490 EIVER EMBANKMENTS. 



the flats adjacent to tlie channel, whether consistmg in dikea 

 which confine the waters, and, at the same time, augment the 

 velocity of the current, or in other means of producing the last- 

 mentioned effect, interfere with the restorative economy of na- 

 ture, and at last occasion the formation of marshes where, if left 

 to herself, she might have accumulated inexhaustible stores of 

 the richest soil, and spread them out in plains above the reach of 

 ordinary floods.* 



Dikes, which, as we have seen, are the means most frequently 

 employed to prevent damage by inundation, are generally parallel 

 to each other and separated by a distance not very much greater 

 than the natural width of the bed.f If such walls are high 

 enough to confine the water and strong enough to resist its pres- 

 sure, they secure the lands behind them from all the evils of in- 

 undation except those resulting from filtration ; but such ram- 

 parts 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 expenditures in increas- 

 ing the height of the banks.:}: They are attended, too, with some 



* The sediment of the Po has filled up some lagoons and swamps in its delta, 

 and converted them into comparatively dry land ; but, on the other hand, the 

 retardation of the current from the lengthening of its course, and the diminu- 

 tion of its velocity by the deposits at its mouth, have forced its waters at some 

 higher points to spread in spite of embankments, and thus fertile fields have 

 been turned into unhealthy and unproductive marshes. — See Botter, SulJa 

 condizione dei Terreni Maremmani nel Ferrarese. Annali di Agricoltura, etc., 

 Fasc. v., 1863. 



f In the case of rivers flowing through wide alluvial plains and much in- 

 clined to shift their beds, like the Po, the embankments often leave a very 

 wide space between them. The dikes of the Po are sometimes three or four 

 miles apart. 



X It appears from the investigations of Lombardini that the rate of elevation 

 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 regarded 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 torrential character, are raised 

 considerably above the level of the adjacent fields. 



The length of the lower course of the Po having been considerably increased 

 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. 



