OBSTKUCTIOlSr OF RIVER MOUTHS. 505 



Obst/mciion of River Mouths. 



The mouths of a laxge proportion of the streams known to an- 

 cient navigation are abeady blocked up by sand-bars or fluviatile 

 deposits, and the maritime approaches to river harbors frequented 

 by the ships of Phenicia and Carthage and Greece and Rome are 

 shoaled to a considerable distance out to sea. The inclination of 

 the lower course of almost every known river-bed has been con- 

 siderably reduced within the historical period, and nothing but 

 great volume of water, or exceptional rapidity of flow, now en- 

 ables a few large streams hke the Amazon, the La Plata, the 

 Ganges, and, in a less degree, the Mississippi, to carry their own 

 deposits far enough out into deep water to prevent the formation 

 of serious obstructions to navigation. But the degradation of 

 their banks, and the transportation of earthy matter to the sea by 

 their currents, are gradually filling up the estuaries even of these 

 mighty floods, and unless the threatened evil shall be averted by 

 the action of geological forces, or by artificial contrivances more 

 efficient than dredging-machines, the destruction of every harbor 

 in the world which receives a considerable river must inevitably 

 take place at no very distant date. 



This result would, perhaps, have followed in some incalculably 

 distant future, if man had not come to inhabit the earth as soon 

 as the natural forces, which had formed its surface, had arrived 

 at such an approximate equilibrium that his existence on the 

 globe was possible ; but the general eflect of his industrial oper- 

 ations has been to accelerate it immensely. Rivers, in countries 

 planted by nature with forests and never inhabited by man, em- 

 ploy the httle earth and gravel they transport chiefly to raise 

 their own beds and to form plains in their basins. In their 

 upper course, where the current is swiftest, they are most heav- 

 ily charged with coarse rolled or suspended matter, and this, in 

 floods, they deposit on their shores in the mountain valleys where 

 they rise ; in their middle course, a hghter earth is spread over 

 the bottom of their widening basins, and forms plains of moder- 

 ate extent ; the fine silt which floats farther is deposited over a 

 still broader area, or, if carried out to sea, is in great part quickly 

 swept far off by marine currents and dropped at last in deep 

 22 



