RIVER EMBANKMENTS. 507 
high enough to confine the water and strong enough to resist 
its pressure, they secure the lands behind them from all the 
evils of inundation except those resulting from filtration ; but 
such ramparts are enormously costly in original construction 
and in maintenance, and, as has been already shown, the filling 
up of the bed of the river in its lower course, by sand and 
gravel, often involves the necessity of incurring new expendi- 
tures in increasing the height of the banks.* They are 
* It appears from the investigations of Lombardini that the rate of eleva- 
tion of the bed of the Po has been much exaggerated by earlier writers, and 
in some parts of its course the change is so slow that its level may be re- 
garded as nearly constant. Observation has established a similar constancy 
in the bed of the Rhone and of many other important rivers, while, on the 
other hand, the beds of the Adige and the Brenta, streams of a more torren- 
tial character, are raised considerably above the level of the adjacent fields. 
The length of the lower course of the Po haying been considerably in- 
creased by the filling up of the Adriatic with its deposits, the velocity of the 
current ought, prima facie, to have been diminished and its bed raised in 
proportion. There are abundant grounds for believing that this has hap- 
pened in the case of the Nile, and one reason why the same effect has not 
been more sensibly perceptible in the Po is, that the confinement of the 
current by continuous embankments gives it a high-water velocity sufficient 
to sweep out deposits let fall at lower stages and slower movements of the 
water. Torrential streams tend to excavate or to raise their beds according 
to the inclination, and to the character of the material they transport. 
No general law on this point can be laid down in relation to the middle and 
lower courses of rivers. The conditions which determine the question of the 
depression or elevation of a river-bed are too multifarious, variable, and 
complex, to be subjected to formule, and they can scarcely even be enu- 
merated. 
The following observation, however, though apparently too unconditionally 
stated, is too important to be omitted. 
Rivers which transport sand, gravel, pebbles, heavy mineral matter in short, 
tend to raise their own beds ; those charged only with fine, light earth, to cut 
them deeper. The prairie rivers of the western United States 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.—BAUMGAR- 
TEN, p. 182. 
In regard to the level of the bed of the Po, there is another weighty con- 
sideration which does not seem to have received the attention it deserves. I 
