gion south of the thirty-fifth parallel of latitude in that State, an area of approximately 72,000 square miles.

The sources of the Gila rise among the eastern spurs of the Mogollon Mountains in New Mexico, at an elevation of from 7,000 to 8,000 feet above sea level. This range forms the Continental Divide between the waters flowing into the Gulf of Mexico on the east and the Gulf of California on the west. Flowing in a generally southwesterly direction, the Gila enters Arizona at latitude 32° 40′ north, at an elevation of about 3,800 feet, and following a course westward across the entire State corresponding closely to the thirty-third parallel, finally joins the Colorado at Yuma, at an elevation of approximately 120 feet above sea level. The distance along the Gila from its source to its confluence with the Colorado is approximately 450 miles.

The principal tributaries entering the Gila between its source and its mouth, with the extent of the respective drainage areas, and the average elevation are shown in the following table:

Total area of Gila Basin (inclusive of tributaries) above San Carlos, 13,455 square miles.

In New Mexico the watershed is for the most part a high rolling plateau, passing through contracted canyons, which occasionally widen into narrow valleys, where small plots of ground are irrigated. About 7 miles east of the Arizona line the canyon broadens into the Duncan Valley. This valley is 30 miles long, with an average width of one-half mile, although its maximum width is 14 miles.

About 10 miles above its lower extremity the valley is crossed by a lava ridge, through which the river has cut a gorge known locally as York Canyon. Below York Canyon the Duncan Valley again broadens out, but it is then gradually contracted by the long, stony ridges extending from mountains known as the Sierre de la Petihaya and the Sierra Nutanes.

The San Francisco River drains the latter range and enters the Gila from the north about 15 miles below York Canyon. A few miles farther west two other tributaries, Eagle and Bonita Creeks, enter from the same direction.

After leaving the Duncan Valley the Gila passes through a narrow box canyon for a distance of about 20 miles, after which it emerges into the Solomonville Valley. This valley terminates at the San Carlos Box Canyon, which, together with the lower or western portion of the valley, is included in the San Carlos Indian Reservation.

The Solomonville Valley is about 70 miles long, that portion which lies east of the San Carlos Reservation having a length of 40 miles and an average width of 2 miles, while the portion which lies within

the San Carlos Reservation is 30 miles long and has an average width of 1 miles.

San Simon Creek, a comparatively unimportant tributary, enters the Gila from the south, a short distance below the head of the Solomonville Valley, while near the lower end of the valley the San Carlos enters the Gila from the north.

About 7 miles below the mouth of the San Carlos the Gila enters the San Carlos Box Canyon, the upper end of which forms the San Carlos Dam site.

The San Carlos Box is one of a series of narrow canyons extending for a distance of 70 miles through the Santa Teresa, Tortilla, and other detached mountain ranges to the head of the Florence Valley. About midway of this distance the San Pedro River enters from the south, and several miles farther west an unimportant tributary, known as Mineral Creek, joins the Gila.

The Gila emerges from the mountains about 16 miles above Florence and enters the broad desert plain which extends from that point to the confluence of the Gila and Colorado Rivers at Yuma, a distance of 243 miles.

Detached mountain ranges, such as the Sacaton, Estrella, Salt River, Gila Bend, Mohawk, and others, rising abruptly from the valley floor, form imperfect topographical divisions or interruptions in the general plain of the desert.

The upper end of this valley embraces the land which may be irrigated from the proposed San Carlos Project. This area includes the irrigated land contiguous to Florence, known as the Florence Valley, and a portion of the irrigable land of the Gila River Indian Reservation, the east line of which is located 11 miles west of Florence. South of the Gila the east line of the reservation practically coincides with the eastern limits of the Sacaton Mountains, which form the division between the Gila and the Santa Cruz watersheds. This stream joins the Gila at the base of the Estrella Range, some 50 miles below Florence. The town of Casa Grande, located on the Southern Pacific Railroad, is within the Santa Cruz watershed and is the center of a large territory that may also be irrigated from the Gila River, the character of country between the two watersheds offering no serious obstacle to the construction of a suitable conduit. On the north the San Tan and Salt River Mountains form the division between the Gila and Salt River Basins. The latter stream is the most important tributary of the Gila and joins it near the western boundary of the Gila River Indian Reservation, which extends for a distance of 52 miles along the Gila River. A little more than a mile below the mouth of the Salt River the Gila is joined by the Agua Fria, which drains the region immediately to the west of the Salt River Basin.

