OBSTRUCTION OF RIVER MOUTHS. 593 
carry their own deposits far enough out into deep water to pre- 
vent 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 fillmg 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 incaleula- 
bly 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 effect of his indus- 
trial operations has been to accelerate it immensely. Rivers, 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 deposit on their shores in the mountain val- 
leys where they rise ; in their middle course, a lighter earth is 
spread over the bottom of their widening basins, and forms 
plains of moderate 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 water. Man’s “improvement” of the 
soil increases the erosion from its surface; his arrangements 
for confining the lateral spread of the water in floods compel the 
rivers to transport to their mouths the earth derived from that 
erosion even in their upper course; and, consequently, the sedi- 
ment they deposit at their outlets is not only much larger in 
quantity, but composed of heavier materials, which sink more 
readily to the bottom of the sea and are less easily removed by 
marine currents. 
