SEDIMENT OP THE PO. 
255 
The bed of the river has been somewhat elevated by the de¬ 
posits in its channel, though not by any means above the level 
of the adjacent plains as has been so often represented. The 
dikes, which confine the current at high water, at the same 
time augment its velocity and compel it to carry most of its 
sediment to the Adriatic. It has, therefore, raised neither its 
own channel nor its alluvial shores, as it would have done if it 
had remained unconfined. But, as the surface of the water in 
floods is from six to fifteen feet above the general level of its 
banks, the Po can, at that period, receive no contributions of 
earth from the washing of the fields of Lombardy, and there is 
no doubt that a large proportion of the sediment it now de¬ 
posits at its mouth descended from the Alps in the form of 
rock, though reduced by the grinding action of the waters, in 
its passage seaward, to the condition of fine sand, and often 
of silt.* 
We know little of the history of the Po, or of the geog¬ 
raphy of the coast near the point where it enters the Adriatic, 
at any period more than twenty centuries before our own. 
Still less can we say how much of the plains of Lombardy had 
been formed by its action, combined with other causes, before 
man accelerated its levelling operations by felling the first 
woods on the mountains whence its waters are derived. But 
we know that since the Boman conquest of Northern Italy, its 
deposits have amounted to a quantity which, if recemented 
into rock, recombined into gravel, common earth, and vege¬ 
table mould, and restored to the situations where eruption or 
upheaval originally placed, or vegetation deposited it, would 
fill up hundreds of deep ravines in the Alps and Apennines, 
change the plan and profile of their chains, and give their 
* In proportion as tlie dikes are improved, and breaches and the escape 
of the water through them are less frequent, the height of the annual inun¬ 
dations is increased. Many towns on the banks of the river, and of course 
within the system of parallel embankments, were formerly secure from 
flood by the height of the artificial mounds on which they were built; but 
they have recently been obliged to construct ring dikes for theii piotec- 
tion.— Batjmgarten, after Lombardini, in the paper last quoted, pp. 141,147. 
