254 
THE RIVER PO. 
during that period, because the wash of the banks consists 
chiefly of fine earth rather than of sand, and after the pond 
was once filled, or nearly so, even this material could no longer 
be deposited in it. The fact of the complete removal of the 
deposit I have described between the two dams in a single 
freshet, shows that, in spite of considerable obstruction from 
roughness of bed, large quantities of sand may be taken up 
and carried off by streams of no great rapidity of inclination ; 
for the whole descent of the bed of the river between the two 
dams—a distance of four miles—is but sixty feet, or fifteen feet 
to the mile. 
The Po and its Deposits. 
The current of the river Po, for a considerable distance 
after its volume of water is otherwise sufficient for continuous 
navigation, is too rapid for that purpose until near Piacenza, 
where its velocity becomes too much reduced to transport 
great quantities of mineral matter, except in a state of minute 
division. Its southern affluents bring down from the Apen¬ 
nines a large quantity of fine earth from various geological 
formations, while its Alpine tributaries west of the Ticino are 
charged chiefly with rock ground down to sand or gravel.* 
* Lombardini found, twenty years ago, that the mineral matter brought 
down to the Po by its tributaries was, in genera], comminuted to about the 
same degree of fineness as the sands of its bed at their points of discharge. 
In the case of the Trebbia, which rises high in the Apennines and empties 
into the Po at Piacenza, it was otherwise, that river rolling pebbles and 
coarse gravel into the channel of the principal stream. The banks of the 
other affluents—excepting some of those which discharge their waters into 
the great lakes—then either retained their woods, or had been so long 
clear of them, that the torrents had removed most of the disintegrated 
and loose rock in their upper basins. The valley of the Trebbia had been 
recently cleared, and all the forces which tend to the degradation and 
transportation of rock were in full activity .—Notice sur les Rivieres de la 
Lombardie, Annales des Fonts et Chaussees , 1847, ler s6mestre, p. 131. 
Since the date of Lombardini’s observations, many Alpine valleys have 
been stripped of their woods. It would be interesting to know whether 
any sensible change has been produced in the character or quantity of the 
matter transported by them to the Po. 
