1

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2008–5113

Prepared in cooperation with the Bureau of Reclamation

Update of the Accounting Surface

Along the Lower Colorado River
FRONT COVER—Looking upstream at Palo Verde Dam on the Colorado River near Blythe, California. 

Photo taken by Sandra J. Owen-Joyce, July 25, 2007.
Update of the Accounting Surface

Along the Lower Colorado River

By Stephen M. Wiele, Stanley A. Leake, Sandra J. Owen-Joyce, and Emmet H. McGuire

Prepared in cooperation with the Bureau of Reclamation

Scientific Investigations Report 2008–5113

U.S. Department of the Interior

U.S. Geological Survey
ii 

U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2008

Revised and printed 2009, version 1.1

This report and any updates to it are available online at:

http://pubs.usgs.gov/sir/2008/5113/

For product and ordering information:

World Wide Web: http://www.usgs.gov/pubprod

Telephone: 1-888-ASK-USGS

For more information on the USGS—the Federal source for science about the Earth, its natural and living 

resources, natural hazards, and the environment:

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Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by 

the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners 

to reproduce any copyrighted materials contained within this report.

Suggested citation:

Wiele, S.M., Leake, S.A., Owen-Joyce, S.J., and McGuire, E.H., 2009, Update of the accounting surface along 

the lower Colorado River: U.S. Geological Survey Scientific Investigations Report 2008-5113, version 1.1, 16 p., 

3 plates in pocket.

Cataloging-in-publication data are on file with the Library of Congress (http://www.loc.gov/).
 iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Legal Framework...................................................................................................................................3

Purpose and Scope ..............................................................................................................................3

Data Collection......................................................................................................................................3

Previous Investigations........................................................................................................................4

Acknowledgments................................................................................................................................4

Accounting-Surface Method.......................................................................................................................4

River Aquifer..........................................................................................................................................6

Generation of the Accounting Surface .............................................................................................6

Criteria for Establishing Reservoir Water-Surface Elevations and Colorado River Flow 

Conditions Used to Generate the Accounting Surface.....................................................6

Areas of the River Aquifer Controlled by Reservoirs......................................................................7

Areas of the River Aquifer Controlled by the Colorado River, Drainage Ditches, or Both........7

River Reaches...............................................................................................................................7

Drainage Ditch Reaches...........................................................................................................10

Ground-Water Flow Models.......................................................................................................................10

Updated Accounting Surface ....................................................................................................................11

Summary........................................................................................................................................................11

References Cited..........................................................................................................................................16

Appendixes [available only online at http://pubs.usgs.gov/sir/2008/5113/appendixes]

1. Drainage ditch water-surface elevations used in the ground-water flow model.

2. Path of the Colorado River used in the ground-water flow models.

3. Digitized surface elevations used to represent the Colorado River in the groundwater 

 flow model south of Eleven-mile gage.

Plates [in pocket]

1. Map showing the accounting surface around Lake Mead and Lake Mohave.

2. Map showing the accounting surface around Lake Havasu and in Mohave, Parker, Palo Verde,

 and Cibola Valleys and in adjacent tributary areas in Arizona, Nevada, and California.

3. Map showing the accounting surface between southern Cibola Valley and the southerly 

 International boundary and in adjacent tributary areas in Arizona and California.

Figures

1. Map showing the lower Colorado River and areal extent of the river aquifer................................2

2. Schematic diagram showing the river aquifer and accounting surface of the 

lower Colorado River.................................................................................................................................5

3. Water surface profile of the Colorado River used in the ground-water flow models ....................7

4. Map showing the accounting surface in the areas surrounding Lake Mead Arizona, 

Utah, and Nevada ....................................................................................................................................12
iv 

Inch/Pound to SI

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Volume

acre-foot (acre-ft) 1,233 cubic meter (m3

)

acre-foot (acre-ft) 0.001233 cubic hectometer (hm3

) 

Flow rate

foot per day (ft/d) 0.3048 meter per day (m/d)

cubic foot per second (ft3

/s) 0.02832 cubic meter per second (m3/s)

Vertical coordinate information is referenced to the “National Vertical Geodetic Datum of 1929 (NGVD 29).”

Horizontal coordinate information is referenced to the “North American Datum of 1927 (NAD 27).”

Elevation, as used in this report, refers to distance above the vertical datum.

Conversion Factors and Datum

Tables

1. Discharges used to determine the water-surface elevation of the Colorado River 

used in the ground-water flow models..................................................................................................7

2. Streamflow-gaging station data used to define the water-surface elevation 

used in the ground-water flow models..................................................................................................8

3. Colorado River water-surface elevation measurements used to define the 

water-surface elevation used in the ground-water flow models......................................................9

4. Properties of ground-water flow models used to compute the accounting 

surface for areas along the lower Colorado River.............................................................................11

5. Map showing the accounting surface in Mohave Valley and adjacent tributary 

areas in Arizona, California, and Nevada ............................................................................................13

6. Map showing the accounting surface in Parker, Palo Verde, and Cibola Valleys 

and adjacent tributary areas in Arizona and California ....................................................................14

7. Map showing the accounting surface in the Yuma area upstream and 

downstream from Laguna Dam and adjacent tributary areas in Arizona and California ............16
 1

Update of the Accounting Surface Along the 

Lower Colorado River

By Stephen M. Wiele, Stanley A. Leake, Sandra J. Owen-Joyce, and Emmet H. McGuire

Abstract

The accounting-surface method was developed in the 

1990s by the U.S. Geological Survey, in cooperation with the 

Bureau of Reclamation, to identify wells outside the flood 

plain of the lower Colorado River that yield water that will 

be replaced by water from the river. This method was needed 

to identify which wells require an entitlement for diversion 

of water from the Colorado River and need to be included 

in accounting for consumptive use of Colorado River water 

as outlined in the Consolidated Decree of the United States 

Supreme Court in Arizona v. California. The method is based 

on the concept of a river aquifer and an accounting surface 

within the river aquifer. The study area includes the valley adjacent to the lower Colorado River and parts of some 

adjacent valleys in Arizona, California, Nevada, and Utah and 

extends from the east end of Lake Mead south to the southerly 

international boundary with Mexico. Contours for the original 

accounting surface were hand drawn based on the shape of 

the aquifer, water-surface elevations in the Colorado River 

and drainage ditches, and hydrologic judgment. This report 

documents an update of the original accounting surface based 

on updated water-surface elevations in the Colorado River 

and drainage ditches and the use of simple, physically based 

ground-water flow models to calculate the accounting surface 

in four areas adjacent to the free-flowing river.

