KIVER EMBANKMENTS. 491 



•collateral disadvantages. Thej deprive the eartli 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 current at high water by confining it 

 to a narrower channel, and it consequently conveys to the sea the 



There are abundant grounds for believing that this has happened 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 condi- 

 tions which determine the question of the depression or elevation of a river- 

 bed are too multifarious, variable and complex, to be subjected to formulae, 

 and they can scarcely even be enumerated. 



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. 133. 



In regard to the level of the bed of the Po, there is another weighty con- 

 sideration that does not seem to have received the attention it deserves. I 

 refer to the secular depression of the western coast of the Adriatic, which is 

 computed at the rate of fifteen or twenty centimetres in a century, and which 

 of course increases the inclination of the bed, and the velocity and transport- 

 ing 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 elevation 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 compared 

 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 increased velocity of 

 ^current which prevents deposits where they would otherwise have happened. 



