256 
SEDIMENT OF THE PO. 
southern and northern faces respectively a geographical aspect 
very different from that they now present. Ravenna, forty 
miles south of the principal mouth of the Po, was built like 
Venice, in a lagoon, and the Adriatic still washed its walls at 
the commencement of the Christian era. The mud of the Po 
has filled up the lagoon, and Ravenna is now four miles from 
the sea. The town of Adria, which lies between the Po and 
the Adige, at the distance of some four or five miles from each, 
was once a harbor famous enough to have given its name to 
the Adriatic sea, and it was still a seaport in the time of Au¬ 
gustus. The combined action of the two rivers has so advanced 
the coast line that Adria is now about fourteen miles inland, 
and, in other places, the deposits made within the same period 
by these and other neighboring streams have a width of 
twenty miles. 
What proportion of the earth with which they are charged 
these rivers have borne out into deep water, during the last two 
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 
* Three centuries ago, when the declivities of the mountains still re¬ 
tained a much larger proportion of their woods, the moderate annual floods 
of the Po were occasioned by the melting of the snows, and, as appears by 
a passage of Tasso quoted by Castellani ( Dell' Influenza delle Selve, i, p. 58, 
note), they took place in May. The much more violent inundations of the 
present century 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. 
