502 RIVER EMBANKMENTS. 
sequently, the water of its floods, which would otherwise 
spread over a wide surface, is confined within narrow limits, 
the velocity of the current and its transporting power are aug- 
mented, and its burden of sand and gravel is deposited at some 
lower point, where the rapidity of its flow is checked by a dam 
or other artificial obstruction, by a diminution in the inclina- 
tion of the bed, by a wider channel, or finally by a lacustrine or 
marine basin which receives its waters. Wherever it lets fall 
solid material, its channel is raised in consequence, and the de- 
clivity of the whole bed between the head of the embankment 
and the slack of the stream is reduced. Hence the current, at 
first accelerated by confinement, is afterwards checked by the 
mechanical resistance of the matter deposited, and by the 
diminished inclination of its channel, and then begins again to 
let fall the earth it holds in suspension, and to raise its bed at 
the point where its overflow had been before prevented by em- 
bankment.* The bank must now be raised in proportion, and 
* In proportion as the dikes are improved, and breaches and the escape of 
the water through them are less frequent, the height of the annual inundations 
is increased. Some towns on the banks of the Po, 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 their protection. 
Lombardini lays down the following general statement of the effects of river 
embankments : 
““'The immediate effect of embanking a river is generally an increase in the 
height of its floods, but, at the same time, a depression of its bed, by reason of 
the increased force, and consequently excavating action, of the current. 
‘Tt is true that coarser material may hence be carried further, and at the 
same time deposit itself on a reduced slope. 
‘“The embankment of the upper branches of a river increases the volume, 
and therefore the height of the floods in the lower course, in consequence of 
the more rapid discharge of its affluents into it. 
‘* When, in consequence of the fiow of a river channel through an alluvial 
soil not yet regulated, or, in other words, which has not acquired its normal 
inclination, the course of the river has not become established, it is natural 
that its bed should rise more rapidly after its embankment. 
‘* The embaukment of the lower course of a river, near its discharge into the 
sea, causes the elevation of the bed of the next reach above, both because the 
swelling of the current, in consequence of its lateral confinement, occasions 
