e example of the manner
in which a great river bears down to the sea the matter poured into it
by a multitude of tributaries descending from lofty chains of mountains.
The changes gradually effected in the great plain of Northern Italy,
since the time of the Roman republic, are considerable. Extensive lakes
and marshes have been gradually filled up, as those near Placentia,
Parma, and Cremona, and many have been drained naturally by the
deepening of the beds of rivers. Deserted river-courses are not
unfrequent, as that of the Serio Morto, which formerly fell into the
Adda, in Lombardy. The Po also itself has often deviated from its
course, having after the year 1390 deserted part of the territory of
Cremona, and invaded that of Parma; its old channel being still
recognizable, and bearing the name of Po Morto. There is also an old
channel of the Po in the territory of Parma, called Po Vecchio, which
was abandoned in the twelfth century, when a great number of towns were
destroyed.

_Artificial embankments of Italian rivers._--To check these and similar
aberrations, a general system of embankment has been adopted; and the
Po, Adige, and almost all their tributaries, are now confined between
high artificial banks. The increased velocity acquired by streams thus
closed in, enables them to convey a much larger portion of foreign
matter to the sea; and, consequently, the deltas of the Po and Adige
have gained far more rapidly on the Adriatic since the practice of
embankment became almost universal. But, although more sediment is borne
to the sea, part of the sand and mud, which in the natural state of
things would be spread out by annual inundations over the plain, now
subsides in the bottom of the river-channels; and their capacity being
thereby diminished, it is necessary, in order to prevent inundations in
the following spring, to extract matter from the bed, and to add it to
the banks of the river. Hence it happens that these streams now traverse
the plain on the top of high mounds, like the waters of aqueducts, and
at Ferrara the surface of the Po has become more elevated than the roofs
of the houses.[339] The magnitude of these barriers is a subject of
increasing expense and anxiety, it having been sometimes found necessary
to give an additional height of nearly one foot to the banks of the
Adige and Po in a single season.

The practice of embankment was adopted on some of the Italian rivers as
early as the thirteenth