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Show as possible, measures should be taken to preserve or improve water quality. 11. Place of Multiple-Purpose Development in the Basin Program The Problem The extent to which multiple-purpose projects should be developed, and the place single-purpose projects have in basin planning. The Situation The limited available water supplies of the basin place a ceiling on future economic development and exercise a high degree of control on future expan- sion in some areas outside the basin. Accordingly, the principal objective of planning in the basin should be to extract the maximum economic and social benefits from the water available. Extent of multiple-purpose projects in the Colo- rado.-Th.e multiple-purpose concept as applied to hydraulic "works fits the objective of maximum use of the basin's waters. Because of the canyon character of the river walls, and the relative lack of settlement along the stream courses, there are a number of high-dam sites. This favors wide appli- cation of tlie multiple-purpose method. Some of the principal dams and reservoirs more recently constructed in the Colorado Basin serve several purposes. For a list of these, see table 4 in chapter 3. There a.lso are many single-purpose dams in the lower basin such as Horse Mesa (power), Cave Creek (flood control), and Laguna (irrigation diversion) . In the u/pper basin, all the dams which have been constructed are largely for the primary purpose of irrigation, except the Electra Lake Dam which is now used for power. The Colorado-Big Thomp- son project is for irrigation and power. Potential sites for multiple-purpose use are numerous both in the lower and the upper basins. In the lower basin, the most outstanding potential projects axe Marble Canyon and Bridge Canyon on the Colorado, Painted Rock on the Gila, Hurri- cane on tlie Virgin River, and Winslow on the Little Colorado system. In the upper basin, the principal potential projects on the Green River sys-tem are for irrigation, power, and flood control and include Sublette, Seedskadee, Little Snake River, Central Utah, Flaming Gorge, 442 Echo Park, Split Mountain, and Gray Canyon. The upper Colorado River includes the Glen Can- yon Project, which will serve power, river regulation, and sediment retention purposes. The San Juan system also has several important potential projects, including Las Animas-LaPlata, South San Juan, Shiprock, and San Juan-Chama Diversion Project. Multiple-purpose versus single-purpose proj- ects.-In general, the multiple-purpose concept promotes full use of the waters of a river basin and permits full use of existing sites, which always are limited. Full use of water is encouraged through multiple-use structures because the water use ob- jectives usually can be served more economically when a dam and reservoir are designed to meet several purposes rather than one. Certain pur- poses alone might not justify the construction of a dam in some instances (for example, flood control storage), but when combined with another purpose (such as power) the basic purpose may be found to return benefits in excess of its share of the costs of the dam and reservoir construction. Similarly, where a structure might be justified on the basis of a single purpose, the eminent desirability of using the site to its full capacity should impel considera- tion of other needs which could be served by a structure designed to accommodate all the purposes. In some instances, as in the Alamo Reservoir on the Bill Williams River, only a single-purpose proj- ect may be justified immediately-in this instance, flood control. In such cases, careful considera- tion should be given to possible future needs for other purposes, and if such investigation appears to warrant the extra cost involved, the structure should be designed to allow for future modification to accommodate the other purposes. In the case of the Alamo Project this procedure was followed to permit future modification for power and water conservation purposes. In still other instances, sites are not suitable for multiple-purpose development, and yet there is a need and economical justification for a single- purpose development. There are a number of po- tential sites both in the upper and lower basins which are economically or physically suited only to a single purpose, primarily flood control or irriga- tion, although in most instances it has been possible to combine these two functions in the planning. Conclusions Multiple-purpose reservoirs for power and con- servation are necessary for the complete develop- |