88 LANDS OF THE ARID REGION OF THE UNITED STATES. 
the irrigable lands should not be selected at such a distance from the source 
of the stream as to be the occasion of a great loss of water by direct and 
indirect evaporation. For general climatic reasons, the lands should be 
selected as low as possible; for economy of water as high as possible; and 
these conditions in the main will cause the selections to be made along the 
middle courses of the streams. But this general rule will be modified by 
minor physical conditions relating to soil and slope—soils that will best 
conserve the water will be selected, and land with the gentlest slopes will 
be taken. 
In general, the descent of the streams in the arid land is very great; for 
this reason the flood plains are small, that is, the extent of the lands adjacent 
to the streams which are subject to overflow at high water is limited. In 
general, these flood-plain lands should not be chosen for irrigation, from the 
fact that the irrigating canals are liable to be destroyed during flood seasons. 
Where the plan of irrigation includes the storage of the water of the non- 
growing season, by which all the waters of the year are held under control, 
the flood-plain lands can be used to advantage, from the fact that they lie in 
such a way as to be easily irrigated and their soils possess elements and 
conditions of great fertility. 
Other locally controlling conditions are found in selecting the most 
advantageous sites for the necessary water works. 
These are the chief physical factors which enter into the problem, and 
in general it will be solved by considering these factors only; but occasion- 
ally artificial conditions will control. 
The mining industries of the Arid Region are proportionately greater 
than in the more humid country. Where valuable mines are discovered 
towns spring up in their immediate vicinity, and they must be served with 
water for domestic purposes and for garden culture. When possible, agri- 
culture will be practiced in the immediate vicinity for the purpose of taking 
advantage of the local market. In like manner towns spring up along the 
railroads, and agriculture will be carried on in their vicinity. For this and 
like reasons the streams of the Arid Region will often be used on lands 
where they cannot be made the most available under physical conditions, 
and yet under such circumstances artificial conditions must prevail. 
