258 
SEDIMENT OF THE PO. 
and the Adda, into the lake of Garda by its affluents, and the 
yet vaster heaps of pebbles, gravel, and earth permanently 
deposited by the torrents near their points of eruption from 
mountain gorges, or spread over the wide plains at lower 
levels, we may safely assume that we have an aggregate of not 
less than four times the quantity carried to the Adriatic by the 
Po, or 220,000,000 cubic yards of solid matter, abstracted every 
year from the Italian Alps and the Apennines, and removed 
out of their domain by the force of running water.* 
The present rate of deposit at the mouth of the Po has con¬ 
tinued since the year 1600, the previous advance of the coast, 
after the year 1200, having been only one third as rapid. The 
great increase of erosion and transport is ascribed by Lombar- 
dini chiefly to the destruction of the forests in the basin of that 
river and the valleys of its tributaries, since the beginning of 
the seventeenth century, f We have no data to show the rate 
of deposit in any given century before the year 1200, and it 
doubtless varied according to the progress of population and 
the consequent extension of clearing and cultivation. The 
transporting power of torrents is greatest soon after their for¬ 
mation, because at that time then* points of delivery are lower, 
and, of course, their general slope and velocity more rapid, 
than after years of erosion above, and deposit below, have 
depressed the beds of their mountain valleys, and elevated the 
channels of their lower course. Their eroding action also is 
most powerful at the same period, both because their mechan¬ 
ical force is then greatest, and because the loose earth and 
* Mengotti estimated the mass of solid matter annually “united 
to the waters of the Po ” at 822,000,000 cubic metres, or nearly twenty 
times as much as, according to Lombardini, that river delivers into the 
Adriatic. Castellani supposes the computation of Mengotti to fall much 
below the truth, and there can be no doubt that a vastly larger quantity 
of earth and gravel is washed down from the Alps and the Apennines than 
is carried to the sea.— Castellani, DelV Immediata Influenza delle Selve 
sul corso delle Acque , i, pp. 42, 43. 
I have contented myself with assuming less than one fifth of Mengotti’a 
estimate. 
t Baumgakten, An. des Fonts et Chaussees , 1847, ler s£mestre, p. 175. 