Introduction

The accounting-surface method was developed in the 

1990s by the U.S. Geological Survey (USGS), in cooperation 

with the Bureau of Reclamation (Reclamation), to identify 

wells outside the flood plain of the lower Colorado River that 

yield water that will be replaced by water from the river (Wilson and Owen-Joyce, 1994; Owen-Joyce and others, 2000). 

Prior to the development of the accounting-surface method, 

water pumped from many wells outside the flood plain was 

not included when accounting for consumptive use of river 

water. A method was needed to identify which wells pump 

water that will be replaced by water from the Colorado River 

and need to be included in accounting for consumptive use of 

Colorado River water as outlined in the Consolidated Decree 

of the United States Supreme Court in Arizona v. California, 

547 U.S.150 (2006). The method is based on the concept of a 

river aquifer and an accounting surface within the river aquifer. The study area includes the valley adjacent to the lower 

Colorado River and parts of some adjacent valleys in Arizona, 

California, Nevada, and Utah and extends from the east end 

of Lake Mead south to the southerly international boundary 

with Mexico (fig. 1). Nearly 15 years have passed since the 

development of the original accounting surface. Prior to the 

issuance of a proposed rule to define the accounting procedure, an update of the accounting surface is needed for use in 

the process of Decree accounting for the following reasons:

1. The original accounting surface was generated on the 

basis of water-surface profiles of the lower Colorado 

River computed for the highest median monthly projected discharge for 1992–2001 and assuming delivery 

of full allocations of river water to users in the United 

States. Since that time, historical data are available that 

represent the current and anticipated future operation of 

the Colorado River for the delivery of full allocation of 

river water to users in the United States and treaty deliveries to Mexico.

2. The original water-surface profiles were generated 

with a surface-water model representing river-channel 

conditions surveyed between 1980 and 1988. More 

recent river stage information is available, and the target 

elevations for Lakes Mohave and Havasu have changed 

slightly since the original accounting surface was developed.

3. The original accounting surface in parts of the Parker and 

Palo Verde areas was based on water-surface elevations 

in drainage ditches or wells along the edge of the flood 

plain that represented regulated flow conditions of the late 

1980s and early 1990s. Furthermore, the elevations from 

the drainage ditches used in the Palo Verde Valley were 

based on a nonstandard vertical datum, adding an error to 

the elevation of the accounting surface in that area.

4. Improved ground-water flow modeling is now available 

that will allow efficient construction of an accounting 

surface tied to the river in reaches not adjacent to reservoirs. An accounting surface computed with a physically 
2 Update of the Accounting Surface Along the Lower Colorado River

Figure 1. Map showing the lower Colorado River and areal extent of the river aquifer.

!(

!(

!(

!(

!(

!(

!(

!(

0 10 20 Miles Base from U.S. Geological Survey

digital data, 1:100,000. 1982,

Universal Transverse Mercator

projection, Zone 11, Datum NAD27,

National Vertical Geodetic Datum of

1929.

Area shown in

Figure 4

Area shown in

Figure 5

Area shown in

Figure 6

15

Lake

Mead

M o h a v e V a ll e y LakeHavasu

Parker Valley

Valley Cibola Val el y

Nevada

California

Utah

Arizona

United States

Mexico

Gila River

C o l o r a

d o R i v e r

Explanation

Yuma Area model grid

Area of river aquifer along river reaches

Area of river aquifer around reservoirs

Imperial Dam to Laguna Dam model grid

Areas shown in figures 4–7

Mohave area model grid

Parker, Palo Verde, and Cibola

area model g rid 

Flood plain

Yuma area model grid extends southward to Gulf of California

C o l o r a d o R i v e r

Verde Palo

¦ ̈§10 ¦ ̈§10

¦ ̈§8

¦ ̈§8

¦ ̈§15

¦ ̈§15

¦ ̈§40

40 ¦ ̈§40

£¤93 

£¤93 

£¤93 

£¤95 

£¤95 

£¤95 

£¤95 

Area shown in

Figure 7

! ! ! Area of plate 3

! ! ! Area of plate 2

! ! ! Area of plate 1 

115° W 114° 113°

37° N

36°

35°

34°

33°

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! !
Introduction 3

based model is an improvement on the original accounting surface, which was hand-drawn based on hydrologic 

judgment, and can be easily replicated and quickly 

updated as required.

Legal Framework

The Colorado River Compact of 1922 apportions the 

waters of the Colorado River between the upper basin and the 

lower basin (U.S. Congress, 1948, p. A17-A22). The requirement for participation of the USGS and Reclamation is stated 

in Article V:

The chief official of each signatory State 

charged with the administration of water rights, 

together with the Director of the United States 

Reclamation Service and the Director of the United 

States Geological Survey shall cooperate, ex-officio:

(a) To promote the systematic determination and 

coordination of the facts as to flow, appropriation, 

consumption, and use of water in the Colorado River 

Basin, and the interchange of available information 

in such matters.

Water in the lower Colorado River is apportioned among 

the States of California, Arizona, and Nevada by the Boulder 

Canyon Project Act of December 21, 1928 (U.S. Congress, 

1948, p. A213–A225) and confirmed by the Consolidated 

Decree (U.S. Supreme Court, 2006) in terms of consumptive 

use. The decree is specific about the responsibility of the Secretary of the Interior to account for consumptive use of water 

from the mainstream. Consumptive use is defined to include 

“water drawn from the mainstream by underground pumping.” 

Article V of the Consolidated Decree (U.S. Supreme Court, 

2006) states in part:

The United States shall prepare and maintain, or 

provide for the preparation and maintenance of, and 

shall make available, annually and at such shorter 

intervals as the Secretary of the Interior shall deem 

necessary or advisable, for inspection by interested 

persons at all reasonable times and at a reasonable 

place or places, complete, detailed and accurate 

records of: * * *

* * * (B) Diversions of water from the mainstream, 

return flow of such water to the stream as is available for consumptive use in the United States or in 

satisfaction of the Mexican treaty obligation, and 

consumptive use of such water. These quantities 

shall be stated separately as to each diverter from the 

mainstream, each point of diversion, and each of the 

States of Arizona, California, and Nevada; * * *

Article I of the decree defines terminology and states in part:

(A) “Consumptive use” means diversions from the 

stream less such return flow thereto as is available 

for consumptive use in the United States or in satisfaction of the Mexican treaty obligation;

(B) “Mainstream” means the mainstream of the 

Colorado River downstream from Lee Ferry within 

the United States, including the reservoirs thereon;

(C) Consumptive use from the mainstream within 

a state shall include all consumptive uses of water 

of the mainstream, including water drawn from the 

mainstream by underground pumping, and including but not limited to, consumptive uses made by 

persons, by agencies of that state, and by the United 

States for the benefit of Indian reservations and 

other federal establishments within the state; * * *

Ground water in the river aquifer beneath the flood plain 

is considered to be Colorado River water, and water pumped 

from wells on the flood plain is presumed to be river water and 

is accounted for as Colorado River water. Drainage ditches 

that lie along the edge of the flood plain contain a mixture of 

river water (recharged on the flood plain from the application 

of diverted irrigation water) and tributary water. 

