RIVER DEPOSITS. 
431 
distance out to sea. The inclination of almost every known 
river bed has been considerably reduced within the historical 
period, and nothing but great volume of water, or exceptional 
rapidity of flow, now enables a few large streams like 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 har¬ 
bor 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 incal¬ 
culably distant future, if man had not come to inhabit the 
earth as soon as the natural forces which had formed its sur¬ 
face had arrived at such an approximate equilibrium that Ins 
existence on the globe was possible; but the general effect of 
his industrial operations has been to accelerate it immensely. 
Bivers, in countries planted by nature with forests and never 
inhabited by man, employ the little 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 heavily charged with coarse rolled or 
suspended matter, and this, in floods, they dej>osit on their 
shores in the mountain valleys where they rise; in their mid¬ 
dle course, a lighter earth is spread oyer the bottom of their 
* Rivers which transport sand, gravel, pebbles, heavy mineral matter 
m short, tend to raise their own beds; those charged only with fine, light 
earth, to cut them deeper. The prairie rivers of the West have deep 
channels, because the mineral matter they carry down is not heavy enough 
to resist the impulse of even a moderate current, and those tributaries of 
the Po which deposit their sediment in the lakes—the Ticino, the Adda, 
the Oglio, and the Mincio—flow, in deep cuts, for the same reason.— Baum- 
gaeten, 1 . c., p. 132. 
