CAUSES OF INUNDATIONS. Pat 
which is the deposit of sand, gravel, and pebbles in their beds 
by torrential tributaries during the floods.* 
In accordance with the usual economy of nature, we should 
presume that she had everywhere provided the means of dis- 
charging, without disturbance of her general arrangements or 
abnormal destruction of her products, the precipitation which 
she sheds upon the face of the earth. Observation confirms this 
presumption, at least in the countries to which I confine my 
inquiries ; for,so far as we know the primitive conditions of 
the regions brought under human occupation within the histori- 
eal period, it appears that the overflow of river-banks was much 
less frequent and destructive than at the present day, or, at 
least, that rivers rose and fell less suddenly, before man had re- 
moved the natural checks to the too rapid drainage of the basins 
in which their tributaries originate. The afiluents of rivers 
draining wooded basins generally transport, and of course let 
fall, little or no sediment, and hence in such regions the 
special obstruction to the currents of water-courses to which I 
* The extent of the overflow and the violence of the current in river-floods 
are much affected by the amount of sedimentary matter let fall in their chan- 
nels by their affuents, which have usually a swifter flow than the main stream, 
and consequently deposit more or less of their transported material when they 
join its more slowly-moving waters. Such deposits constitute barriers which 
at first check the current and raise its level, and of course its violence at 
lower points is augmented, both by increased volume and by the solid material 
it carries with it, when it acquires force enough to sweep away the obstruc- 
tion.—RIsLeR, Sur VInfluence des Foréts sur les cours @eau, in Revue des 
Eaux et Foréts, 10th January, 1870. 
In the flood of 1868 the torrent Illgraben, which had formerly spread its 
water and its sediment over the surface of a vast cone of dejection, having 
been forced, by the injudicious confinement of its current to a single channel, 
to discharge itself more directly into the Rhone, carried down a quantity of 
gravel, sand, and mud, sufficient to dam that river for a whole hour, and in 
the same great inundation the flow of the Rhine at Thusis was completely 
arrested for twenty minutes by a similar discharge from the Nolla. Of course, 
when the dam yielded to the pressure of the accumulated water, the damage 
to the country below was far greater than it would have been had the currents 
of the rivers not been thus obstructed.—Marcuanp, Les Torrents des Alpes, 
in Revue des Hauz et Foréts, Sept., 1871. 
