A river emerging from a mountain region and meandering through a valley may receive small tributaries along its valley course. These small streams will usually be taken out first, and the lands which they will be made to serve will often lie low down in the valley, because the waters can be more easily controlled here and because the lands are better; and this will be done without regard to the subsequent use of the larger stream to which the smaller ones are tributary. But when the time comes to take out the larger stream, it is found that the lands which it can be made to serve lying adjacent on either hand are already in part served by the smaller streams, and as it will not pay to take out the larger stream without using all of its water, and as the people who use the smaller streams have already vested rights in these lands, a practical prohibition is placed upon the use of the larger river. In Utah, church authority, to some extent at least, adjusts these conflicting interests by causing the smaller streams to be taken out higher up in their course. Such adjustment is not so easily attained by the great body of people settling in the Rocky Mountain Region, and some provision against this difficulty is an immediate necessity. It is a difficulty just appearing, but in the future it will be one of great magnitude. INCREASE OF IRRIGABLE AREA BY THE STORAGE OF WATER. Within the Arid Region great deposits of gold, silver, iron, coal, and many other minerals are found, and the rapid development of these mining industries will demand _pari passu_ a rapid development of agriculture. Thus all the lands that can be irrigated will be required for agricultural products necessary to supply the local market created by the mines. For this purpose the waters of the non-growing season will be stored, that they may be used in the growing season. There are two methods of storing the waste waters. Reservoirs may be constructed near the sources of the streams and the waters held in the upper valleys, or the water may be run from the canals into ponds within or adjacent to the district where irrigation is practiced. This latter method will be employed first. It is already employed to some extent where local interests demand and favorable opportunities are afforded. In general, the opportunities for ponding water in this way are infrequent, as the depressions where ponds can easily be made are liable to be so low that the waters cannot be taken from them to the adjacent lands, but occasionally very favorable sites for such ponds may be found. This is especially true near the mountains where alluvial cones have been formed at the debouchure of the streams from the mountain cañons. Just at the foot of the mountains are many places where ancient glaciation has left the general surface with many depressions favorable to ponding.