re 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
are the first to suffer from the drouths that follow and, during the
period of floods, whole cities are often inundated. Fig. 128 shows
such a scene. The history of Forestry is full of horrible incidents
of the loss of life and property from floods which are directly
traceable to the destruction of the local forests and, on the other
hand, there are many cases on record where flood conditions have
been entirely obviated by the planting of forests. France and
Germany have suffered from inundations resulting from forest
devastation and, more than a hundred years ago, both of these
countries took steps to reforest their mountain slopes, and thereby
to prevent many horrible disasters.

[Illustration: FIG. 129.--Planting a Forest with Seedling Trees on the
Nebraska National Forest. The man on the right is placing the tree in a
slit just made with the spade. The man on the left is shoveling the dry
sand from the surface before making the slit for the tree.]

[Illustration: FIG. 130.--Diagrammatic Illustration of a Selection
Forest.]

How forests are established: New forests may be started from seed or
from shoots, or suckers. If from seed, the process may be carried on
in one of three ways:

First, by sowing the seed directly on the land.

Second, by first raising young trees in nurseries and later setting
them out in their permanent locations in the forest. This method is
applicable where quick results are desired, where the area is not
too large, or in treeless regions and large