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Show and Goachella Valleys of southern California. This latter project is by far the largest in the entire basin. When completed the Ail-American Canal system will furnish irrigation water to 475,000 acres. The AU-American Canal irrigates 400,000 acres in the Imperial Valley as well as supplying some water for Mexican lands, and, when com- pleted, the Coachella Canal will serve 75,000 acres in the Coachella Valley. The principal Federal irrigation projects using Colorado River water in Arizona are in the vicinity of the G-ila and the Yuma Projects, where the irrigable acreage is about a quarter of a million. These are all made possible by the conservation storage at Hoover Dam. The other large irriga- tion developments in Arizona are on the Gila, Salt, and Verde Rivers. Program for Future Development The Bureau of Reclamation plans for regula- tion of the Colorado River dwarf all the projects, public and private, completed to date. To maie water deliveries to the lower basin called for by the Colorado River Compact and still have reliable water supplies available for use in the upper basin, regulation and storage of the high flow discharges are needed. To accomplish this the Bureau has proposed the Colorado River Storage Project. It contemplates the construction of 10 dams on the Colorado and its principal tribu- taries, ha>dng 48.5 million acre-feet of storage, of which 37.5 million would be active storage. These would all be multiple-purpose projects. Irrigation plans of the Bureau for the upper basin, eitJher directly associated with the dams of the Colorado River Storage Project or as sepa- rate projects, call for irrigating over 380,000 acres of new la.nd and furnishing a supplemental sup- ply for 435,000 acres of land now under ditch. Over half the new and supplemental water would be exported from the Colorado Basin to lands in the Utah section of the Great Basin by the facili- ties of the Central Utah Project. In the lower basin several of the projects now under construction would provide water for a large additional acreage of irrigated land now dry or having an inadequate water supply. At the present time the Bureau of Reclamation is recommending only a few additional projects. The only major one is the Central Arizona Project, in which the diver- sion of 1.2 million acre-feet of water by pumping is proposed, using power generated at the proposed Bridge Canyon Dam. The water would be used as a supplemental supply for presently irrigated lands and for municipal water supply. No new land would be irrigated under this project. Numerous additional projects have been investi- gated, but the Bureau of Reclamation has recom- mended either further study or deferral until a later date for most projects. Watershed Management An important function of nearly all lands of the Colorado Basin is their use as water sources. Be- cause of its aridity, rough topography, or poor soils, most of the land is of low productivity. Many lands produce a little forage for cattle or game animals or minor amounts of timber, but nearly all have utility as water sources. The generally low productivity of the basin lands is evidenced by the fact that about 70 percent* of the total basin area is in Federal ownership despite large grants of railroad lands and the homesteading of most lands suitable for irrigation and large acre- ages of the best pasture lands. The attempt to exploit the forage and timber resources of the basin has caused deterioration of the watershed lands and adverse effects on the character and volume of stream flow, the quality of water, and the amount of sediment transported. The Present Situation About 58 million acres of the total area are in forest or related cover and of these, 28 million are in national forests. This forest land is the water- yielding area of the basin. It receives 18 inches or more of precipitation, 75 percent coming in the cooler portion of the year. The higher lands carry- ing the commercial forest (about 6 percent of the basin) receive more than 25 inches, half of it in snow. A major portion of the remaining arid or semi- arid sections of the basin is in public ownership. It receives from as little as 4 inches annually up to 18 inches of precipitation, with a general average of 10 inches. The vegetation makes use of prac- tically all the soil moisture, so very little water is available for runoff to feed the streams except dur- ing heavy rains. The forest areas produce 7.2 1 Because some lands have not been surveyed and the status of some lands is not definite, all figures regarding land areas in the Colorado River Basin are approximate. 366 |
