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...) . .' For the determination of municipal and ~dustrial uses, 4iversion and return flow cr~cords were obtained where readily . ~ailable. However, because of the relatIVely small magnitude :of these items in the Upper Basin. many of the reported values are estimated. ! Throughout thi~ study, considerable use was made of the ~echniques and data bases developed for the jUpper Colorado Region Comprehensive Framework Study. No attempt waslmade to deal with the question of channel losses and salvage. The values of consump~ive use presented herein for the Upper Basin r4present onsite uses and losses and are not recessarily equivalent to the corresponding depletion of flow at Lee Ferry, Arizona. LOWER qOLORADO RIVER The consumptive iuse of water from the Colorado River mai~stream and the New Mexico portion of the Gila ~iver Basin was taken from annual reports prep~red pursuant to articles V and VII of the decre'e of the Supreme Court of the United Stateslin Arizona v. California, dated March 9, 196~. In response to the State's request for credit of unmeasured subsurface flows ret~rning to the mainstream, a. preliminary estimatel has been made and credited arbitrarily to Arizona and California. A joint study is currend,ly being conducted by the Geological Survet 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 rep- , resentatives, to determine the location and amounts of subsurface return flow. Until these studies are completed. any estimate of subsurface return flo~s must be considered preliminary and subject to revision. Surface water return flows thr<\ugh Las Vegas Wash from Lake Mead diver~ions were estimated and shown in the 1975 Article V accounting of mainstream use. Base~ on the same method, the 1971-74 return fl~ws are included in this report. Other unmeasured return flows from Nevada diversions also occur but have not been accounted fo~ 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 -uepleted 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. ~. ~.;. -~: ~' .'tt- ~, ;'.c >.', Study Areas The estimated drainage area of the Colorado River system in the United States is about 242.000 square miles, of which t ~ i, r t I ~ .. 5