RIVER EMBANKMENTS. 505 
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. The raising of the channels 
* IT do not mean to say that all rivers excavate their own valleys, for 1 
have no doubt that in the majority of cases such depressions of the surface 
originate in higher geological causes, such as the fissures and other irregu- 
larities of surface which could not fail to accompany upheaval, 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, will discharge its waters along the 
lowest lines of the surface, and cut those lines deeper, and so on with every 
successive rain. The disintegrated rock from the upper part of the basin 
forms the lower by alluvial deposit, which is constantly transported farther 
and farther until the resistance 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, accord- 
ing 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 southwards, at certain points, in consequence 
of the mechanical force of its northern afiluents. A diversion of these trib- 
utaries from their present beds, so that they should enter the main stream at 
other points and in different directions, might modify the whole course of that 
great river. But the mechanical force of the tributary is not the only element 
of its influence 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. 
+ 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 difficult practical solution 
than almost any other in hydraulics. The elements—such as straightness or 
crookedness of channel, character of bottom and banks, volume and previous 
velocity of current, mass of water far above the obstruction, extraordinary 
drought or humidity of seasons, relative extent to which the river may be 
affected by the precipitation in its own basin, and by supplies received 
through subterranean.channels from sources so distant as to be exposed to 
very different meteorological influences, effects of clearing and other im- 
provements always going on in new countries—are all extremely difficult, and 
some of them impossible, to be known and measured. In the American 
States, very numerous water-mills have been erected within a few years, and 