Purpose and Scope

This report documents the updates to the data and method 

used to generate the accounting surface in previous reports 

(Wilson and Owen-Joyce, 1994; Owen-Joyce and others, 2000) 

and presents the updated accounting surface needed to identify 

wells outside the flood plain of the lower Colorado River that 

yield water that will be replaced by water from the Colorado 

River. The report describes the process to update the accounting surface using simple, physically based ground-water flow 

models and contains maps (figs. 4–7 and plates 1–3) that show 

the elevation and contours of the updated accounting surface. 

Site-specific data were collected where needed to update the 

accounting surface. 

Data Collection

The USGS collected hydrologic data for the study 

during 2007–08. Most field work was done along the drainage ditches on the flood plain in Parker and Cibola Valleys 

in Arizona, in Palo Verde Valley in California, and in the 

Yuma area in Arizona and California. Additional data were 

collected along reaches of the river between Parker and 

Headgate Rock Dams and from upstream of Imperial Dam 

to the northerly international boundary (NIB) with Mexico. 

Water-surface elevations in drainage ditches were determined by use of Global Positioning System (GPS) surveys 

(Remondi, 1985). The data are stored in a database of the 

Arizona Water Science Center of the U.S. Geological Survey, Tucson, Arizona.

Precise GPS was used to collect water-level elevation 

data in the drainage ditches of agricultural areas along the 
4 Update of the Accounting Surface Along the Lower Colorado River

lower Colorado River in Parker, Cibola, and Palo Verde Valleys and in the Yuma area. Field collection of data for the Palo 

Verde Valley drainage-ditch survey was conducted during 

the weeks of August 13 and 27, 2007. Data for the drainage 

ditches in Parker Valley were collected during the weeks of 

August 27 and September 10, 2007. Data for the Cibola Valley 

drainage ditches were collected during the week of November 

5, 2007. Data for the drainage ditches in the Yuma area were 

collected January 30–31, 2008. Precise GPS was also used 

to collect data for specific reaches along the Colorado River. 

Data for the river between Parker Dam and Headgate Rock 

Dam were collected January 24, 2008. Data for the river in the 

Yuma area were collected the week of February 4, 2008.

Survey methods included collecting survey data by using 

Real-Time Kinematic (RTK)-Infill and static GPS. RTK GPS 

was used to collect edge-of-water or staff-gage elevations in 

the drainage ditches. RTK base-station positions were located 

at higher topographic locations near the drainage ditches. The 

base-station positions were selected by virtue of line-of-sight 

capability with the area of the drainage ditch to be surveyed. 

Because most survey points within the drainage ditches were 

obscured from the base station by the embankments, two 

technicians conducted the survey for safety and to ensure lineof-site radio link between the RTK base station and rover unit. 

One technician entered the drainage ditch to place the rover 

GPS antenna pole at the edge of water or, when available, on 

top of a staff gage, while the second technician remained at the 

top of the drainage ditch with the rover radio receiver. Downto-water measurements were made from the top-of-staff gage 

or other measuring-point positions.

Static GPS methods included the occupation of surrounding survey benchmarks that have coordinates published by 

the National Geodetic Survey (NGS). Data collected from the 

static occupations were used to tie-in, correct, and check the 

coordinates of individual RTK base-station positions for each 

of the individual drainage-ditch surveys. In addition, individual base-station positions from each of the drainage ditches 

were surveyed to a single benchmark located just west of the 

right bank cableway tie-back at the Colorado River below Palo 

Verde Dam (USGS 09429100) streamflow-gaging station. 

Selected top-of-staff measuring points and surrounding NGS 

benchmarks, when available, were reoccupied with RTK GPS 

to check for survey accuracy and repeatability. The accuracy 

of the surveyed elevations was ±0.20 feet.

Various precise GPS methods were used to collect waterlevel elevation data depending on the conditions that existed 

in those areas. Traditional RTK and faststatic techniques were 

not feasible due to line-of-sight problems and the absence of 

an established faststatic base station in the area between Parker 

and Headgate Rock Dams. Data were collected at eleven 

points along this reach of the river by treating each point as a 

base station and obtaining an Online Positioning User Service 

(OPUS) solution for each point. In the Yuma area, where there 

is an established base station surveyed in at the Yuma USGS 

office, data were collected at 28 points in drainage ditches and 

at 6 wells using the faststatic technique. Along the river in the 

reach upstream from Laguna Dam, 15 elevation points were 

collected by using the faststatic technique with the Yuma base 

station at the USGS office. Along the river in the Yuma area 

downstream from Laguna Dam to the NIB with Mexico, the 

RTK technique was used to collect data at 14 points by using 

both the AMVD and COCO base stations, which are developed benchmarks established by the City of Yuma.

Previous Investigations

The accounting-surface method is described for two areas 

in separate reports—the area upstream from Laguna Dam 

in Wilson and Owen-Joyce (1994) and the area downstream 

from Laguna Dam in Owen-Joyce and others (2000). Previous 

geohydrologic studies of the lower Colorado River valley from 

Davis Dam to Yuma defined and described the formations that 

constitute the river aquifer, discussed the geologic structures and 

framework of the lower Colorado River valley, and described 

the occurrence and movement of ground water (Metzger, 1965, 

1968; Metzger and Loeltz, 1973; Metzger and others, 1973; 

Olmsted and others, 1973). The major emphasis of these studies 

was the ground-water flow system beneath the flood plain and its 

relation to the Colorado River because few wells were available 

outside the flood plain to provide water levels or samples for 

chemical analysis. Refinement of the hydrogeologic framework, 

updated maps of ground-water flow, estimates of ground-water 

storage in the mound under Yuma Mesa, water-chemistry analyses, and water-budget components are topics covered in a recent 

study of the Yuma area (Dickinson and others, 2006). Additional 

work to develop procedures to apply the accounting-surface 

method to water-level data from wells applied geographic 

information system (GIS) methods to identify areas where wells 

pump water that will be replaced by water from the Colorado 

River (Spangler and others, 2007).

Acknowledgments

Jeffrey C. Addiego, Douglas B. Blatchford, William B. 

Greer, Ruth M. Thayer, Shana G. Tighi, and Dennis E. Watt 

of the Bureau of Reclamation provided information regarding 

discharges and water-surface profiles in the Colorado River 

and guidance concerning the use of drainage ditches in the 

ground-water modeling. The Colorado River Indian Tribes 

kindly provided access to their reservation. Palo Verde Irrigation District provided maps and water-level data for their staff 

gages in drainage ditches. 

