1938, about 75 percent were artesian. The depth of the wells drilled varied from 150 to over 1,150 feet. Lack of conservation of the artesian water has depleted the available supply, especially in the upper artesian sands.

The development of the Coachella Valley, also an artesian-well basin, has been almost continuous since the first well was drilled in 1898. However, the placing of new lands under irrigation recently has caused a serious drawdown of the ground-water level in the basin.

Other developments of irrigated lands by wells exist in the Colorado and lower Gila River Valleys, the most important of which is the Wellton-Mohawak area in southwestern Arizona. Supplies of ground water in this area are replenished by the flood waters of the Gila River. Developments on the upper Gila watershed have greatly reduced the amount of flood water reaching these downstream areas. In recent years the ground water supply has decreased and its salt content has increased. As a consequence, an unsatisfactory economic condition exists in this area.

Quality of water.-Water of the lower Colorado River is suitable for irrigation use. Before the completion of Boulder Dam and the subsequent storage of water in Lake Mead, the content of dissolved material in the water tended to fluctuate with the various flows. Generally, high flows were relatively low in total dissolved solids and low flows were high. Since the lake has filled, the outflow water is fairly constant in dissolved solid content, averaging annually about 680 parts per million. As it progresses downstream, however, the content of dissolved solids in the river water increases somewhat until at Yuma, the total dissolved solids average about 700 parts per million. Water of this quality is suitable for irrigation and domestic use. Although the calcium carbonate content makes the water hard, thus requiring more soap for cleansing purposes, it is of benefit to the sandy soils of the southwestern desert lands. When water of this quality is used for irrigation, the calcium ions present in the water replace to some extent the sodium ions in the soil. This tends to improve soil structure by making it more granular or flocculent, as well as more friable.

The silt content of the waters of the Colorado River has been greatly reduced by the construction of Boulder, Parker, and Imperial Dams. During the years 1911 through 1934, before the construction of these dams, the river carried an average annual silt load of 179,920,000 tons at the Yuma sampling station. For the years 1936 through 1942, the average annual silt load of the river at the same point was 13,100,000 tons.

Silt problem.-Water entering the Boulder division at Lee Ferry carries immense quantities of silt. The San Juan River, largest of the southern tributaries of the upper basin, contributes about one-quarter of the silt passing Grand Canyon, and the northernmost tributary

of the lower basin, the Little Colorado, accounts for about one-sixth more. From Lee Ferry to the upstream end of Lake Mead, the Colorado River, falling approximately eight feet per mile, is continuously cutting its course, even through the hard rocks of the canyon. Silt originates not only from stream cutting in the channels of the river and its important tributaries, but also from general erosion. The rate of land reduction through erosion in the Colorado River Basin is the highest of any stream basin in the United States.

Lake Mead receives the silty load of both the Colorado and the Virgin Rivers. Storage capacity in Lake Mead is being reduced an estimated 137,000 acre-feet a year by the deposition of this silt.

The clear water discharged from Lake Mead picks up and transports downstream a considerable amount of river bed material, progressively lowering the bed for the first 88 miles. An estimated 100 million cubic yards had been removed by the end of 1943. From 1941 through 1943 about 35 million cubic yards were deposited in the next 32-mile stretch downstream. A large but unknown amount of material has been carried beyond this stretch and deposited in Havasu Lake, the reservoir formed by Parker Dam. For a long period prior to Boulder Dam the river deposited some of its silt load in these same sections, causing a rise in the water surface elevation averaging annually about 0.35 feet in the vicinity of Needles and about 0.55 feet at the Topock gaging station.

A similar condition of cutting out and redepositing has taken place below Parker Dam. Clear water leaving Parker Dam has scoured out bottom material and lowered the river bed elevation for 93 miles downstream, and beyond that point the river bed has been raised.

By early 1944 the water surface at the diversion of the Palo Verde irrigation district had been lowered by river retrogression to such an extent that a full diversion into the district's canal became impractical.

Between Imperial Dam and Laguna Dam little change in river conditions has taken place, but below Laguna Dam some 30 million cubic yards of material have been removed from the river banks and bed since January 1940. All material passing the Imperial Dam sluiceway, as well as that from the Gila River, has been transported downstream.

Controlling the silt load of the Colorado River and its tributaries to prevent damage is an important phase of water conservation for beneficial use. Silt control is especially desirable upstream from Lake Mead, where the Colorado River alone carries each year about 180 million tons of silt, which, when deposited, occupies a volume estimated at 110,000 acre-feet. Obviously, therefore, any plan of development must provide adequately for sediment storage. The possibility of removing sediment deposits by sluicing holds little prospect for success.

