PORT JACKSON SILT BEDS. 177 



deposition in the channels can only be accurately guaged by 

 periodical and careful investigations with the sounding line and 

 boring rod. Though the soundings recorded on Captain Sydney's 

 Chart show greater depths in some spots than the latest chart 

 gives, yet as the soundings were not probably taken on exactly 

 the same lines any comparison would be unreliable. 



There is no doubt however as to the ooze being quite a recent 

 deposit since the settlement of Sydney began, and that its presence 

 evidences the loss to a certain degree of scouring action in the tidal 

 currents. Therein lies the danger, for as the silt beds rise the 

 power of the tidal currents for scouring it will diminish, and the 

 volume of tidal water running up the Parramatta River will 

 sensibly decrease. Thus whilst owing to the clearance and 

 settlement of land all over the watershed of the harbour there 

 will ever be increasing elements for silt deposit in its bed, the 

 power of resistance from tidal currents will proportionately decrease. 

 The cause and prevention of this silt formation it therefore a 

 question of considerable importance. Its cause may be traced to 

 different actions which are constantly at work, and as constantly 

 increasing in force, such as city and suburban drainage, erosion of 

 country within the watershed by heavy wind and rain storms, to 

 the low rise and fall of the tide, and lastly to accumulation from 

 littoral currents. There need be but little apprehension of any 

 material addition to the silt beds from outside influences, such as 

 littoral currents or the action of gales, because of the rocky 

 precipitous character of the coast line and the great depth of water 

 immediately beyond it. The great danger to be guarded against 

 -arises from inside influences, such as the erosion of the cleared 

 country within the watershed of the Parramatta River and the 

 drainage of Sydney and suburbs, which tend to fill in the river 

 and harbour from Parramatta downwards, and so reduce the 

 volume of water entering the harbour, and the range of tidal 

 influences. Thus the action of scour on the bar will be continually 

 weakened whilst the degree of silt accumulation during slack water 

 will be proportionately increased. 



In considering the measures to prevent these silt beds it is 

 necessary to investigate ; first, the quantity of silt being delivered 

 into the harbour ; second, the power that yet remains in the tidal 

 currents to transport this silt ; third, to what extent it can be 

 prevented from entering the harbour ; fourth, in what degree the 

 volume and velocity of the tidal currents can be increased so as to 

 preserve present channels. The quantity of silt delivered into 

 the harbour cannot, without the most exact knowledge and minute 

 examination of the country, be even approximately estimated, but 

 it must be considerable. The approximate area of the watershed 

 draining into the Parramatta River and Port Jackson is 188 



