508 RIVER EMBANKMENTS. 
attended, too, with some collateral disadvantages. They deprive 
the earth of the fertilizing deposits of the waters, which are 
powerful natural restoratives of soils exhausted by cultivation; 
they accelerate the rapidity and transporting power of the cur- 
rent at high water by confining it toa narrower channel, and it 
consequently conveys to the sea the earthy matter it holds in 
suspension, and chokes up harbors with a deposit which it 
would otherwise have spread over a wider surface; they in- 
terfere with roads and the convenience of river navigation, and 
no amount of cost or care can secure them from occasional 
rupture, in case of which the rush of the waters through the 
breach is more destructive than the natural flow of the highest 
inundation.* 
For these reasons, many experienced engineers are of opinion 
refer to the secular depression of the western coast of the Adriatic, which is 
computed at the rate of fifteen or twenty centimétres in a century, and which 
of course increases the inclination of the bed, and the velocity and trans- 
porting power of the current of the Po, unless we assume that the whole 
course of the river, from the sea to its sources, shares in the depression. Of 
this assumption there is no proof, and the probability is to the contrary. 
For the evidence, though not conclusive, perhaps, tends to show an eleya- 
tion of the Tuscan coast, and even of the Ligurian shore at points lying farther 
west than the sources of the Po. The level of certain parts of the bed of 
the river referred to by Lombardini as constant, is not their elevation as com- 
pared with points nearer the sea, but relatively to the adjacent plains, and 
there is every reason to believe that the depression of the Adriatic coast, 
whether, as is conceivable, occasioned by the mere weight of the fluviatile 
deposits or by more general geological causes, has increased the slope of the 
bed of the river between the points in question and the sea. In this instance, 
then, the relative permanency of the river level at certain points may be, not 
the ordinary case of a natural equilibrium, but the negative effect of an in- 
creased velocity of current which prevents deposits where they would cther- 
wise have happened. 
* To secure the city of Sacramento, in California, from the inundations to 
which it is subject, a dike or levée was built upon the bank of the river and 
raised to an elevation above that of the highest known floods, and it was 
connected, below the town, with grounds lying considerably above the river. 
On one occasion a breach in the dike occurred above the town at a very high 
stage of the flood. The water poured in behind it, and overflowed the lower 
part of the city, which remained submerged for some time after the river had 
