DEFECTS TO BE OVERCOME 165 



tlid before. Finally, they find their way by some shorter cut 

 to the margin of the land, and, for the reason of tlie shorter 

 distance, the slope over which the)- tlow is steeper than that 

 they have before pursued. Moving rapidly down the decline, 

 the water soon cuts open a new^ channel through which it 

 has for a time a freer exit ; the old and longer way then 

 gradually silts up, until it has no value as a pathway for the 

 stream or as a port. 



The changes of channel which occur in a delta district 

 are often of formidable commercial importance. Thus the 

 Yellow River of China in 1853 opened a new outlet to the 

 sea, the mouth of which was some hundreds of miles from 

 that of its former path. At the present time the main out- 

 let of the Mississippi tends to change its place in the manner 

 and for the reasons above suororested. In cjeneral, it mav be 

 said that a few centuries represent the time during which a 

 delta mouth will normally maintain one position. 



It is possible by means of engineering devices to overcome 

 some of the most considerable defects and dangers which 

 beset delta ports. The obstacles to upward navigation due 

 to the current of the stream, a hinderance which is often of a 

 serious nature to sailing vessels, may be met by the use of 

 steam tugs. Such accidents [is the change of channel, which 

 may at any time cut off the city which develops at the port, 

 and leave it in an unprofitable position, can be indefinitely 

 postponed by engineering devices. The most serious objec- 

 tion to this class of harbors, that due to the broad platform 

 of mud forming widespread shallows just beyond the margin 

 of the delta land, is more difficult to meet. For a long time 

 it seemed an insuperable difficulty, for the reason that, 

 although a channel miglit be cut open by artificial dredging, 



