408 
RIVER DEPOSITS. 
The deposit of slime by rivers upon the flats along their 
banks not only contributes greatly to the fertility of the soil 
thus flowed, but it subserves a still more important purpose in 
the general economy of nature. All running streams begin 
with excavating channels for themselves, or deepening the 
natural depressions in which they flow ; * but in proportion as 
their outlets are raised by the solid material transported by 
their currents, their velocity is diminished, they deposit gravel 
and sand at constantly higher and higher points, and so at last 
elevate, in the middle and lower part of their course, the beds 
they had previously scooped out.f The raising of the chan- 
* I do not mean to say that all rivers excavate their own valleys, for I 
have no doubt that in the majority of cases such depressions of the surface 
originate in higher geological causes, and hence the valley makes the river, 
not the river the valley. But even if we suppose a basin of the hardest rock 
to be elevated at once, completely formed, from the submarine abyss where 
it was fashioned, the first shower of rain that falls upon it after it rises to 
the air, though its waters will follow the lowest lines of the surface, will 
cut those lines deeper, and so on with every successive rain. The disin¬ 
tegrated rock from the upper part of the basin forms the lower by alluvial 
deposit, which is constantly transported farther and farther until the re¬ 
sistance of gravitation and cohesion balances the mechanical force of the 
running water. Thus plains, more or less steeply inclined, are formed, in 
which the river is constantly changing its bed, according to the perpetually 
varying force and direction of its currents, modified as they are by ever- 
fluctuating conditions. Thus the Po is said to have long inclined to move 
its channel southward in consequence of the superior mechanical force of 
its northern affluents. A diversion of these tributaries from their present 
beds, so that they should enter the main stream at other points and in dif¬ 
ferent directions, might modify the whole course of that great river. But 
the mechanical force of the tributary is not the only element of its influ¬ 
ence on the course of the principal stream. The deposits it lodges in the 
bed of the latter, acting as simple obstructions or causes of diversion, are 
not less important agents of change. 
t The distance to which a new obstruction to the flow of a river, 
whether by a dam or by a deposit in its channel, will retard its current, 
or, in popular phrase, “ set back the water,” is a problem of more diffi¬ 
cult practical solution than almost any other in hydraulics. The elements 
—such as straightness or crookedness of channel, character of bottom and 
hanks, volume and previous velocity of current, mass of water far above 
the obstruction, extraordinary drought or humidity of seasons, relative 
