DAMAGE BY FLOODS IN RIVERS PREVENTION. 637 
reach the sea for ages, being used up in raising the bed of 
the river and making new land. or being reduced by wear 
and tear to smaller materials. 
Although a river-bed may be encumbered by many great 
banks of deposits ali the way to its mouth, yet it displays 
much uniformity from year to year, and only careful 
Measurements May show any changes; yet the deposits are 
constantly travelling, and like the strange phenomenon of 
drifting beaches, there is no apparent change, because the 
material removed is balanced by the supply. 
Although the deposits are constantiy travelling along its 
bed, yet every river has reaches and. channels in which the 
depth and width do not vary perhaps for centuries; from 
which it may be inferred that there are positions and con- 
ditions of flow which are not affected by any deposits, 
whether in great or small quantities; and in seeking to 
improve a river with the view of abating the effect of floods 
and maintaining its bed in the improved condition one 
naturally seeks to imitate the good points found to exist in 
the river as it is. This would not be so difficult were it 
not for the reverse curves in every river, and the great 
irregularities in the widths. Irregular width can of course 
be dealt with, but reverse curves, called “cross-overs” by 
the Americans, have always proved to be difficult to deal 
with, especially in large rivers. Any one can see where the 
difficulty lies, and that it is a matter of comparison. For 
instance, the channel of a river flowing round a curve may 
be 40 feet deep, but the same stream “flowing in a straight 
line may not give a channel more than 10 feet deep. In 
the space between two reverse curves the river is flowing in 
a straight line, and any attempt to train this part would 
tend to block up the river, and therefore cannot be done. 
The German engineers have found in their practice, or one 
of them has asserted as an axiom, that by intelligent study 
and observation, the natural or even the artificial axis of 
the current can be ascertained at any part of a river, and 
that any ftmprovements, such as dredging, if done in this 
axis, will be permanently maintained by -the river. 
This seems likely to be true, because we see that the river 
has in various places natural channels which are permanent 
and never vary in width or depth, but I can conceive the 
difficulty of deciding upon an axis of currents along which 
all artificial channels will be permanent, seeing that the 
least alteration in the bed or banks may start changes 
which would tend to entirely alter the conditions. I have 
seen costly dredging entirely obliterated through being 
