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Show APPENDIX XII XII-51 For the determination of municipal and Industrial uses, diversion and return flow records were obtained where readily available. However, because of the relatively small magnitude of these items in the Upper Basin, many of the reported values are estimated. Throughout this study, considerable use was made of the techniques and data bases developed for the Upper Colorado Region Comprehensive Framework Study. No attempt was made to deal with the question of channel losses and salvage. The values of consumptive use presented herein for the Upper Basin represent onsite uses and losses and are not necessarily equivalent to the corresponding depletion of flow at Lee Ferry, Arizona. LOWER COLORADO RIVER The consumptive use of water from the Colorado River mainstream and the New Mexico portion of the Gila River Basin was taken from annual reports prepared pursuant to articles V and VII of the decree of the Supreme Court of the United States in Arizona v. California, dated March 9, 1964. In response to the State's request for credit of unmeasured subsurface flows returning to the mainstream, a preliminary estimate has been made and credited arbitrarily to Arizona and California. A joint study is currently being conducted by the Geological Survey and the Bureau of Reclamation with the advice and guidance of the Task Force on Ground-Water Return Flows, which consists of State and Federal representatives, to determine the location and amounts of subsurface return flow. Until these studies are completed, any estimate of subsurface return flows must be considered preliminary and subject to revision. Surface water return flows through Las Vegas Wash from Lake Mead diversions were estimated and shown in the 1975 Article V accounting of mainstream use. Based on the same method, the 1971-74 return flows are included in this report. Other unmeasured return flows from Nevada diversions also occur but have not been accounted for herein. In addition to the mainstream, six tributary areas were selected for the study: Little Colorado River, Arizona-New Mexico; Virgin River, Utah-Arizona; Muddy River, Nevada; Bill Williams River, Arizona; Gila River, Arizona-New Mexico; and remaining areas in Arizona, Nevada, and Utah. Selected outflow points monitored by gaging stations and drainage areas are shown in table C-l. Within these selected areas, particularly in the Gila River Basin, numerous records of diversions are available; however, few return flows are recorded. For the most part, return flows are subsurface and not amenable to direct measurement. It is usually necessary to estimate consumptive use in these areas by empirical means. The land use, population, and production data from which estimates were made are from various current and past reports. This data base is believed to be generally adequate for the tributary areas of the Lower Colorado River system. Since much of this routinely published data follows political subdivision, considerable disaggregation of data is necessary to conform to the reporting areas selected. Certain types of water use, such as recreation, fish and wildlife, etc., are difficult to estimate because of a lack .of current information and methodology. Ground water overdrafts occur in Arizona and Nevada. For the purpose of this report, tributary consumptive use has not been modified to take into account that a major portion of these uses are supplied by ground water overdraft, nor were channel losses and salvage evaluated. Values of tributary consumptive use presented are for onsite uses and losses. It is recognized that under depleted conditions significant losses occurred on the tributaries by evaporation from water surfaces and transpiration from native vegetation prior to their confluence with the Colorado River mainstream. Study Areas The estimated drainage area of the Colorado River system in the United States is about 242,000 square miles, of which |