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~ ~.... W -1 ) \ , . . USGS Accomplishments Accounting for Diversions of Colorado River Water Drawn from the Mainstream by Underground Pumping in the Lower Colorado River Basin - Accounting for the use of Colorado River water is required by the U.S, Supreme Court decree, ] 964, Arizona v. California. Water pumped from wells on the flood plain and from certain wells on alluvial slopes outside the flood plain is presumed to be river water and is accounted for as Colorado River water. The U.S. Geological Survey, in cooperation with the Bureau of Reclamation, developed a method to identify wells outside the flood plain of the lower Colorado River that yield water that will be replaced by water from the river. The method provides a uniform criterion of identification for all users pumping water from wells by determining if the elevation of the static water table at a well is above or below the accounting surface. Wells that have a static water-level elevation equal to or below the accounting surface are presumed to yield water that will be replaced by water from the river. -\;1.J- Wells that have a static water-level elevation above the accounting surface are presumed to /" 1 t yield water that will be replaced by water from precipitation and inflow from tributary valleys~ S t,V f.-:\o~ l-D"" q,., The method i/based on the concept of a river aquifer and an accounting surface within the river aquifer. The river aquifer consists of permeable, partly saturated sediments and sedimentary rocks that are hydraulically connected to the Colorado River. The subsurface limit of the river aquifer is the nearly impermeable bedrock of the bottom and sides of the basins. The accounting surface represents the elevation and slope of the unconfined static water table in the river aquifer outside the flood plain and reservoirs that would exist if the river were the only source of water to the river aquifer. The accounting surface extends outward from the edges of the flood plain or a reservoir to the subsurface boundary of the river aquifer. Nineteen maps at a scale of I: 100,000 show the extent and elevation of the accounting surface from the area surrounding Lake Mead to Laguna Dam near Yuma, Arizona. Project is complete. Four publications are available; the method report, including 19 maps, was published in FY94. This project also included a partial inventory of wells (about 850), located between the edge of the flood plain and the river-aquifer boundary between Davis Darn and Laguna Dam, Dissolved Solids Estimation Project The objectives of this project were to: (I) determine the availability and completeness of discharge and water-quality records for selected sites on the lower Colorado River from Imperial Dam to the southerly International Boundary with Mexico; (2) develop techniques to ~eStimate missing periods of records for discharge and dissolved solids; and (3) present monthly discharge and monthly dissolved-solids discharge for sites on the lower Colorado River from 1935 to the present. The District estimates the final report will be approved and published during FY95. 12