FORESTS AND FLOODS
Forests are necessary at the headwaters of streams. The trees
break the force of the rain drops, and the forest floor, acting
as a large sponge, absorbs rainfall and prevents run-off and
floods. Unless there are forests at the sources of streams and
rivers, floods occur. The spring uprisings of the Mississippi,
Ohio and Missouri Rivers are due largely to the lack of forests
at their headwaters. In the regions drained by these streams the
run-off water is not absorbed as it should be. It flows unimpeded
from the higher levels to the river valleys. It floods the river
courses with so much water that they burst their banks and pour
pell-mell over the surrounding country. Many floods which occur
in the United States occur because we have cut down large areas
of trees which formerly protected the sources of streams and
rivers.
A grave danger that threatens western farming is that some time
in the future the greater part of the vegetation and forest cover
on the watersheds of that section may entirely disappear. Such a
condition would cause floods after every heavy rain. The
available supplies of rainwater which are needed for the thirsty
crops would be wasted as flood waters. These floods would cause
great damage in the valleys through which they rushed. The
freshets would be followed by periods of water famine. The
streams would then be so low that they could not supply the
normal demands. Farmers would suffer on account of the lack of
irrigation water. Towns and cities that depended on the mountain
streams for their water supplies would be handicapped severely.
In a thousand and one ways, a deficient water supply due to
forest depletion would cause hardships and suffering in the
regions exposed to such misfortune.
The important part which forests play in the development of our
country is shown by the fact that from the streams of the
National Forests over 700 western cities and towns, with an
aggregate population of nearly 2,500,000, obtain their domestic
water supply. The forests include 1266 irrigation projects and
325 water-power plants, in addition to many other power and
irrigation companies which depend on the Government timberlands
for water conservation and the regulation of rain water run-off
and stream flow.
The National Forests aid greatly in conserving and making
available for use the precious limited rainfall of the arid
regions. That is why settlers in irrigated districts are deeply
interested in the cutting of timber in the Federal woodlands.
Destructive lumbering is never practiced in these forests. In its
place has been substituted a system of management that assures
the continued preservation of the forest-cover. Uncle Sam is
paying special attention to the western water-sheds which supply
reclamation and irrigation projects. He understands that the
ability of the forest to regulate stream flow is of great
importance. The irrigation farmers also desire a regular flow,
evenly distributed, throughout the growing season.
One of the chief reasons for the establishment of the National
Forest was to preserve the natural conditions favorable to stream
flow. In a treeless country, the rise of the streams is a very
accurate measure of the rainfall. In the region where forests are
frequent, an ordinary rain is scarcely noticed in its effect on
the stream. In a denuded district no natural obstacles impede
the raindrops as they patter to the ground. The surface of the
soil is usually hard. It is baked and dried out by the sun. It is
not in condition to absorb or retain much of the run-off water,
consequently, the rain water finds little to stop it as it swirls
down the slopes. In torrents it rushes down the stream beds, like
sheets of water flowing down the steep roof of a house.
Conditions are very different in a region where forest cover is
abundant. In the forests, the tops of the trees catch much of the
rain that falls. The leaves, twigs, branches and trunks of the
trees also soak up considerable moisture. The amount of rainfall
that directly strikes the ground is relatively small. The upper
layer of the forested ground consists of a network of shrubs, and
dead leaves, branches, and moss. This forest carpet acts like an
enormous sponge. It soaks up the moisture which drops from the
trees during a storm. It can absorb and hold for a time a
rainfall of four or five inches. The water that finally reaches
the ground sinks into the soil and is evaporated or runs off
slowly. The portion that is absorbed by the soil is taken up by
the roots of the trees and plants or goes to supply springs and
watercourses.
The power of the trees and forest soil to absorb water regulates
the rate at which the rainfall is fed to the streams and rivers.
Frequently it takes weeks and even months for all the waters of a
certain rain to reach these streams. This gradual supplying of
water to the streams regulates their flow. It prevents floods and
freshets. Careful observation and measurements have shown that
unforested regions will discharge rain water at least twice as
fast as will forested districts.
The stealing of soil by erosion occurs where run-off waters are
not obstructed by forest growth. Silt, sand, and every other kind
of soil are swept from their natural positions and spritted away
by the foaming waters as they surge down the steep slopes. The
stream or river which is flooded by these rushing waters roars
down its narrow channel, tearing loose and undermining the
jutting banks. In some cases, it will break from its ordinary
course to flood exposed fields and to carry away more soil. As
the speed of the stream increases its power to steal soil and
carry it off is increased. Engineers report that the carrying
power of a stream is increased 64 times when its rate of flow is
doubled. If the flow of a river is speeded up ten times, this
raging torrent will be able to carry one million times as much
foreign material as it did when it was flowing at a normal rate
of speed, causing inexpressible damage and destruction of life
and property.
The protection afforded by forests on the water-sheds of streams
furnishing the domestic water supply for cities and towns is
becoming more fully realized. A large number of cities and towns
have purchased and are maintaining municipal or communal forests
for this very reason.