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rate of nearly 5 percent. A large portion of the population resides within Maricopa and Pima Counties, Arizona, and in Clark County, Nevada. Sixty percent of the Upper Basin and about 20 percent of the Lower Basin population were classified as rural with a significantly smaller per capita use of water. Both the urban and rural areas have the mutual problem of providing an adequate current and future water supply for a growing population in a water-short area. As a result of almost continuous studies concerning these problems, adequate production and effluent records are usually available to adequately assess water use. Exports: Nearly all the transbasin exports from the Colorado Basin were measured and reported by the Geological Survey, or local water commissioners and users. The remainder were esti- mated on the basis of past records and capacity of facilities. Due to the high degree of measurement, this area of basin consumptive use is considered to be quite accurately determined. Lower Colorado River Mainstream The annual consumptive use of water from the Colorado River mainstream by the States and exports from the system were taken from the Bureau annual report entitled "Compilation of Records in Accordance with Article V of the Decree of the United States in Arizona v. California." The estimated Colo- rado River component of the combined surface and subsurface return flows accruing to Las Vegas Wash and dis- charging into Lake Mead, as taken from the report, is credited to Nevada's municipal and industrial water uses. Unmeasured subsurface return flows were estimated below Hoover Dam, based partially on preliminary information supplied by the Task Force on Ground- Water Return Flows. All unmeasured subsurface return flows were credited to the irrigation water use taken from the Article V report, and were divided between California and Arizona based on their respective irrigation diversions. Gross evaporation from Lake Mead is estimated by the USGS and published in its annual Water Resources Data reports. Net evaporation for Lake Mead is estimated by subtracting precipita- tion at nearby Boulder City, Nevada, from the gross evaporation. Net evaporation from Lakes Mohave and Havasu and Senator Wash Reservoir was derived from available evaporation and precipitation records and operating data. Annual channel losses were estimated as the inflow or outflow necessary to balance a simplified water budget for the Lee Ferry to Hoover Dam and Hoover Dam to International Boundary reaches. Channel losses include evaporation, seepage, phzeatophyte consumptive use, and bank storage. The accuracy of flow measurements of the Colorado River mainstream for use in determining the channel losses values in table LC-1 is in question. The gage error of an "excellent" USGS flow gage is 5 percent. Actual flow at Lee Ferry, Hoover Dam, or to Mexico may therefore vary approximately 400,000 acre-feet per year from values supplied by the USGS. Such inaccuracies, though a very small percent of total flow, will have dramatic effects on apparent channel loss computations. The annual land use, water supply, and water use information being gath- ered for the operation, maintenance, and administration of the Colorado River mainstream below Lee Ferry is believed to be generally adequate in quantity, quality, and extent. These data are under constant review and are being continually upgraded. Studies and programs are in progress to remedy a lack of data on return flows from mainstream diversions. 15