Accounting-Surface Method

The accounting-surface method was developed to identify wells outside the flood plain of the lower Colorado River 

that yield water that will be replaced by water from the river 

(Wilson and Owen-Joyce, 1994; Owen-Joyce and others, 

2000). The method is based on the concept of a river aquifer 
Accounting-Surface Method 5

and an accounting surface within the river aquifer. The method 

provides a uniform criterion for all users pumping water from 

wells by determining whether the elevation of the static water 

table at a well is above or below the accounting surface. The 

elevation of the static water table at a well is determined by 

measuring the elevation of the static water level in the well. 

The static water level is the level of the water in a well that is 

not being affected by ground-water withdrawal or the level to 

which water will rise in a tightly cased well under its full pressure head. 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. Wells that have 

a static water-level elevation above the accounting surface are 

presumed to yield water that will be replaced by water from 

precipitation and inflow from tributary valleys (fig. 2). Ground 

water in the river aquifer beneath the flood plain is considered 

to be Colorado River water regardless of water levels. Water 

pumped from wells on the flood plain is presumed to be river 

water and is accounted for as Colorado River water. 

The accounting surface is defined to represent the elevation and slope of the static water table in the river aquifer 

outside the flood plain and the reservoirs of the Colorado 

River that would exist if the water in the river aquifer were 

derived only from the river (Wilson and Owen-Joyce, 1994). 

The accounting surface extends outward from the edges of 

the flood plain or a reservoir to the subsurface boundary of 

the river aquifer. Initial attempts to compare the water level in 

wells to the accounting surface were stymied by the inability 

to obtain water levels in every well (Spangler and others, 

2007). Consequently, a method was devised by Spangler and 

others (2007) to estimate the water surface from available data 

and a new category—near the accounting surface—was added 

to the existing categories of well water levels—at, below, or 

above the accounting surface. GIS methods were used to create maps from measured water-level data that were then used 

to delineate areas where the water levels in wells were above 

or below the accounting surface. Estimations of water elevation can be made for wells without a measured water level 

(Spangler and others, 2007) from these maps. Water levels in 

wells were measured with calibrated steel or electrical tapes 

that are accurate to within tenths or hundredths of a foot. A 

differential GPS was used to determine land-surface elevations to within an operational accuracy of ±0.43 ft, resulting in 

calculated water-level elevations having a 95-percent confidence interval of ±0.84 ft. GIS interpolation tools were used 

to delineate areas within the river aquifer where water-level 

elevations are presumed to be above, below, and near (within 

±0.84 ft at the 95-percent confidence interval) the elevation of 

the accounting surface. 

The criterion in the accounting-surface method for all 

users pumping water from wells was changed by Spangler 

and others (2007) to determining whether the elevation of 

the static water table at a well is above, near, or below the 

accounting surface. Wells that have a static water-level 

Figure 2. Schematic diagram showing the river aquifer and accounting surface (red line) of the lower Colorado River. 

Wells labeled “R” have a static water-level elevation equal to or below the accounting surface and are presumed to 

yield water that will be replaced by water from the river. Wells labeled “T” have a static water-level elevation above 

the accounting surface and are presumed to yield water that will be replaced by water from precipitation and inflow 

from tributary valleys (Modified from Wilson and Owen-Joyce, 1994).

R R T

T

C

River

olora od

FLOOD PLAIN

RESERVOIR

SEDIMENTS AND SEDIMENTARY ROCKS

TRIBUTARY WASH

ALLUVIAL

SLOPE

DAM

WATER TABLE

WATER

TABLE

RIVER AQUIFER

ACCOUNTING SURFACE

BEDROCK

RIVER AQUIFER BOUNDARY

DRAINAGE

DITCH

BEDROCK
6 Update of the Accounting Surface Along the Lower Colorado River

elevation near, equal to, or below the accounting surface are 

presumed to yield water that will be replaced by water from 

the river. Wells that have a static water-level elevation above 

the accounting surface are presumed to yield water that will 

be replaced by water from precipitation and inflow from 

tributary valleys.

River Aquifer

The boundary of the area that contains the accounting surface was defined as the river aquifer and delineated in the previous studies (Wilson and Owen-Joyce, 1994; Owen-Joyce and 

others, 2000). The river aquifer consists of permeable, partly 

saturated 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 Colorado River valley 

and adjacent tributary valleys, which is a barrier to groundwater flow. For this study the boundary of the river aquifer 

remains the same as defined previously (fig. 1).

The river aquifer beneath the area where the accounting 

surface exists can be divided into two areas. The first area is 

where the water table is controlled by reservoirs, and the second 

area is where the water table is controlled by the Colorado 

River, drainage ditches on the flood plain, or both. In areas controlled by reservoirs, the accounting surface is set at a constant 

elevation defined by a representative reservoir level specified by 

Reclamation. In areas controlled by the Colorado River, drainage ditches, or both, the accounting surface varies depending on 

the shape of the aquifer and the surface-water elevations.

Generation of the Accounting Surface

The accounting surface adjacent to free-flowing reaches 

of the river between reservoirs published by Wilson and 

Owen-Joyce (1994) and Owen-Joyce and others (2000) was 

represented by hand-drawn contours based on surface-water 

profiles. In Parker and Palo Verde Valleys, drainage ditches or 

wells along the edge of the flood plain were used to define the 

level of the accounting surface. Reclamation considers the water 

levels in the drainage ditches to represent the level of Colorado 

River water beneath the flood plain. Adjacent to reservoirs, the 

accounting surface is flat, and is set to an elevation of the adjacent reservoir defined by the annual high water-surface elevation used by Reclamation to operate the reservoirs under normal 

flow conditions.

The general strategy for updating the accounting surface 

was as follows:

1. The extent of the river aquifer and area over which the 

accounting surface was defined by Wilson and OwenJoyce (1994) and Owen-Joyce and others (2000) was 

retained.

2. Water-surface profiles of the Colorado River and drainage ditches used in defining the accounting surface were 

updated using the most recent information available. 

Drainage ditches were used in Parker, Palo Verde, and 

Cibola Valleys in defining the accounting surface.

3. Water-surface elevations in reservoirs were updated 

on the basis of current operating conditions for Lakes 

Mead, Mohave, and Havasu.

4. Contours of the accounting surface adjacent to freeflowing reaches of the Colorado River were generated 

using simple steady-state ground-water models that 

simulate two-dimensional flow, using a constant transmissivity value, with river and drainage-ditch elevations 

as boundary conditions.

The discharges along the free-flowing reaches of the 

Colorado River and the water-surface elevations in reservoirs 

used to define the accounting surface were specified by Reclamation. Implementation of this general strategy is discussed in 

more detail in the following sections.