PRESENT DEVELOPMENT OF WATER RESOURCES

General. Boulder Dam and Lake Mead, with over 32 million acre-feet of storage capacity, provide the key to present and future development of water resources in the division. Water stored here is released as required for irrigation, for power development, and for domestic uses. The lake acts as a desilting basin, clarifying the muddy Colorado and making it fit for human consumption; it catches and, to a great extent, subdues the floods which pour down the river channel; and in addition it furnishes an unexcelled water playground in the desert. For complete development and regulation of the river, however, other dams should be built above Boulder.

Davis Dam, on the Colorado River 67 miles below Boulder with its reservoir of 1,600,000 acre-feet capacity, will serve many purposes by reregulating the releases from Boulder Dam. Construction of this dam, temporarily halted by an order of the War Production Board, is scheduled to be resumed in 1946.

Below Davis Dam are four diversion dams: Parker, for the Metropolitan Water District of Southern California; Headgate Rock, for the Colorado River Indian Reservation; Imperial, for the All-American Canal System and the Gila project; and Laguna, for the Yuma project. Although the latter three are primarily for diversion purposes, small amounts of power are generated at plants either at the dams or in the canals below. The Parker power plant is an important unit in the power network of the area.

Prior to the construction of Boulder Dam, all irrigation from the river was dependent on natural stream flow; now, however, storage water is used to supplement natural flow for most of the irrigated areas.

Irrigation. The Colorado River Indian irrigation project is the first major irrigation development downstream from Boulder Dam. Irrigation has been practiced on this reservation since the seventies, first by gravity ditch diversions which later failed, then by pumping, and since June 1942 by diversion at the newly completed Headgate Rock Diversion Dam and Main Canal. Works are now completed to irrigate 9,400 acres. The system is designed ultimately to irrigate 100,000 acres.

Palo Verde irrigation district lands are located along the Colorado River in California mainly in southeastern Riverside County but with a small area extending into Imperial County. Distribution works have now been constructed by the district to deliver water to 75,000 acres. This district in recent years has experienced difficulty in diverting its required water because of silt deposits in the intake canal and the lowering of the river channel at the headgate by scouring. By 1943 the channel had lowered to such an extent that it was evident the district would not be able to maintain its gravity diversion, and an appeal was made to the Bureau of Reclamation for assistance.

The Bureau subsequently constructed a temporary rock weir to raise the water surface to a sufficient height for diversion into the district's canal, pending a permanent solution of the problem.

Prior to 1940, lands in the Imperial irrigation district were served by the Imperial Canal, which diverted water from the Colorado River below Yuma. A part of this canal looped into Mexico, which led to complications and made it highly desirable to have a canal located entirely within the United States. The All-American Canal svstem of the Bureau of Reclamation's Boulder Canyon project answers this purpose. In 1943, there were 400,400 acres irrigated in the Imperial irrigation district from the All-American Canal. The project is still under construction and will irrigate ultimately lands in Imperial Valley, Coachella Valley, East and West Mesas, and Pilot Knob Mesa. The area now irrigated in Coachella Valley by ground water will be served supplemental water from the Colorado River.

The Yuma project, located in Yuma County, Ariz., and Imperial County, Calif., was one of the first irrigation developments of the Bureau of Reclamation and its earliest on the Colorado River. Construction was authorized in 1904, and the first water was delivered during the 1907 season. Water is diverted at Laguna Dam for a portion of the California lands and from the All-American Canal at the Siphon Drop power plant for the remainder of the project. In 1943, 58,800 acres were irrigated including 7,800 acres of Indian land, reported by the Office of Indian Affairs as the maximum development possible.

The Gila project is located in the southwest corner of Arizona and borders on the east side of the Yuma project. As originally conceived, this project contemplated irrigation of 585,000 acres of land. Although considered as one development, the location of the lands suggests certain groupings within the project itself. These are designated as the Yuma Mesa, South Gila Valley, North Gila Valley, and Wellton-Mohawk divisions. Construction of facilities for the irrigation of 150,000 acres in the Yuma Mesa, South Gila Valley, and North Gila Valley divisions has been started. Imperial Dam, already completed, is the diversion dam for both the All-American Canal and the Gila Gravity Main Canal. Water is now being delivered to North Gila Valley and to a small acreage of the Yuma Mesa. Originally it was planned to develop 139,000 acres in the Yuma Mesa. It now appears more desirable to limit the irrigated area to 70,000 acres thus permitting greater development in the other three divisions where it is believed the water could be used to better advantage. A new authorization will be required, however, for any development in the Wellton-Mohawk division. The extent of ultimate development in the Gila project will depend upon the final allocation of water between this project and the potential central Arizona project,

discussed under the Gila division, or other possible developments within the State of Arizona.

