ia6 THE IMPROVEMENT (/' RIVERS. 



ment of the mouth of the Danube (1858-61), one of the successful examples in this 

 field, the Sulina branch was chosen, although smaller than the other principal one, St. 

 George's, and but little trouble has been caused so far by the deposit of sediment. In 

 this case two jetties were built, and an increase of depth of n feet was obtained by 

 1872, and has since been maintained. The littoral drift being from the north, a gradual 

 building up of the foreshore has taken place under the lee of the southern jetty. 



At the mouth of the Rhone two jetties were completed in 1856, leading through 

 one of the main outlets into the Gulf of Foz. This gulf is sheltered from the littoral 

 current, and as a result the great amount of sediment brought down by the river was 

 deposited in quiet water, and the general level of the bottom was raised so that the 

 jetties in later years had to be extended considerably. Finally, owing to the fact that 

 certain coast towns would have been cut off from communication by any further exten- 

 sion, the work had to be stopped with an average depth of only 6J feet on the bar, 

 quite inadequate to the needs of navigation.* 



A striking natural example of the effects of volume and sediment is afforded by 

 the river Amazon, where the principal outlet, discharging enormous quantities of 

 water, is obstructed by constantly shifting mud-banks and shoals, while the Para mouth, 

 with a much smaller discharge, and consequently less sediment and disturbance of 

 the bottom, affords at all times a good channel for navigation. 



Where the outlets are crooked and flow over sandy beds, they are usually trained 

 within fixed limits, gradually enlarging into deep water. This increases the tidal 

 flow and facilitates the discharge of the river in such a manner as to render mainte- 

 nance less difficult. 



If properly located, canals solve the problem so far as sediment is concerned, but 

 their locks are a hindrance to navigation. Those canals cut at the outlets of the Rhone, 

 Nile, and Tiber, by the ancients, were mere derivations without gates, and the sedi- 

 ment of the rivers entered at flood-time and bars were formed at the sea extremity. 

 Those of the present day have locks connecting them with the river unless the outlet 

 is so situated that it does not fill with material. 



Among the larger tidal rivers there are many whose condition is such that no 

 extensive improvements are required, but there are many others where extensive 

 works for their improvement and maintenance have been installed, not all of which 

 have been wholly successful. In some of these the depths are maintained by dredg- 

 ing. The methods usually adopted for improving this class of streams are those of 

 projecting jetties located at suitable points, or longitudinal jetties following the course 

 of the river. The distance apart these training-banks should be placed has ever luvn. 

 and is still, a fruitful source of discussion; as well as the height to which they should 

 be built. Vernon-Harcourt states that it is generally unwise to raise the banks above 

 the half-tide level, and that the training of a river by longitudinal banks always con- 

 duces to the improvement of an irregular and shifting channel, and gives as instances 



* Rivieres et Canaux, Guillemain. 