Criteria for Establishing Reservoir Water-Surface 

Elevations and Colorado River Flow Conditions 

Used to Generate the Accounting Surface

The water-surface elevations in the Colorado River, reservoirs, and drainage ditches satisfy the following criteria (Jeffrey 

C. Addiego, Bureau of Reclamation, written commun., 2007):

•	 The Colorado River is flowing under normal operating conditions. Normal operating conditions exist 

when releases from the reservoirs are being made to 

accommodate downstream requirements where each 

State is using its full apportionment (consumptive use 

in Arizona + California + Nevada equals 7.5 million 

acre-feet) and a treaty-specified 1.5 million acre-feet 

is being delivered to Mexico (approximately 1.36 

million acre-feet at the NIB with Mexico and 0.14 

million acre-feet at the land boundary near San Luis). 

Flow and (or) river stage values can be either historical or modeled values, and should exclude flood 

flows from the lower basin tributaries and side-wash 

inflows.

•	 The hydraulic influence of the Colorado River under 

normal operating conditions is defined by the mean 

stage of the Colorado River (excluding reservoirs) during the highest flow month of the year (the flow that 

should be used to calculate the river stage is the mean 

monthly flow for the highest flow month of the year).

•	 The elevations used for the reservoirs (Lakes Mohave 

and Havasu) are the high monthly target elevation for 

the year used when operating under normal operating 

conditions—644 feet for Lake Mohave and 448.7 feet 

for Lake Havasu. 
Accounting-Surface Method 7

•	 The maximum elevation of the accounting surface for 

Lake Mead is the top of the spillway gates in their 

fixed (down) position (1,205.4 feet). This elevation 

corresponds to an elevation (and corresponding area) 

in the vicinity of Lake Mead where a well would have 

the potential to pump Colorado River water. Whether 

a well would be considered to pump Colorado River 

water in the Lake Mead area would depend upon the 

actual lake elevations during the accounting year.

•	 The flows and river stage values account for major 

diversions from and return flows to the river at their 

respective locations. These diversions and return flows 

include, at minimum, the diversion at Headgate Rock 

Dam and major drainage ditches from the Colorado 

River Indian Reservation, the diversion at Palo Verde 

Diversion Dam and major drainage ditches from the 

Palo Verde Irrigation District, the diversions from Lake 

Havasu by the Central Arizona Project and the Metropolitan Water District canals, the diversions at Imperial 

Dam and major returns below Imperial Dam. As many 

diversions and return flow points are used as practical given the available data and the practical influence 

upon the resultant values. 

Areas of the River Aquifer Controlled by 

Reservoirs

The accounting surface elevations in the river aquifer surrounding Lake Havasu, Lake Mohave, and Lake Mead are determined by the reservoir levels. Reclamation has determined that 

the accounting-surface elevations are 448.7 ft for Lake Havasu, 

644.0 ft for Lake Mohave, and 1205.4 ft for Lake Mead. 

Areas of the River Aquifer Controlled by the 

Colorado River, Drainage Ditches, or Both

River Reaches

Along reaches of the Colorado River without irrigation 

on the flood plain where the river loses water to the aquifer, the 

accounting surface is determined by the water surface of the 

Colorado River. Under predevelopment conditions and where 

the flood plain is not irrigated with diverted river water, groundwater levels in areas outside the flood plain that are higher than 

the Colorado River are caused only by tributary ground-water 

inflow. In this case, the river controls the elevation of the water 

table under the flood plain, and the accounting surface would be 

lower than the higher water level caused by tributary groundwater inflow. Water pumped from a well with a static water 

level above the accounting surface would be deemed tributary 

water, and an entitlement would not be needed.

A calibrated and documented step-backwater model was 

not available for the study area, and development of such 

a model was beyond the scope of this study. Reclamation 

Colorado River 

gaging station

River 

mile

Discharge, in 

cubic feet per 

second

Below Hoover Dam 342.0 17,634

Below Davis Dam 275.5 17,069

At Big Bend 264.7 19,567

Below Parker Dam 192.2 12,370

Forebay at Headgate Rock Dam 177.7 11,402

At Parker 175.3 11,970

At Water Wheel 151.5 11,157

Below Palo Verde Diversion Dam 132.7 10,924

At Taylor Ferry 106.4 9,825

At Lower Cibola Bridge 86.9 10,399

Above Imperial Dam 49.2 10,222

Below Imperial Dam 49.2 549

Below Laguna Dam 41.7 716

Below Yuma Main Canal Wasteway 29.4 1,527

Table 1. Discharges used to determine the water-surface elevation 

of the Colorado River used in the ground water-flow models.

[River miles start at the southerly international boundary with Mexico and 

increase upstream (Bureau of Reclamation, 2001)]

Figure 3. Water surface profile of the Colorado River used in 

the ground-water flow models. The approximate extent of the 

accounting surface in each of the four modeled areas is also 

displayed as a function of river mile. River miles (Bureau of Reclamation, 2001) start at the southerly international boundary with 

Mexico and increase upstream.

0

200

400

600

0 100 200 300

Colorado River water-surface elevation

Mohave Valley

Parker Valley, Cibola Valley, and 

Palo Verde Valley

Above Imperial Dam to Laguna Dam

Yuma area

Water-surface elevation, in feet

River mile

Lake Mohave

Lake Havasu
8 Update of the Accounting Surface Along the Lower Colorado River

surface elevations used to represent the Colorado River south 

of Eleven-mile gage are in the appendixes (available only 

online at http://pubs.usgs.gov/sir/2008/5113/appendixes/).

Mohave Valley—The water-surface profile was determined from the stage-discharge relations at four streamflowgaging stations at river miles 275.4 (Colorado River below 

Davis Dam), 264.7 (Colorado River at Big Bend), 243.4 (Colorado River below Needles Bridge), and 233.6 (Colorado River 

near Topock [at RS41]), and the elevation of Lake Havasu. 

Parker Valley and Palo Verde Valley—The linearly interpolated profile was based on streamflow-gaging station data at 

river miles 192.2 (Colorado River below Parker Dam), 177.7 

(Colorado River Forebay above Headgate Rock), 175.3 (Colorado River at Parker), 151.5 (Colorado River at Water Wheel), 

132.7 (Colorado River below Palo Verde Diversion Dam), 106.4 

(Colorado River at Taylor Ferry), and 86.9 (Colorado River at 

Lower Cibola Bridge) and Reclamation GIS coverages of the 

extent of the Palo Verde Dam and Headgate Rock Dam forebays 

(Shana Tighi, Bureau of Reclamation, written commun. 2008). 

