506 RIVER EMBANKMENTS. 
is compensated in part by the simultaneous elevation of their 
banks and the flats adjoining them, from the deposit of the 
finer particles of earth and vegetable mould brought down 
from the mountains, without which elevation the low grounds 
bordering all rivers would be, as in many cases they in fact 
are, mere morasses. 
All arrangements which tend to obstruct this process of 
raising the flats adjacent to the channel, whether consisting in 
dikes 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 
nature, 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.t If such walls are 
there is scarcely a stream in the settled portion of the country which has not 
several mill-dams upon it. When a dam is raised—a process which the grad- 
ual diminution of the summer currents renders frequently necessary — or 
when a new dam is built, it often happens that the meadows above are flowed, 
or that the retardation of the stream extends back to the dam next above. 
This leads to frequent law-suits. From the great uncertainty of the facts, 
the testimony is more conflicting in these than in any other class of cases, 
and the obstinacy with which ‘‘ water causes’’ are disputed has become 
proverbial. 
* 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 diminution 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 BorreR, Sulla condizione dei Terrent Maremmani nel Ferrarese. Annali 
di Agricoltura, etc., Fase. v., 1863. 
+ In the case of rivers flowi ing 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. 
