36 THE GROWTH OF THE YANGTZE DELTA 



positing its sediment upon wide breadth of country, this 

 wonderful Yangtze does its work in its Estuary quite as 

 effectually, by the help of the powerful tides of the Pacific, 

 which enable its mud-laden waters to enter the thousand 

 little canals that have been made by man for* the irrigation 

 of the fields even after the main dykes are built shutting 

 o-ff the large river itself and the tides. The decay of vege- 

 table matter combined with artificial irrigation raises the 

 surface, and would, if continued sufficiently long, render 

 the whole region at length entirely independent of the em- 

 bankments made round the entire sea coast for the security 

 of agriculture." 



The rate of advance of the coast is, as I have said, 

 somewhere about 1 mile in 60 years, on an average. In 

 some places the coast may be stationary — in others advanc- 

 ing at double the rate. The figure is just a general average 

 indication. 



Only for very short periods ahead is it possible to predict 

 exactly where channels are tending to form, where any way 

 the islands or coasts grow, and again disappear, only to 

 form again. 



One thing is certain. The work of erosion will proceed 

 in the upper drainage basin. As the intermediate basins 

 are fairly well filled, the mouth will receive a good deal, 

 perhaps half, perhaps much less, of the matter eroded. This 

 process will probably continue for many thousands of years 

 and the present delta grow. 



If we suppose that one-third of the whole watershed 

 ( = 250,000 square geographical miles) consists of hillsides 

 and other erodible area, the silt passing Wuhu corresponds 

 to a reduction in height of one foot in 625 years, or of one 

 mile of height in 3 J million years. It would certainly seem 

 reasonable to suppose that the actual denudation is at least 

 twice that which is indicated by the Wuhu silt content, i.e., 

 that at least half the matter eroded is left above Wuhu. 



As the high upland of the Yangtze is many thousands 

 of feet high — there are many millions of years to look forward 

 to, before the Yangtze has succeeded in levelling down its 

 entire basin to the level of a plane. 



Before this has happened, however, some new move- 

 ment in the surface of the earth may change the map again 

 and the delta formation will begin all over again on new 

 lines in a different part o<f the continent, and the Estuary 

 of the Yangtze may then be at the present Ichang or still 

 further up. 