Water-surface elevation measurements were made between 

Colorado River

gaging station

UTM coordintate1

, in meters Elevation,

in feet River mile Agency Easting Northing

Below Davis Dam (09423000) 721369 3895914 503.17 275.40 USGS

Big Bend 717750 3884573 486.60 264.70 Reclamation

Below Needles Bridge 721649 3855318 462.20 243.30 Reclamation

RS41 (below Topock Marsh) 731394 3844023 454.14 233.60 Reclamation

Below Parker Dam (09427500) 763366 3798537 370.45 192.20 USGS

Forebay at Headgate Rock Dam 750315 3783939 362.62 177.70 Reclamation

Parker 748190 3781783 344.00 175.30 Reclamation

Water Wheel 728171 3756367 302.63 151.50 Reclamation

Below Palo Verde Diversion Dam 732289 3732777 267.02 132.70 Reclamation

Taylor Ferry (TFLC) 720531 3701245 231.57 106.40 Reclamation

Lower Cibola Bridge 716492 3676582 208.38 86.90 Reclamation

Below Imperial Dam (09429500) 736985 3640727 180.72 49.20 USGS

Below Laguna Dam (0942600) 732742 3633016 127.06 41.7 USGS

Below Yuma Main Canal Wasteway (09521100) 720849 3623858 113.22 29.50 USGS

Above Rockwood Weir 713707 3622116 106.80 23.10 IBWC

Above Morelos Dam 712976 3620783 105.00 22.10 IBWC

Below Morelos Dam 712939 3620723 100.20 22.11 IBWC

Eleven-mile gage 711163 3616163 92.20 18.80 IBWC

1 Universal Transverse Mercator, Zone 11 coordinates, North American Datum of 1927.

reevaluated the discharges below dams and streamflow-gaging 

stations along the river used to establish the water-surface 

elevations according to the criteria described above (Douglas B. Blatchford, Bureau of Reclamation, written commun., 

2007) and produced the discharges in table 1. The watersurface profile of the Colorado River was based on a profile 

linearly interpolated between streamflow-gaging stations that 

was provided by Reclamation (Shana G. Tighi, written commun., 2008) that included streamflow data collected at gaging 

stations operated by Reclamation, USGS, and the International 

Boundary and Water Commission (IBWC). That profile was 

modified by additional water-surface elevation measurements 

made by the USGS. Water-surface measurements were made 

where the linearly interpolated profile deviated significantly 

from the profiles used in the previous studies (Wilson and 

Owen-Joyce, 1994; Owen-Joyce and others, 2000). The watersurface profile of the Colorado River used in the ground-water 

flow models is shown in figure 3. Data from streamflowgaging stations and USGS measurements are listed in tables 

2 and 3. Tables containing the water-surface elevations in the 

drainage ditches, the path of the Colorado River in Universal 

Transverse Mercator (UTM) coordinates, and the digitized 

Table 2. Streamflow-gaging station data used to define the water-surface elevation used in the ground-water flow models.

[Agency: USGS, U.S. Geological Survey; Reclamation, Bureau of Reclamation; IBWC, International Boundary and Water Commission]
Accounting-Surface Method 9

River mile

UTM Coordinates1

, in meters Elevation,

Easting Northing in feet

191.74 763447 3798135 368.3

190.77 763714 3796614 367.1

189.00 763681 3794281 362.2

188.56 763001 3794254 365.1

187.31 761528 3793982 365.0

186.14 760340 3793027 364.9

184.34 758613 3790855 365.2

182.92 757106 3789138 365.0

181.33 755824 3787245 364.8

179.68 754201 3785508 364.8

178.07 752000 3784263 365.0

47.86 736669 3639365 156.2

47.72 736624 3639178 150.8

46.81 736281 3638125 150.8

44.43 735168 3635338 150.8

40.98 731882 3631582 2

122.9

38.47 731591 3627923 2

121.3

35.93 731039 3624107 2

119.8

34.46 729226 3622800 2

118.6

34.31 728990 3622783 2

118.4

33.39 727531 3622993 2

117.7

31.55 724802 3623457 2

114.7

31.41 724567 3623385 2

114.8

30.98 723908 3623357 2

114.1

29.70 721982 3623768 3

112.0

28.88 720667 3623899 3

111.1

28.20 719580 3623936 3

110.1

26.69 717318 3624355 3

108.5

25.83 716056 3624828 3

107.7

1

 Universal Transverse Mercator, Zone 11 coordinates, North American Datum of 1927.

2 These elevations were increased by 0.8 ft for use in the ground-water 

model to account for difference in discharge specified for the accounting 

surface and the discharge during the stage measurements. See the section 

Yuma Area for further explanation.

3

 These elevations were increased by 1.2 ft for use in the ground-water 

model to account for difference in discharge specified for the accounting 

surface and the discharge during the stage measurements. See the section 

Yuma Area for further explanation.

Table 3. Colorado River water-surface elevation 

measurements used to define the water-surface elevation 

used in the ground-water flow models.

Parker Dam and Headgate Rock Dam during this study because 

the interpolated profile showed the forebay behind Headgate 

Rock Dam extending upstream to around river mile 189, 

whereas the earlier profile had a sloping water surface, resulting 

in higher water-surface elevations. The measurements supported 

the extent of the forebay represented in the interpolated profile.

Above Imperial Dam to Laguna Dam—The linearly interpolated profile was based on a streamflow-gaging station at river 

mile 49.2 (Colorado River below Imperial Dam) and Reclamation GIS coverages of the Imperial and Laguna Dam forebays. 

Water-surface elevation measurements were made above Imperial Dam and showed that the linearly interpolated profile overestimates the extent of the forebay. The measurements, which show 

a sloping water surface, were used in the ground-water model. 

The measurements were made at a discharge of about 6,000 ft3

/s, 

whereas the discharge specified for the accounting surface in this 

reach was 10,222 ft3

/s. Stage-discharge relations were not available for this reach and a correction for the difference between 

the two discharges was not made. Water-surface measurements 

made between Imperial Dam and Laguna Dam supported the 

linearly interpolated profile drawn from the Reclamation GIS 

coverage of the Laguna Dam forebay.

