eristing Physical Causes during stated Periods of Time. 221 
Winter-freshets following a sudden fall of rain would raise the 
water-level of rivers rapidly, and carry it above the tanks before 
the augmented current has time to scour the river-channel and 
raise it to its former capacity. Accumulations of silt, small at 
any one place, must each raise the water a little above its proper 
level, and the point of overflow will be where the sum of these 
sinall elevations amounts to more than the height of the banks, 
above last year’s level. But floods leave a deposit of silt, &c. 
upon the banks they pass over, which increases the capacity of 
the channel ; and until new deposit has again reduced the area 
of the stream below its proper size, inundation will not occur. 
_ Aseach flood raises only the part of the bank it flows over, it 
1S easy to see that the point of overflow will be changed from 
time to time; and every part of the alluvial plains through which 
a river flows will be visited in turn by floods, provided there are 
ho artificial banks. These banks assist the scouring power of 
tivers in winter, because they retain more water in the river; but, 
on the other hand, silt that would have been carried over the 
banks is kept within the channel, and this may be the reason why 
the beds of all navigable rivers have become so much elevated 
during the historical period. The contraction of water-channels 
mM stimmer, and their enlargement in winter, is thus directly traced 
to the nnequal supply of rain at different periods of the year. 
uy is being admitted, we have an explanation of the manner in 
which rivers may, by a succession of floods, build upon alluvial 
Ge posits along their courses, at the same time raising their beds 
10 preportion to the height of their plains. 
_ 4 river-channels were perfectly symmetrical in form, the iden- 
Heal sediment that had fallen in summer might he removed again 
Winter. It is, however, well known that river-channels are 
Pon one side and shallow on the other. ‘The principal de- 
Posit therefore takes place on the shallow or quiet side, and the 
Pincipal removal occurs from the deep side where the current 
Tus more quickly. : 
is may explain why the traveller on the Mississippi sees for 
hundreds of miles a caving bank on one side, and an advancing 
ing of curves instead of straight lines must be 
Produced. When each curve, however, had assumed the com- 
'o that on the inner or shallow side. ‘The current would thus 
dual te 
dition of equilibrium might last for some time. ; 