About 1,200 and 500 acres are irrigated by gravity diversion from Big Sandy River and Kirkland Creek, respectively. Both streams are tributaries of the Williams River.

Some 1,200 acres near Las Vegas, Nev., are now irrigated from ground water sources. About 16,000 acres in Coachella Valley, 6,600 acres in the South Gila Valley, and 7,800 acres in the Mohawk Municipal Water Conservation District near Roll, Ariz., are also irrigated from ground water. The two areas last named may be furnished a water supply under the Gila project.

Municipal and industrial use. The Colorado River aqueduct in southern California is the only large municipal diversion out of the Boulder division. The aqueduct was constructed by the Metropolitan Water District of Southern California, composed at present of the cities of Anaheim, Beverly Hills, Burbank, Compton, Fullerton, Glendale, Long Beach, Los Angeles, Pasadena, San Marino, Santa Ana, Santa Monica, and Torrance and of the Coastal Municipal Water District. The Metropolitan District's first objective in constructing the Colorado River aqueduct was to supply Colorado River water for domestic, industrial, and other beneficial uses to the area within its boundaries, and to such additional surrounding areas as may later desire to join the district. Water is pumped from Havasu Lake and conveyed by the aqueduct to the southern California area.

Because of the critical nature of the water situation at San Diego, President Roosevelt on November 29, 1944, directed that the Bureau of Reclamation complete plans and specifications for an aqueduct to take 50 million gallons of water per day from the Colorado River aqueduct of the Metropolitan Water District of Southern California and deliver it to San Vicente Reservoir of the San Diego water system. The President directed that the Bureau of Yards and Docks perform the necessary construction. Contracts for construction of some parts of the aqueduct were awarded during the summer of 1945.

Water is pumped from Lake Mead and conveyed by pipe line to Henderson, Nev., for municipal and industrial use, mainly by Basic Magnesium, Inc.,

Drainage and overflow protection. The cultivated areas included in the Colorado River Indian Reservation, Palo Verde Irrigation District, Yuma project, and Imperial Irrigation District are protected by levees from river overflow. In these districts the lands next to the river are higher than those farther removed and bordered by table lands. Seepage from canals and storm run-off from these higher lands cause drainage problems on the lower lands. As yet the canal seepage from higher lands has not been serious, but some protective measures have been taken. All four areas are dependent on artificial drains to remove

excess water.

In recent years it has been difficult to maintain a satisfactory discharge from the drain-ditch system in the lower part of the Palo Verde Valley.

Drainage conditions on the upper Colorado River Indian Reservation and the Yuma project have been benefited by retrogression of the river channel opposite the lands.

Water drained from California lands of the Yuma project is pumped over the levee to the river when the river is high; water drained from Arizona lands of the project is pumped over the levee into Mexico.

Some of the levees and drains protecting lands in the division are:

1 Maintained by the district Length of open drains in private ownership not available. Levees are in Mexico.

Power.-Power production in the Boulder division is of utmost importance to the lower basin and to southern California. It is here that the waters of the Colorado River are utilized to generate the tremendous electric energy output of the Boulder and Parker Dam power plants.

Thirteen transmission lines extend from Boulder Dam power plant to power market areas in Arizona, Nevada, and southern California. The two largest electric utilities in southern California, the Southern California Edison Co., Ltd., and the City of Los Angeles Department of Water and Power, obtained 25 and 78 percent, respectively, of their total energy from this one source in 1943. Average firm power production at the Boulder power plant is about 4.5 billion kilowatt-hours annually, but during 1944 about 6 billion kilowatt-hours were generated.

The Parker Dam power plant is connected to the Boulder Dam power plant by transmission facilities which permit an interchange of electric energy, thereby conserving water. During the fiscal year 1944 the Parker Dam plant generated 781,642,000 kilowatt-hours. Transmission lines run from the Parker Dam plant to load centers in Arizona, to the Metropolitan Water District pumping plants, and to the All-American Canal power plants in California.

Other hydroelectric plants in the area include those on the All-American Canal, which, although comparatively small, are very important because of their location near the power market. market. Some plants on the canal are not yet