The Hassayampa, the last tributary of any importance, enters the Gila from the north at a point 23 miles further west. A short distance below the Hassayampa the Gila makes a long bend to the south and west around the Gila Bend Mountains. West of Gila Bend there are more detached ranges of mountains, and the valley lands along the river alternate with the mountain areas in about equal proportions. These conditions are not such as to admit of tributary drainage basins of any magnitude. However, between the Hassayampa and the Colorado numerous small washes are found which con

tribute to the water supply of the Gila only during times of heavy rains.

GEOLOGY.

Considerable variation in geologic structure is to be expected in an area as large as the Gila River watershed. Unfortunately, as far as can be learned, the geology of this region has not as yet received the attention of a trained geologist.

The upper reaches of the rivers forming the Gila system drain the eastern portion of the great plateau occurring in western New Mexico and eastern Arizona.

This plateau, which is of sedimentary origin, has been covered throughout a large part of its area by lava flow, which has since been greatly eroded. From these high regions the Gila and its tributaries rapidly descend through a broken country, which ́has been subject to extensive volcanic action, until the river emerges into the lower desert region in the vicinity of Florence.

The area drained by the Gila and its tributaries west of Florence is characterized by broad, level valleys, separated by low, abrupt mountain ranges, usually of volcanic formation. The valleys were formed by a process of aggradation, in which the original topography has become so obliterated under immense deposits of presumably Pleistocene sands and gravels that only the higher ridges of what were once mountain ranges are visible.

As a consequence, of the topography of the Gila watershed the streams change from eroding agents in the mountains of the upper reaches and the intermediate valleys to aggrading agents in the desert region to the west. In the intermediate valleys it is apparent that in former times the character of the river has changed from an eroding to an aggrading agent.

This change was evidently due to volcanic action, since the sedimentary rocks in this region have been subject to considerable upheaval, while in the higher mountain regions, where the rivers have been continually eroding, very little displacement has taken place. Consequently it is seen that the upper reaches of the Gila and its principal tributaries have cut through the Tertiary gravels, conglomerates, and lavas, and to some extent the earlier formations; while further west the canyons are eroded through the tilted limestones and quartzites of the Carboniferous and even earlier areas, as well as the later formation and more recent lava flows.

The streams of the mountain region are characterized by deep canyons, which occasionally widen out into long, narrow valleys, the bottom of which have been filled by recent alluvial deposits. The intermediate valleys, namely, the Duncan and Solomonville Valleys, have been partially filled by large deposits of sand and clay, evidently belonging to the Pleistocene period. These earlier deposits have been partially eroded through, and again partly filled with recent alluvial sand and silt. It is the more recent valley fill that constitutes the land now suitable for cultivation.

Where the Gila or any one of its tributaries breaks through an intervening mountain range, an attractive dam site frequently results. This is notably the case where the canyon occurs just below the confluence of the main stream and one of its tributaries, which per

mits the use of basins of both streams for storage purposes. This condition exists on Salt River, where the Roosevelt Dam is located, immediately below the mouth of Tonto Creek, and also at the San Carlos dam site, which is located on the Gila, a short distance below the mouth of the San Carlos River.

Several other excellent dam sites are found along the Gila, both below and above the San Carlos Dam site; those below at the Buttes, at Riverside, and possibly at Sentinel; those above at Guthrie and Red Rock, on the Gila proper, and at Alma, on the San Francisco. The three sites above San Carlos may form a solution of the difficult silt problem, which now seems seriously to limit the life of the proposed San Carlos project. If the construction of these reservoirs would so regulate the flow of the Gila as to prevent destructive floods, which now invariably erode and carry down the river large sections of cultivated land, it seems that, while the silt problem would not be entirely solved, the life of the proposed reservoir might be increased to compare favorably with that at Roosevelt.

