Forests prevent soil erosion and floods: Forests help to regulate the
flow of streams and prevent floods. Most streams are bordered by
vast tracts of forest growths. The rain that falls on these forest
areas is absorbed and held by the forest soil, which is permeated
with decayed leaves, decayed wood and root fibers. The forest floor
is, moreover, covered with a heavy undergrowth and thus behaves like
a sponge, absorbing the water that falls upon it and then permitting
it to ooze out gradually to the valleys and rivers below. A forest
soil will retain one-half of its own quantity of water; i.e., for
every foot in depth of soil there can be six inches of water and,
when thus saturated, the soil will act as a vast, underground
reservoir from which the springs and streams are supplied (Fig.
125). Cut the forest down and the land becomes such a desert as is
shown in Fig. 126. The soil, leaves, branches and fallen trees dry
to dust, are carried off by the wind and, with the fall of rain, the
soil begins to wash away and gullies, such as are shown in Fig. 127,
are formed. Streams generally have their origins in mountain slopes
and there, too, the forests, impeding the sudden run off of the
water which is not immediately absorbed, prevent soil erosion.

[Illustration: FIG. 128.--Flood in Pittsburgh, Pa.]

Where the soil is allowed to wash off, frequent floods are
inevitable. Rain which falls on bare slopes is not caught by the
crowns of trees nor held by the forest floor. It does not sink into
the ground as readily as in the forest. The result is that a great
deal of water reaches the streams in a short time and thus hastens
floods. At other periods the streams are low because the water which
would have fed them for months has run off in a few days. The farms