Yuma area—The following six streamflow-gaging stations were used for this reach: Colorado River below Laguna 

Dam, below Yuma Main Canal Wasteway, above Rockwood 

Weir at the NIB (International Boundary and Water Commission (IBWC) 095-219.00), immediately above Morelos Dam 

(IBWC 09-5220.21), immediately below Morelos Dam (IBWC 

09-5220.41), and at Eleven-mile gage (IBWC 09-5221.00). The 

previous water-surface profile (Owen-Joyce and others, 2000) 

showed considerable variability in the water-surface profile 

between Laguna and Morelos Dams that was not represented 

by the linearly interpolated profile, so water-surface elevation 

measurements were made in this reach. The measurements were 

made at lower discharges than specified for the updated accounting surface. Corrections to water-surface elevations that account 

for the differences in discharge were estimated from the stagedischarge rating curves at the below Laguna Dam streamflowgaging station and the below Yuma Main Canal Wasteway 

streamflow-gaging station. Between Laguna Dam and the below 

Yuma Main Canal Wasteway streamflow-gaging station, the 

updated accounting surface discharge was 716 ft3

/s, whereas the 

discharge measured during the water-surface elevation measurements was 440 ft3

/s. The water-surface profile between Laguna 

Dam and the below Yuma Main Canal Wasteway streamflowgaging station was defined in the ground-water model by the 

measurements plus a correction of 0.8 ft, based on the stagedischarge rating curve at the below Laguna Dam streamflowgaging station. In the reach between the below Yuma Main Canal 

Wasteway streamflow-gaging station and Morelos Dam, the 

discharge specified for the updated accounting surface was 1,526 

ft3

/s. The discharges measured during the water-surface elevation

measurements were 670 and 778 ft3/s. The water-surface profile 

between the below Yuma Main Canal Wasteway streamflowgaging station and the above Rockwood Weir streamflow-gaging 
10 Update of the Accounting Surface Along the Lower Colorado River

station was defined in the ground-water model by the measurements plus a correction of 1.2 ft, based on the stage-discharge 

rating curve at the below Yuma Main Canal Wasteway streamflow-gaging station and the average of the measured discharges 

in that reach. An analysis of streamflow-gaging station records 

by Jeffrey C. Addiego (Bureau of Reclamation, written commun., 2008) provided the water-surface elevation at the above 

Rockwood Weir streamflow-gaging station. Just above and 

below Morelos Dam and at the Eleven-mile gage (at river mile 

18.8), the average monthly high stages at the IBWC streamflowgaging stations were used to establish the water-surface elevations. Below Eleven-mile gage, the land surface digitized along 

the path of the Colorado River was used as the water-surface 

boundary condition in the ground-water flow model. 

Drainage Ditch Reaches

Along reaches of the Colorado River where water is 

diverted for irrigation on the flood plain, drainage ditches intercept return flow to the river and the river gains water from the 

aquifer. In these reaches, the accounting surface is defined by 

using the water-surface elevation in the drainage ditches along 

the edge of the flood plain. Flood-plain irrigation with diverted 

Colorado River water causes a higher ground-water level under 

the flood plain because irrigation water not consumptively 

used by crops percolates down to the water table and causes 

the water table to rise. There is a constant flow of irrigation 

with diverted river water, percolation to the drainage ditches 

or river, and flow in the drainage ditches to the river. Interception of the percolated irrigation water by a network of drainage 

ditches connected to the Colorado River keeps the water table 

from rising up into the root zone and this level is higher than it 

would be if controlled by the river. Because water in the drainage ditches is considered Colorado River water for accounting surface purposes, it warrants the same level of protection 

from depletion without an entitlement as water in the Colorado 

River and stored in reservoirs. Where drainage ditches intercept 

percolated irrigation water, the water-surface elevations in the 

drainage ditches were used to define the accounting surface. 

Drainage ditches along the edge of the flood plain could not be 

used in the Yuma area because the elevation of the water surface 

is controlled mainly by recharge from the unlined canals that 

are above the flood-plain elevation and run parallel to the edge 

of the flood plain rather than by percolation from irrigation on 

the flood plain. Water-surface elevations in the drainage ditches 

were determined from USGS measurements in 2007 and 2008.

Ground-Water Flow Models

In the previous studies, the accounting surface was handdrawn using hydrologic judgment to extend water-surface 

elevations into the river aquifer based on the shape of the river 

aquifer. This study refines that procedure by using simple 

physically based steady-state numerical models to calculate 

the updated accounting surface. 

The river aquifer was represented as a single model layer 

of uniform aquifer thickness and spatially invariant or constant 

transmissivity. Because transmissivity is the product of the 

aquifer thickness and hydraulic conductivity, hydraulic conductivity in the model also is constant. With the assumption of 

a spatially invariant or constant transmissivity throughout the 

model domain, the governing equation of steady-state flow in 

two dimensions is:

 

 (1)

where h is hydraulic head and x and y represent Cartesian coordinates along orthogonal axes. Aquifer thickness and conductivity are not present in equation 1; the distribution of heads in the 

modeled river aquifer depends only on the aquifer boundaries 

and the specified water-surface elevations in the drainage ditches 

in the flood plain and in the Colorado River. Flow rates through 

the aquifer would depend on the aquifer transmissivity, but flow 

rates are not considered in this study. 

The river aquifer adjacent to the Colorado River is unconfined. The assumption of constant transmissivity neglects spatial 

variations in transmissivity that would arise from the spatial 

variations in the vertical position of the water table. The assumption also neglects variations in transmissivity that occur from 

variations in the vertical position of the aquifer bottom as well 

as spatial variations in hydraulic conductivity. A more rigorous 

approach would be to use the nonlinear Boussinesq equation 

instead of the simpler linear Laplace equation (equation 1). That 

approach, however, would have required unavailable information 

on aquifer geometry and hydraulic properties. Use of equation 

1 is in keeping with a long history in the field of ground-water 

hydrology of using simple linear equations with the assumption 

of homogeneous properties to approximate ground-water conditions and responses and is consistent with the overall parsimonious approach taken in the concept, definition, and application of 

an accounting surface.

The accounting surface in the four areas was modeled with 

MODFLOW 2000 (Harbaugh and others, 2000) using the watersurface elevations in the Colorado River and drainage ditches as 

constant-head boundaries. The grid spacing in the models was 

0.25 mi along model rows and columns. General characteristics of the model grids are given in table 4 and the extent of the 

model grids is shown in figure 1. The path and distribution of 

Colorado River and drainage ditch water-surface elevations were 

established on the model grids using the RIVGRID program 

(Leake and Claar, 1999). The water-surface elevations defined 

by RIVGRID were then incorporated into the models as nodes 

with a constant head. 

Areas of the river aquifer adjacent to the Colorado River for 

which the accounting surface was modeled include (1) Mohave 

Valley; (2) Parker, Palo Verde, and Cibola Valleys; (3) Imperial 

Dam to Laguna Dam; and (4) the Yuma area. Each area was 

modeled with a single horizontal layer of cells of thickness 500ft 

and hydraulic conductivity 39.2 ft/day; however, as pointed out 

in the discussion of equation 1, the model predictions of the 

accounting surface are independent of thickness and hydraulic 

∂

∂

+

∂

∂ =

2

2

2

2 0 h

x

h

y
Summary 11

conductivity. Tests were carried out by varying hydraulic conductivity and thickness to verify that computed head distributions were independent of these parameters. Rows and columns 

of the model grids were oriented in east-west and north-south 

directions in the UTM, Zone 11, coordinate system. 

