638 PROCEEDINGS OF SECTION H. 
wrongly placed as regards the currents, and other dredged 
channels, presumably properly placed, which are good to 
this day; a good example of this latter may be seen in the 
new cut recently made from the great middle channel of 
the Fitzroy River into the fine basin of Broadmouth, which 
it was predicted would silt up as fast as excavated; but the 
current has taken to it properly, and it seems quite - 
permanent. 
This is not beside the question under discussion, because 
any improvement in the channel of a river will mitigate 
the effect of the floods, and in some cases may be all that is 
required to be done. 
Another, and possibly the most common method of 
avoiding the damage caused by floods, is to raise dykes or 
embankments along the banks of sufficient height to prevent 
the floods overtopping them. If this is done on a large 
scale, it is often followed by effects which are generally 
overlooked at the time, but are sure to be felt in the future. 
The valley of a river seldom offers evidence of any 
sudden or violent changes; but most valleys do undergo © 
changes, which become evident if careful observation is 
continued for sufficient length of time; and the most usual 
change is found to be that the bed of the river slowly 
rises by the deposits brought down from the mountains. 
If the river is left in its natural state this effect may 
remain unnoticed for ages, because the flat meadow land 
bordering the river is raised also by the yearly deposits 
left in it by the floods. The amount of this raising may 
vary greatly, but as a rule it corresponds roughly to the 
rate at which the river-bed rises. Now the river-bed rises 
according to the rate at which the heavier material of its 
deposits creeps down towards the sea, and however slow 
in most cases, this advance is ceaseless as long as the 
relative level of sea and land remains the same. In most 
rivers the deposits carried to the sea slowly advance the 
shore-line, which has the same effect in raising the river-bed. 
In the case of a river like the Amazon, unnumbered 
centuries may elapse before the heavy material of its 
upper waters approaches the sea, but in small rivers, like 
those of New Zealand, the process is sufficiently evident 
in a reasonable number of years. Of course, the coarser 
and heavier the material comprising the deposits of the 
bed of the river, the steeper will be the slope of the bed. 
Thus. in the Amazon and Parana the heavy deposits are 
still thousands of miles from the sea. The bed of these rivers 
has therefore as yet no slope; it is on the contrary scooped 
