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Method to identify wells that yield water that will be replaced by water from the Colorado River downstream from Laguna Dam in Arizona and California

June 1, 2001

Accounting for the use of Colorado River water is required by the U.S. Supreme Court decree, 1964,
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 accounting-surface method developed for the area upstream from Laguna Dam was modified for use
downstream from Laguna Dam to identify wells outside the flood plain of the lower Colorado River that
yield water that will be replaced by water from the river. Use of the same method provides a uniform
criterion of identification for all users pumping water from wells by determining if the static water-level
elevation in the well is above or below the elevation of 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 Colorado River. Wells that have a static water-level elevation above the
accounting surface are presumed to yield river water stored above river level.

The method is based on the concept of a river aquifer and an accounting surface within the river
aquifer. The river aquifer consists of permeable sediments and sedimentary rocks that are hydraulically
connected to the Colorado River so that water can move between the river and the aquifer in response to
withdrawal of water from the aquifer or differences in water-level elevations between the river and the
aquifer. The subsurface limit of the river aquifer is the nearly impermeable bedrock of the bottom and
sides of the basins that underlie the Yuma area and adjacent valleys. The accounting surface represents
the elevation and slope of the unconfined static water table in the river aquifer outside the flood plain of
the Colorado River that would exist if the river were the only source of water to the river aquifer. The
accounting surface was generated by using water-surface profiles of the Colorado River from Laguna
Dam to about the downstream limit of perennial flow at Morelos Dam. The accounting surface extends
outward from the edges of the flood plain to the subsurface boundary of the river aquifer. Maps at a scale
of 1:100,000 show the extent of the river aquifer and elevation of the accounting surface downstream from
Laguna Dam in Arizona and California.