here men have destroyed the earth cover,
have little to detain them and are soon on their way back to their home.
In their hasty journey they do much damage to the unprotected soil.

If the drops fall upon gentle slopes, or where there are marshes and
lakes, or upon the forest with its decaying vegetation, or upon deep
beds of gravel and sand, they are a long time getting back to the ocean.

[Illustration: _George J. Young_
The cool and shady stream before men came and cut the trees away so that
the hot sun could get at it.]

We can in no way change the amount of rain that falls upon any part of
the earth. We cannot call up a storm when we wish it, nor can we send it
away when there has been rain enough. But there are many ways in which
we can hasten or delay the return of the water to the ocean. Nature
shows us some of these. The spongelike carpet underneath the forest
holds the water until it has had time to soak into the earth from which
it later emerges as springs. Nature forms basins on the heads of the
rivers where a part of the water, instead of immediately flowing away,
collects in the form of lakes. From these lakes the water runs away
slowly instead of in torrential floods.

[Illustration: _H. W. Fairbanks_
The rotting tree trunks take up the rainwater like a sponge.]

Only a few places in our country have more rain than is really needed.
One of these is the region about the mouth of the Mississippi River
upon the Gulf of Mexico. Another is upon the Northwest coast. Throughout
the central part of the country the summer rains are sometimes too light
to afford a full harvest. The rainfall upon the plains and valleys of
the Southwest is so small that the only plants that can live there are
those strange and curious forms that have become used to desert
conditions. The only way in which these lands can be made useful to the
farmer is by means of irrigation. To obtain water for irrigation we have
either to go to the distant mountains and build reservoirs to collect
the rains which fall there and then dig canals to carry the water to the
desert valleys, or to make use of some river flowing through them, if
they are fortunate enough to have such a river. Can you think of any
rivers that are used in this way?

[Illustration: _Brown Brothers_
The great Roosevelt Dam, in the Salt River irrigation project,
Arizona.]

Although water sometimes seems the greatest blessing that we have, yet
it may pr