carried down to bed rock, which, in the deepest place, is about 30 feet. An outer shell of carefully laid dry rock wall is to be built, which will be carried down to bed rock at the down-stream toe. Spillways will be excavated at each end of the dam, and it is from these that rock will be obtained for the construction of the dam. Little is known of the hydrographic possibilities of this basin, owing to the almost total absence of data on the subject. A record of rainfall ten months in duration was kept at Silver King, but its results appear to be of doubtful utility. Measurements of discharge were begun at Whit

low's ranch in July, 1896. It is very difficult to make these measurements with any considerable degree of accuracy, owing to the extremely flashy character of the stream. Almost the entire discharge at this point is in the form of violent floods. It is estimated that a supply of about 10,000 acre-feet per annum can be depended upon from this project, sufficient to reclaim about 5,000 acres of land at a cost somewhere about $200,000. A table and diagrams of discharge of this stream may be found on page 42. An abundance of excellent land lies near at hand, with perfectly smooth surfaces and a maximum slope of about 40 feet to the mile.

THE LOWER GILA STORAGE RESERVOIR.

This project contemplates the construction of a large dam on the Lower Gila in the gorge below Oatman and Cottonwood flats, not far from the railroad station of Sentinel. So great diversity of climate

and topography is embraced by the drainage tributary to this reser voir that it is likely that it would receive a large run-off available for storage for irrigation and very materially increase the area reclaimable after all reservoirs in the upper portion of the basin are utilized.

Work on this reservoir was begun in 1892, but was discontinued, owing to financial difficulties, in the year 1893. It is the plan at present to construct at this point a dam 50 feet in height, a section of which is shown in Fig. 15. It is to be an overflow dam, protected on both faces with asphalt concrete. Seepage under the dam is to be cut off, so far as possible, by three rows of fluted sheet piling, one row at the toe, one at the axis, and one at the heel of the dam. This dam is to be utilized also as a diversion dam to raise the waters into the proposed canal, which will be taken out about 30 feet above the natural bed of the river, leaving 20 feet storage capacity above the bottom of the canal. The amount of this storage capacity is not known, but is undoubtedly great.

FUTURE DEVELOPMENTS.

Further development of irrigation in Arizona by the simple diversion of water from the Gila River and its tributaries is impossible. As already indicated, the dry-weather flow of these streams is overappropriated. The area irrigated from such streams can be increased by more economical use of the water now claimed, but the actual water supply for irrigation can be increased only by storage and in some degree by the development of underground sources.

NATURAL ADVANTAGES.

The vast extent of land in southern Arizona of surpassing fertility, admirably situated for irrigation, and with a climate the aridity and warmth of which make it at once exceptionally healthy and marvelously productive, and depending for its development and reclamation solely upon the practicable water supply, renders the item of water supply of vital importance to the future history of the Commonwealth. It is impossible in the present state of knowledge regarding this question to make even an approximate computation of the extent to which this water supply can be increased. Enough is known, however, to give to a summary of the known possibilities considerable interest and some scientific value. Unquestionably the main reliance for the increase in water supply is the storage of storm and winter waters and those of the season of melting snows, to be held and allowed to flow upon the land only as needed, instead of flowing to the sea, or of being applied for a short time in excess only to evaporate.

STORAGE OF FLOODS.

In some respects the topographic and climatic conditions in Arizona are peculiarly favorable for the complete utilization of its hydrographic possibilities. Few sections of the country are so well supplied with excellent reservoir sites favorably situated for the conservation of the surplus waters well above the areas to be irrigated. These favorable