372 
THE ECOXOMIC ASPECTS OF SOIL EROSION 
of the rain-water enters into the earth and thus is preserved 
from doing erosive work. The result is that even in times of 
flood the streams draining from these fields covered b}" glacial 
debris are never very muddy ; they have no imi)ortant alluvial 
j)lains and characteristically lack the deltas or detritic cones 
which are such a prominent feature of the streams which flow 
from non-glaciated regions. In such fields soil erosion is so slight 
that it maj^ give us no concern, except on the .steeper mountain 
slopes, where the slii>ping of the deposited soils into the torrents 
may occasionally exi)ose the bare rocks. 
On the alluvial plains — at least .so long as the down-cutting of 
the stream bed does not leave them above the level of the floods — 
the annual inundations constantly bring down layers of fertilizing 
sediment, and this at a rate which is pretty sure to compensate for 
any waste which the most reckless agriculture can bring about. 
Such soils, unlike those of our ordinary fields, grow by accessions 
on the surface and not by the decay of the bed-rocks. It is to this 
constant superficial gain in fertile materials that they owe their 
j)eculiar value to man. Alluvial lands are, however, subjected to 
a ])eculiar kind of erosion — that brought about by the lateral 
swinging of the river channels to and fro in their flood-plains. 
These peculiar, iiendulum-like movements of the great rivers 
through their delta accumulations are important for the rea.son 
that they are not easily controlled and are often disastrous to 
the interests of men who dwell upon their banks. The move- 
ments are often made in a very raj)id manner; moreover, where 
the streams change their courses in one portion of the alluvial 
]»lains the regimen of the currents is so altered that the curves 
thence downward to the mouth are subject to ra])id modifications. 
In this manner the Missis.sipj)i has been endlessly wandering 
over the flood-plains between Cairo and the sea. 
The natural check to the divagations of a river is found in the 
protective action due to the growth of trees upon its banks. 
There are many species which have habits of growth that permit 
them to flourish in places where their roots and stems are bathed 
by the floods for a considerable part of the year. These forms 
have fine roots which entangle the sediments deposited at high 
water, and they have a habit of growing in close order, so that 
their thick-set stems arrest the current and cause a plentiful 
deposition of sediment on the gravel which they occupy. If 
such a forest develops on one side of a river while the banks on 
the oi)posite border are not thus protectee^, the result usually is 
