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seasons being almost devoid of rainfall. During these dry months flows of the rivers are usually small, but they increase enormously after a big storm or when sudden warm temperatures rapidly melt the mountain snow. About 45 percent of the annual run-off of the Gila River occurs during February, March, and April. Storage is imperative for efficient utilization of the available water. merous reservoirs already store the waters of the Gila River and its tributaries, which are almost completely utilized by existing irrigation projects. In fact, during protracted dry years the water available within the basin is inadequate to meet the demand. The Gila Basin, therefore, must look to other basins for an additional supply of surface water.

Ground water. There is no law in Arizona regulating the development of percolating ground waters. In the absence of legal protection, development of ground water resources for any purpose would be hazardous, and this factor should be recognized in planning future develop

ments.

Considerable portions of the broad basins of southwest

ern Arizona are underlain with uncemented valley-fill material, generally several hundred feet deep. This porous material absorbs much of the flow of streams as they enter the valley areas, thus creating great underground reservoirs. Irrigation seepage contributes substantial recharge to the reservoirs, but the scanty precipitation on valley lands adds little to the ground water supply.

Large quantities of water for irrigation and domestic use are obtained from ground water. Artesian wells of importance have been drilled in the upper Gila Valley, and small artesian flows have been encountered in the upper San Pedro and Santa Cruz Valleys. Yields of individual wells within the division range in discharge from .5 to 2,250 gallons a minute.

Small to moderate-sized springs are scattered through the watershed area. Only a small amount of land is irrigated directly from springs, but springs contribute a substantial amount of water to the perennial flows of the larger rivers. Spring water is particularly valuable in the semiarid outlying ranges, where creeks used for watering livestock are dry during large parts of the year.

With few exceptions, ground water within the basin has been developed beyond its economic limit, and in all but a few areas,`ground-water withdrawals exceed replenishments.

Arizona lacks comprehensive legislation regulating the use of ground water, but the State Water Code provides that "water flowing in definite underground channels" is subject to appropriation. Since it is difficult to prove that such water does flow in a definite channel, little regulation of ground water exists in the State. In New Mexico the State Engineer can declare any area with underground water, the boundaries of which can be reasonably determined, to be an underground water basin. Underground water within the area must then be appropriated in much the same manner as that of surface streams. Present water users are thus protected, and expansion is permitted only where supplies are more than adequate for existing developments.

Quality of water. Surface waters of Gila River and its tributaries carry considerable quantities of dissolved solids, chiefly sodium chloride and the sulphates and bicarbonates of sodium, calcium, and magnesium; however, the percentage of sodium is reasonably low. Low flows of the Gila River at Gillespie Dam in the lower part of the division carry over 6,000 parts per million total dissolved solids, while flood flows carry as little as 300 parts per million.

The quality of the ground water obtained from artesian wells and springs varies with location. Some waters have less than 100 parts per million dissolved salts, while others range as high as 5,000 parts per million. The total hardness of these waters, expressed as calcium carbonate, ranges from less than 5 to over 700 parts per million.

a few small tributary flows, are almost completely utilized by lands now under irrigation; and an estimated 1,600,000 acre-feet of ground water is pumped annually for irrigation and domestic use. Ground-water pumping greatly increases during dry years when reservoirs are low, throwing a tremendous strain on generating facilities already curtailed by low water conditions. The output of power plants in the division has been augmented recently by energy from Parker Dam on the Colorado River.

Rivers of the area transport large quantities of silt each year. Although river flows in dry weather are fairly clear, a load of sand constantly is being moved along the river bed even during the lowest river stages. High discharges, resulting from torrential rains and rapid run-off, carry in suspension heavy loads of fine silt and clay in addition to enormously increased bed loads.

The fine material carried in suspension presents no problem in canal maintenance, except when flows are exceptionally low. Coarser bed-load material, however, settles in canals and ditches and must be removed to maintain the capacity of the conduits. Both bed loads and suspended loads settle in storage reservoirs. This accumulation is important in determining the length of life of a reservoir. Records of the Salt River Valley Water Users' Association show a total silt accumulation of 108,000 acrefeet in Roosevelt Reservoir on Salt River during the period 1905 to 1934, inclusive. Other streams in the basin notably the Gila, are known to carry greater proportions of silt than Salt River, so this figure is lower than the average to be expected.

The suspended material found in the waters of the Gila River and its tributaries contains considerable organic matter and but little coloidal clay. When applied to sandy land, this material gives body to the soil and is beneficial. When spread on tight land, however, it clogs the pores of the soil, reducing its permeability and making its cultivation more difficult. In municipal water works, it clogs intakes and makes water clarification costly.

The only practicable solution of the silt problem lies in providing adequate silt storage capacity in reservoirs on major streams contributing silt and in limiting erosion by better watershed control.

Irrigation

Gila River. Irrigated areas along the Gila River and its tributaries, San Francisco River, San Simon Creek, Queen Creek, and Centennial Wash, total 213,400 acres. Irrigation projects located above Coolidge Dam have no water-storage facilities and must depend on diversions from the unregulated flow of the Gila River supplemented by pumping from ground water for their irrigation supplies. These upstream projects cover an area of approximately 51,000 acres and require additional water to irrigate adequately all project lands.

The San Carlos Reservoir, with a capacity of 1,200,000 acre-feet formed by Coolidge Dam (Office of Indian Affairs), stores water used for irrigation on several downstreams projects. The San Carlos project, largest of these irrigation developments, serves 100,500 acres, about half of which is farmed by Indians. Project lands require more water than is supplied to them by existing irrigation developments on the Gila River.

Other downstream developments include those made by the Buckeye Water Conservation and Drainage District, Arlington Canal Co., Gillespie Land & Irrigation Co., and others. These lands obtain much of their water supply by pumping from underground sources. At the present time ground-water depletions exceed recharges to a considerable degree, and unless replacement water is supplied, some land must be taken out of cultivation within a few years.

San Pedro River.-About 2,600 acres of land are irrigated in the Benson-St. David area of the upper San Pedro River watershed and a few hundred acres are dryfarmed. No surface water storage is available and irrigation supplies depend on the extremely variable flow of the river, the output of several small flowing wells, and a limited amount of pumping from ground water.

All of the dependable surface water supply of the San Pedro is now divided among irrigators in the San Pedro Valley and in the Gila River Valley downstream from the confluence of the two streams. The only possibility of irrigating additional lands in the San Pedro Valley is through the importation of water to the Gila Basin, so that San Pedro River water now used there may be released for use in the San Pedro Valley. Ground-water resources along the San Pedro River are not fully developed, and increased pumping would yield valuable but limited supplemental water supplies.

Salt and Verde Rivers.-Diversions of irrigation water from Salt River were first made by white settlers in 1867. Because of erratic river flows and lack of storage facilities, water supplies during dry years were inadequate to supply the demands of the lands in cultivation. The Bureau of Reclamation constructed Roosevelt Dam and power plant to provide storage and regulation of Salt River. Diversion works, canals, laterals, and other power plants were also built by the Bureau before turning the project over to the Salt River Valley Water Users' Association in 1917, subject to payment of the unpaid balance of construction charges. During the years between 1922 and 1930, the association constructed the Horse Mesa, Mormon Flat, and Stewart Mountain Dams for irrigation and power, and the Cave Creek Dam for flood control. The Bureau of Reclamation, during the 1936-39 period, built Bartlett Dam on the Verde River, principal tributary of Salt River.

Large areas of fertile land surrounding the Salt River