DEPOSITS OF THE TUSCAN RIVERS 
415 
almost absolutely depopulated, and tbe malarious fevers have 
extended their ravages far into the interior. 
These results are certainly not to be ascribed wholly to 
human action. They are, in a large proportion, due to geo¬ 
logical causes over which man has no control. The soil of 
much of Tuscany becomes pasty, almost fluid even, as soon as 
it is moistened, and when thoroughly saturated with water, it 
flows like a river. Such a soil as this would not be completely 
protected by woods, and, indeed, it would now be difficult to 
confine it long enough to allow it to cover itself with forest 
vegetation. Nevertheless, it certainly was once chiefly wooded, 
and the rivers which flow through it must then have been 
much less charged with earthy matter than at present, and 
they must have carried into the sea a smaller proportion of 
their sediment when they were free to deposit it on their banks 
than since they have been confined by dikes.* 
* For the convenience of navigation, and to lessen the danger of inun¬ 
dation by giving greater directness, and, of course, rapidity to the current, 
bends in rivers are sometimes cut off and winding channels made straight. 
This process has the same general effects as diking, and therefore cannot 
be employed without many of the same results. 
This practice has often been resorted to on the Mississippi with advan¬ 
tage to navigation, but it is quite another question whether that advantage 
has not been too dearly purchased by the injury to the banks at lower 
points. If we suppose a river to have a navigable course of 1,600 miles 
as measured by its natural channel, with a descent of 800 feet, we shall 
have a fall of six inches to the mile. If the length of channel be reduced 
to 1,200 miles by cutting off bends, the fall is increased to eight inches per 
mile. The augmentation of velocity consequent upon this increase of in¬ 
clination is not computable without taking into account other elements, 
such as depth and volume of water, diminution of direct resistance, and 
the like, but in almost any supposable case, it would be sufficient to 
produce great effects on the height of floods, the deposit of sediment in 
the channel, on the shores, and at the outlet, the erosion of banks and 
other points of much geographical importance. 
The Po, in those parts of its course where the embankments leave 
a wide space between, often cuts off bends in its channel and straightens 
its course. These short cuts are called salti, or leaps, and sometimes 
reduce the distance between their termini by several miles. In 1777, the 
ealto of Cottaro shortened a distance of 7,000 metres by 5,000, or, in other 
