eased
by artificial means. Dikes or levees are built to regulate the spread and
flow of the water and to protect the land from destructive floods. Dams
and reservoirs are constructed for the storage of water, which is led by a
system of canals and ditches to irrigate large tracts of land which would
be otherwise worthless. By means of irrigation, the farmer has control of
his water supply and is able to get larger returns than are possible where
he depends upon the irregular and uncertain rainfall. It is estimated that
in the arid regions of western United States there are 150,000 square
miles of land which may be made available for agriculture by irrigation.
Perhaps in the future the valley of the lower Colorado may become as
productive as that of the Nile.

Streams are the easiest routes of travel and commerce. A river usually
furnishes from its mouth well up toward its source a smooth, graded
highway, upon which a cargo may be transported with much less effort than
overland. If obstructions occur in the form of rapids or falls, boat and
cargo are carried around them. It is often easy to pass by a short portage
or "carry" from one stream system across the divide to another. In regions
which are not very level the easiest grades in every direction are found
along the streams, and the main routes of land travel follow the stream
valleys. In traversing a mountainous region, a railroad follows the
windings of some river up to the crest of the divide, which it crosses
through a pass, or often by a tunnel, and descends the valley of some
stream on the other side.

Man is largely indebted to streams for the variety and beauty of scenery.
Running water itself is attractive to young and old. A landscape without
water lacks its chief charm. A child instinctively finds its way to the
brook, and the man seeks beside the river the pleasure and recreation
which no other place affords. Streams have carved the surface of the land
into an endless variety of beautiful forms, and a land where stream
valleys are few or shallow is monotonous and tiresome. The most common as
well as the most celebrated beauty of scenery in the world, from the tiny
meanders of a meadow brook to the unequaled grandeur of the Colorado
canyons, is largely due to the presence and action of streams.

--Dryer: _Lessons in Physical Geography_.

In the above selection we find that each group of sentences is related to
some main topic. A more extended observati