IMPROVEMENT OF RIVER OUTLETS. 123 



tidal channel, so that it may have the fullest possible effect in reinforcing the ebb 

 throughout the whole of the tidal course of the river, as the power of the outflowing 

 current to maintain the channel depends upon the additional force thus furnished to 

 the ebb. 



" (3) The form of the estuary should be regulated so as to enlarge gradually as it 

 approaches the sea, and thus promote regularity of flow without unduly restricting 

 the tidal capacity above the outlet. This may be sometimes accomplished by low 

 training banks which, whilst directing and concentrating the latter half of the ebb, do 

 not materially impede the admission of the flood-tide up the estuary. Where the 

 estuary is very wide and irregular, and the main river channel through it is very tortuous 

 and shifting, high embankments may be formed, on each side, widening out toward 

 the sea, and the land behind them reclaimed." 



The principles governing non-tidal rivers differ to some extent from those given 

 above because of the lack of tidal influences capable of affecting the maintenance of 

 their outlets, and because of the difference in form of the mouths themselves. 



In this class of stream the current always flows in the same direction toward the 

 sea upon reaching which it is suddenly checked, and, as before stated, deposits its 

 load of sediment. This gradually builds up a bar which forces the water in various 

 directions through separate outlets across the foreshore, and forms what is known as 

 a delta. This division into various arms or outlets tends to reduce the scouring effect 

 of the current, and the channels become too shallow for navigation by reason of the 

 deposit of the matter brought down by the river. These deltas gradually extend into 

 the sea as this material is progressively deposited at the mouths of the outlets. 



The following principles are laid down by the authority just quoted for improving 

 non-tidal outlets.* 



"(i) The only method of deepening the outlet of sediment-bearing rivers flow- 

 ing into tideless seas is to prolong one of their delta channels by parallel jetties out to 

 the bar, so that the prolonged current, being concentrated across the bar, may scour a 

 deeper channel, and carry its burden of sediment into deep water further out. 



" (2) One of the minor outlets should be selected for improvement, if its delta 

 channel is adequate, or can easily be made adequate for the requirements of navigation ; 

 and the discharge of the other outlets should not be interfered with. The advance 

 of the delta at one of the minor outlets is slower, and the distance out to the bar is less, 

 and consequently the jetty works are less costly; whilst an increased discharge, pro- 

 duced by impeding the flow through the other outlets, would also increase the volume 

 of sediment, and therefore quicken the rate of advance of the delta, and hasten the 

 necessity of prolonging the jetties. 



" (3) The success of the jetty system depends on a rapid deepening of the sea in 

 front; on the fineness and lightness of the sediment brought down; and on the exist- 

 ence of a littoral current, its velocity, and the depth to which it extends. Any erosive 



* Improvement of the Maritime Portion of Rivers. 



