so full that they frequently run over their banks and do great
damage.

How to preserve the water thus going to waste and have it at hand
for summer use has been an important problem in regions where every
particle of water is valuable. Study of the question has led to
the examination of the streams with reference to the building of
reservoirs to hold back the flood waters. A reservoir may be formed
of a natural lake in the mountains in which the stream rises, by
placing a dam across its outlet and so making it hold more water.
If this cannot be done, a narrow place in the canon of the stream
is selected, above which there is a broad valley. At such a place
the dam which is built across the canon is held firmly in place
by the walls of rock upon each side, and an artificial lake or
reservoir is made. Ditches lead away from this reservoir, and by
means of gates the water is supplied when and where it is needed.

[Illustration: FIG. 116.--SWEETWATER RESERVOIR, NEAR SAN DIEGO,
CALIFORNIA]

The streams which furnish the water for irrigation in the arid
region rise in mountains with steep rocky slopes, and until the
water issues from these mountains it is confined to canons with
bottoms of solid rock, so that no water is lost except by evaporation.

After the streams emerge from the canons upon the long, gentle
slopes of gravel and soil which lie all about the bases of the
mountains, they begin immediately to sink into the porous material.
They frequently disappear entirely before they have flowed many
miles. Some of this water can be brought to the surface again by
digging wells and constructing pumping plants, but the greater
part is lost to the thirsty land.

To prevent the water from sinking into the gravel, ditches lined
with cement are often made to carry it from the canons to the points
where it is needed. Sometimes iron pipes or wooden flumes are used
in place of the ditches.

What a transformation irrigation makes in the dry and desert-like
valleys of the West! Land which under Nature's treatment supports
only a scanty growth of sagebrush or greasewood, and over which
a few half-starved cattle have roamed, becomes, when irrigated,
covered with green fields and neat homes, while sleek, well-fed
herds graze upon the rich alfalfa. Ten acres of irrigated land
will in many places support a family, where without irrigation a
square mile would not have sufficed.

One might suppose that the soil of these natural