The area drained by the Gila and its tributaries west of Florence does not lend itself favorably to the construction of storage works, owing to the absence of reservoir sites and the great depth to bedrock, although one reservoir site, located near Gila Bend, has received considerable attention. This region is, however, admirably adapted to the storage of underground water in the voids of immense deposits of débris. It is safe to predict that considerable development may be expected in this section through pumping. Over a considerable portion of this area, however, the deposits are very deep, and the water-bearing gravels lie at too great a depth for economical pumping.

The rich alluvial bottoms of the valleys about Florence and the broad, fertile plains to the west only require the application of water to make them of great agricultural value.

CLIMATE.

The great difference in elevation existing in the Gila River watershed induces great climatic variations. Along the headwaters of the Gila, at elevations ranging from 5,000 to 7,000 feet, the climate is comparable to that of Virginia. The average maximum temperature is 100 degrees, the average minimum is -8 degrees.

In the Solomonville Valley, which includes San Carlos, and which is situated at an elevation of 2,000 to 3,000 feet, the average maximum temperature is 106 degrees, the average minimum temperature is 8 degrees, while the normal precipitation is 13 inches.

From Florence to the mouth of the Gila at Yuma the elevation drops from 1,500 feet to 140 feet above sea level, and true desert conditions prevail. Here the average maximum temperature is 113° F., the average minimum temperature is 21° F., while the normal annual precipitation does not exceed 10 inches. It is this section that is responsible for Arizona's reputation for intensely hot summers; rather unjustly so, however, as a temperature of 120° F. at Yuma, owing to the low humidity, is scarcely more uncomfortable than 90° F. at Washington. D. C.

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Throughout the entire watershed radiation is very great and produces a wide range in temperature from day to night, averaging 30° F. in the upper region and 10° F. in the lower valley.

There are two wet seasons, one in summer (July and August), the other in winter (October to March), which are separated by much drier periods. The summer rainy season is characterized by frequent violent local showers, which are due to convectional atmospheric currents and are usually accompanied by severe electrical

storms.

The winter is marked by steady, gentle rains and by snowstorms at elevations above 4,000 feet. The winter precipitations are due to general widespread cyclonic disturbances.

Rainfall records have been kept at a number of different points in the Gila Basin. These records have been published in several previous reports, notably in Water-Supply Paper No. 33, page 20, and in the more recent report of the Army board elsewhere referred to, published as House Document No. 791, Sixty-third Congress, second session.

The local summer storms give rise to and are frequently accompanied by heavy winds, but during the remainder of the year very little wind occurs. An average wind movement at Phoenix is given

at 2.4

miles an hour.

The atmospheric humidity over the entire area is unusually low. According to United States Department of Agriculture Bulletin 235, page 17, "Relative humidities of less than 10 per cent are often recorded in June, the annual average for four years at Phoenix being 35 per cent."

Owing to the prevailing high normal temperatures and low humidity, evaporation even in the absence of wind is very rapid and must be taken into consideration on planning any irrigation project. But little observed data are available. Deductions must be drawn from results obtained in regions where similar conditions prevail. In the report of the Army board the conclusion has been made that the mean annual evaporation at San Carlos is 60 inches.

VEGETATION.

The great range in elevation encountered in the Gila watershed, and the consequent variation in climate and rainfall, bring about striking changes in vegetation.

The upper region, above the elevation of 5,500 feet, are clothed in a magnificent forest of pine, fir, spruce, and juniper. Below this elevation, to 3,000 feet, the mountain sides are blanketed with a dense thicket of brush, scrub oak, etc., while occasional groves of juniper are found. Below 3,000 feet the usual desert vegetation is in evidence, consisting of mesquite, paloverde, ironwood, greasewood, sage, and the several varieties of cactus. The stream courses of the upper regions are fringed with cottonwood, walnut, and willow. The lower Gila is bordered by dense mats of "water motes" and willow, while the uncultivated tracts of the valley land are generally covered with thick groves of mesquite.

1 Arizona Sta. Bul. 20, p. 36, 37.