SEDIMENT OF THE PO. 279 
thousand years, we do not know, but as they still transport 
enormous quantities, as the North Adriatic appears to have 
shoaled rapidly, and as long islands, composed in great part of 
fluviatile deposits, have formed opposite their mouths, it must 
evidently have been very great. The floods of the Po occur 
but once, or sometimes twice, in a year.* At other times, its 
waters are comparatively limpid and seem to hold no great 
amount of mud or fine sand in mechanical suspension; but at 
high water it contains a large proportion of solid matter, and, 
according to Lombardini, it annually transports to the shores 
of the Adriatic not less than 42,760,000 cubic metres, or very 
nearly 55,000,000 cubic yards, which carries the coast-line out 
* In the earlier medixval centuries, when the declivities of the mountains 
still retained a much larger proportion of their woods, the moderate annual 
floods of the Po were occasioned by the melting of the snows on the lower 
slopes, and, according to a passage of Tasso quoted by Castellani (Dell? In- 
Jluenea delle Selve, i., p. 58, note), they took place in May. The usually more 
violent inundations of later ages are due to rains, the waters of which are no 
longer retained by a forest-soil, but conveyed at once to the rivers—and they 
occur almost uniformly in the autumn or late summer. Castellani, on the 
page just quoted, says that even so late as about 1780, the Po required a heavy 
rain of a week to overflow its banks, but that forty years later it was some- 
times raised to full flood in a single day. 
Pliny says: ‘‘ The Po, which is inferior to no river in swiftness of current, 
is in flood about the rising of the dog-star, the snow then melting, and though 
so rapid in flow, it washes nothing from the soil, but leaves it increased in fer- 
tility.’"— Natural History, Book iii., 20. 
The first terrible inundation of the Po in 1872 took place in May, and ap- 
pears to have been occasioned by heavy rains on the southern flank of the 
Alps, and to have received little accession from snow. The snow on the higher 
Alps does not usually thaw so as to occasion floods before August, and often 
considerably later. The more destructive flood of October, 1872, was caused 
both by thaws in the high mountains and by an extraordinary fall of rain. 
See River Hmbankments ; post. 
Pliny’s remark as to enrichment of the soil by the floods appears to be veri- 
fied in the case of that of October, 1872, for it is found that the water has 
left very extensively a thick deposit of slime on the fields. 
See a list of the historically known great inundations of the Po by the 
engineer Zuccholli in TORELLI, Progetto di Legge per la Vendita di Beni 
incoitt. Roma, 1872. 
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