
rs DYNAMICAL GEOLOGY. ——‘[Boox IIL. 
of the surrounding plains as at / (Fig. 119). This tendency is 
displayed by the Adige, Reno, and Brenta, which, descending from _ 
the Alps well supplied with detritus, debouch on the plains of the Po. 

Fie. 118.—Fans or ALLUviuM. Mapison River, Monrana. 
The Po itself has been quoted as an instance of a river continuing 
to heighten its bed, while man in self-defence heightens its embank- 
ments, until the surface of the river becomes higher than the plains 
on either side. It has been shown by Lombardini, however, that the 
bed of this river has undergone very little change for centuries; that — 

Vann 
Fig. 119.—SxEctTIonN or A RIVER PLAIN, SHOWING HEIGHTENING OF CHANNEL BY 
DEPOSIT OF SEDIMENT (B.). 
only here and there does the mean height of the Po rise above the 
level of the plains, being generally considerably below it, and that 
even in a high flood the surface of the river is scarcely ten feet above 
the pavement in front of the palace at Ferrara. The Po and its 
tributaries have been carefully embanked, so that much of the 
sediment of the rivers, instead of accumulating on the plains of 
Lombardy as it naturally would do, is carried out into the Adriatic. 
Hence, partly, no doubt, the remarkably rapid rate of growth of the 
delta of the Po. But in such cases man needs all his skill and labour 
to keep the banks secure. Even with his utmost efforts the river will 
now and then break through, sweeping down the barrier which it has 
itself made, as well as any additional embankments constructed by 
him, and carrying its flood far and wide over the plain. Left to itself, 
the river would incessantly shift its course, until in turn every 
part of the plain had been again and again traversed. It is indeed 
in this way that a great alluvial plain is gradually levelled and 
heightened.’ 
(c) On River-banks and Flood-plains—As is partly implied in 
1 It is in the north of Italy that the struggle between man and nature in this de- 
partment has been most persistently waged. See on this subject Lombardini, in Ann. 
des Ponts et Chaussées, 1847, Beardmore’s “ Tables,” p. 172. ' 
