642 PROCEEDINGS OF SECTION H. 
As the front of a flood travels faster than its rear, it is 
usually found that a flood coming down a river carries its 
sediments before it, and this action in time would clear out 
‘the river-bed ; but the rear of the flood is more sluggish, the 
-sediments trailing along with it are soon left stranded, and 
the sheals and sand-banks, after being partly stripped by 
the first of the flood, are replenished by it as it subsides. 
This suggests the question, whether the river annually 
delivers to the sea the same quantity of sediment that is 
put into it by its tributaries; and,it seems highly probable 
that in most, if not all, cases, it does not; the observed 
rising of the bed, and the silting up of adjacent lands, are 
proofs that to such extent at least the river does not take 
as much to sea as. it receives. 
There are many places in most rivers where defined deep 
channels are found which all sediments pass over without 
any effect, for in ’such parts sand-banks are not accumulated. 
Here the velocity of the current keeps the bed clear; and 
one would think, that if this condition of things could 
be brought about over all parts of the river, then there 
would be no deposits, and flooding of the country would be 
avoided. But the existence of deposits indicates that the 
necessary velocity is not to be found at all parts of the 
river, and to procure it artificially by traiming-walls and 
levees requires that the flat gradients of the river shall be 
made steeper, which would have the effect of raising the 
entire river above the adjacent land. Such a proceed- 
ing is equivalent to raisimg the banks by levees, and 
involves great risk of accident, and mm the lohg run becomes 
ineffectual from the gradual flattening of the gradient of 
the river-bed, caused partly by the deposits pushing the 
shore-line further out to sea, partly by the retention of all 
heavier and coarser sediments, which gradually raise the 
river-bed in spite of the scouring action. Grand examples 
of this kind of action are seen in many of the rivers of New 
Zealand, where steep mountain slopes of, say, two to one 
end abruptly in a wide river-bed of shingle. There can be 
no doubt that these mountain slopes are continued below 
the river-bed, and end in -original rock-bound river-banks, 
hundreds of feet below the present flat, shingly river-bed. 
But-the shingle, sand, and boulders have pushed the sea- 
beach twenty to thirty miles further out to sea, thus flatten- 
ing the gradient to such an extent that the present river- 
bed is raised hundreds of feet above its original rock-bound 
ravine, in which it may have flowed hundreds of thousands 
of years ago. Thus, the magnitude and the diversity of 
