the cost of construction, rather than attempt to supply water to lands already under ditch and partly irrigated.

Some of the best storage projects, however, are so situated that the water can not be taken readily to new areas, and it appears probable that these, although among the most important in the country, can only be constructed by the adoption of some scheme by which all of the lands of the valley benefited directly or indirectly will be forced to contribute an amount equal to the benefits derived. This is possible only under some system similar to that of the district organization in California, where all of the lands receiving water from a given source are assessed. Here it is not necessary to consider whether the enterprise will be money making in itself. Thus it is possible to push forward the construction of beneficial works which otherwise never could be built.

F. H. N.

IRRIGATION NEAR PHOENIX, ARIZONA.

BY ARTHUR P. DAVIS.

TOPOGRAPHIC FEATURES OF ARIZONA.

The Territory of Arizona covers approximately 113,000 square miles, of which about 39,000 lie below the altitude of 3,000 feet, about 27,000 lie between the contours of 3,000 and 5,000 feet, and about 47,000 lie above the elevation of 5,000 feet. The highest point in the Territory is San Francisco Mountain, in the northern part, which reaches an altitude of nearly 13,000 feet. The Territory is sharply divided into two characteristic portions by the trend across it of the main axis of the great Colorado Plateau, from the northwestern corner of the Territory in a nearly southeasterly direction. This plateau slopes gently to the northward, but on the southwestern side breaks off suddenly throughout most of its course, and its steep slope is deeply carved by lines of erosion. Almost the whole of that portion of the Territory which is below an elevation of 3,000 feet lies to the southward of this escarpment.

To the north of the escarpment the temperature ranges from that of the temperate zone to that where snow is nearly perpetual, on the summits of the San Francisco and White mountains. The southern portion of the Territory is characterized by temperatures which may be designated as ranging from temperate, along the foothills of the Colorado Plateau, to semitropic, in the lower valley of the Gila and Colorado. The southern portion of the Territory may be again subdivided into two portions, that draining directly into the Colorado and lying to the westward of Prescott, and the greater portion to the south and east, which forms the great Gila river system. The Colorado Plateau is partly of igneous origin, and a great portion of it is somewhat pervious to water. Its northern slope for a considerable distance from the summit is very gentle, and though the precipitation is greater than in most portions of the Territory, it is very meagerly marked by

drainage lines and almost destitute of water. Sharply contrasted with these facts are the conditions on the southern slope. Here, through most of its course, the plateau drops off with a very steep slope, which is deeply cut with drainage lines in which are living creeks and rivulets of clear, beautiful water, such as San Francisco River, Black Creek, Bonito Creek, White River, Carrizo Creek, Cibicu Creek, Box Creek, Cherry Creek, Tonto Creek, Wild Rye Creek, East Verde River, Pine Creek, Fossil Creek, Clear Creek, Beaver Creek, etc.

The region of high altitude, as before remarked, lies largely north of the divide, while the great bulk of the water flowing from the plateau, as proved both by erosion of drainage lines and by the volume of permanent streams, flows away to the south. The explanation of this is partly the porosity of the strata composing the plateau, which allows the water to sink instead of flowing off the surface. Once underground, its egress to the south is favored by the shorter distance which it must percolate on a given grade before reaching a surface, due to the more abrupt slope.

Another partial explanation is found in the meteorological condition. The moisture of this region is brought from the Pacific Ocean and the Gulf of California by the prevailing southwest wind. As this wind ascends the elevations toward the Colorado Piateau, its temperature is lowered, which reduces its capacity for holding moisture and increases its relative humidity. When this quantity reaches 100 per cent in any part, precipitation occurs. This influence continues until

the wind passes the summit, where the process is reversed.

As might be expected, therefore, the hydrographic resources of the country immediately southwest of the Colorado Plateau are disproportionately great when compared with those to the northward. For instance, the precipitation at Fort Apache, as shown by a mean of twenty years' observations, is 19.75 inches, the elevation being 5,050 feet, while the precipitation at Holbrook, at an elevation of 5,047 feet, on the northern slope, is 8.47 inches, as indicated by the mean of ten years' observations. This is an important fact, especially when taken in connection with the fact that the great areas of valley land with a semitropic climate lie in the southwestern portion of the Territory, and are easily covered by the streams which are formed by the conditions above described, and which constitute the main features of the great Gila river system.

GILA BASIN.

The drainage area of Gila River, including a number of small lost basins which are topographically tributary but which seldom or never furnish any run-off to the main stream, is about 72,000 square miles, of which nearly 57,000 lie in the Territory of Arizona, about 14,000 in New Mexico, and something over 1,000 in Mexico. The areas in