhurst to Moleville exceeds that when 

 the river is low by about two feet two inches per mile, so that at 

 such times the surface fall between these places cannot be less 

 than about two and a-half feet per mile. With such a river as 

 the Clarence this fall gives a high rate of velocity which is only 

 checked by degrees by the tidal waters. When the river is low, 

 or even during moderate floods, the resistance of the tidal waters 

 is suflficient to cause the deposit of all, or nearly all the silt before 

 the Heads are reached ; but in the highest floods the velocity is 

 sufiicient to retain considerable scouring power in the current 

 even at the river's mouth. 



The Clarence is here specially referred to; but a similar 

 description, with only slight variations, would apply to all the 

 coastal rivers. Their fall is in all cases at first rapid till they 

 reach the alluvial deposits with which, in a large measure, they 

 have filled their estuaries. The rivers are tidal ; and the upper 

 ends of the alluvial deposits represent approximately the limits 

 reached by the tides. With few exceptions every river has a 

 sandy bar at its mouth — representing the result of the conflict 

 between the tidal and upland waters. To these characteristics of 

 the coastal rivers is to be added that deltas in the ordinary 

 acceptation of the term are not found on the coast of New South 

 Wales. Enormous quantities of silt are carried out to sea during 

 floods, but it is evident that the ocean currents are sufiiciently 

 strong to prevent its deposition near the coast. 



Classification of our rivers according to their silt-carrying 

 capabilities naturally suggests inquiry as to whether we have any 

 information on which can be based an approximate estimate of 

 the quantity of silt carried. This is a subject which has been 

 dealt with by careful experiments in several important instances ; 

 but so far as I am aware its investigation has not yet been entered 

 on in these colonies. It will, however, give some conception of 

 the magnitude of the work being done by our rivers if we apply 

 to them the results obtained elsewhere under somewhat analagous 

 circumstances. 