Updated Accounting Surface

The accounting surface around reservoirs was updated using 

a reservoir elevation. The accounting surface is set at its maximum possible level of 1,205.4 ft in the river aquifer around Lake 

Mead (fig. 4 and plate 1) and has not changed from the original 

accounting surface. The accounting surface is set at 644.0 ft in 

the river aquifer around Lake Mohave (fig. 5 and plate 1), and at 

448.7 ft in the river aquifer around Lake Havasu (fig. 5 and plate 

2), the current high monthly target elevations for these reservoirs. 

These elevations are slightly different from the high monthly target elevation used for the original accounting surface. In the river 

aquifer between the major reservoirs, ground-water flow models 

with boundary conditions set by Colorado River and drainage 

ditch water-surface elevations were used to contour the accounting surface. The models computed water-level elevations over the 

entire river aquifer; however, only contours in the river aquifer 

where the accounting surface exists are shown for modeled areas 

(figs. 4–7 and plates 1–3). The updated accounting surface is 

shown on maps for Mohave Valley and adjacent tributary areas 

(fig. 5 and plate 2); for Parker, Palo Verde, and Cibola Valleys 

and adjacent tributary areas (fig. 6 and plate 2); and for the Yuma 

area upstream and downstream from Laguna Dam and adjacent 

tributary areas (fig. 7 and plate 3). The model grid in the Yuma 

area extends to the south of the accounting surface, but only contours in the area with the accounting surface are shown.

Summary

An update of the accounting surface developed in the 

1990s to identify wells outside the flood plain of the lower 

Colorado River that yield water that will be replaced by 

water from the river was required as a result of changes in 

the ground and surface water systems and a datum correction to the water surface elevations in drainage ditches. The 

updated accounting surface will be used to identify which 

wells need an entitlement for diversion of water from the 

Colorado River and need to be included in accounting for 

consumptive use of Colorado River water as outlined in the 

Consolidated Decree of the United States Supreme Court in 

Arizona v. California, 547 U.S.150 (2006). Contours of the 

original accounting surface were hand drawn based on the 

shape of the aquifer, water-surface elevations in the Colorado River and drainage ditches, and hydrologic judgment. 

The original accounting surface was updated based on 

updated water-surface elevations in the Colorado River and 

drainage ditches, and the use of a simple, physically based 

ground-water flow model to calculate the accounting surface. The water-surface elevation of the Colorado River was 

determined for discharges specified by Reclamation. The 

water-surface elevations were derived from a linearly interpolated profile between USGS and Reclamation streamflowgaging stations and supplemented by IBWC streamflowgaging stations downstream from the northerly international 

boundary with Mexico. In addition, water-surface elevations were measured where the linearly interpolated profile 

deviated significantly from the water-surface profile used 

to develop the original accounting surface. The USGS also 

measured water-surface elevations in drainage ditches in the 

Parker, Palo Verde, and Cibola Valleys. 

The accounting surface was modeled with MODFLOW 

2000 (Harbaugh and others, 2000) using the water-surface 

elevations in the Colorado River and drainage ditches as 

constant-head boundaries. Reaches of the river aquifer 

adjacent to the Colorado River for which the accounting 

surface was modeled include (1) Mohave Valley, (2) Parker, 

Palo Verde, and Cibola Valleys, (3) Imperial Dam to Laguna 

Dam, and (4) the Yuma area. The development and application of computer models will make further updating of the 

accounting surface, if necessary, a straightforward task. 

In the river aquifer adjacent to reservoirs, the accounting 

surface was determined by a reservoir elevation specified by 

Reclamation. 

Area modeled 

UTM Coordinates of northwest 

corner of model grid1

, in meters Number of 

model rows

Number of 

model columns

Number of 

active cells2

Easting Northing

Mohave Valley 706260.7 3897829.0 160 139 13,264

Parker, Palo Verde, and Cibola Valleys 636449.1 3797916.0 329 388 87,176

Imperial Dam to Laguna Dam 730975.8 3672261.3 103 88 4,702

Yuma Area 640414.6 3691950.0 511 340 69,814

1

 Universal Transverse Mercator, Zone 11 coordinates, North American Datum of 1927.

2

 Includes cells with computed head and constant-head cells used to represent water-surface features.

Table 4. Properties of ground-water flow models used to compute the accounting surface for areas along the lower Colorado River.
12 Update of the Accounting Surface Along the Lower Colorado River

Figure 4. Map showing the accounting surface in the areas surrounding Lake Mead, 

Arizona, Utah, and Nevada.

114°30' W 114°

37° N

36°30'

36°

Base from U.S. Geological Survey

digital data, 1:100,000. 1982,

Universal Transverse Mercator

projection, Zone 11, Datum NAD27,

National Vertical Geodetic Datum of 1929.

Hoover

Dam

1205.4

1205.4

1205.4

1205.4

1205.4

1205.4

1205.4

644

Lake

Mead

U t a

h

N e v

a

d

a

A r i z o n

a

Explanation

Area of river aquifer around reservoirs –

number is the elevation of the accounting

surface, in feet.

E River mile

0 4 8 Miles

3

1205.4

15

§ ̈¦

£¤

15

93

£¤95

342

340

335

330

325

320

315

310

342

340

335

330

325

320

315

310 E

E

E

E

E

E

E

E

Co ol rado

River
Summary 13

Figure 5. Map showing the accounting surface in Mohave Valley and adjacent tributary areas in Arizona, California, and Nevada.

!

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#

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320

315

310

305

300

295

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285

280

275

270

265

260

255

250

245

240

235

230

225

220

215

205

200

195

190

185

Needles

Lake Havasu City

Bullhead City

210

456

492 490

480

482

476

478

458

494

472

488

474

468

466

484

496

470

486

464

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500

464

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468

488

498

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492

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498

358

500

464

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468

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458

482

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478

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466

476

486

462

E

E

E

E

E

E

E

E

E

E

E

E

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£

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0 4 8 Miles

40

§ ̈¦40

93

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Base from U.S. Geological Survey

digital data, 1:100,000. 1982,

Universal Transverse Mercator

projection, Zone 11, Datum NAD27,

National Vertical Geodetic Datum of

1929.

644

644

644

448.7

448.7

448.7

Parker

Dam

Lake Mohave

Lake

Havasu

Co ora l do

River

N e v a d a

C a l i f o r n i a

A

r

i

z

o

n

a

Davis

Dam

Explanation

Area of river aquifer along river reaches—

contours define accounting surface, in feet.

Area of river aquifer around reservoirs—

number is the elevation of the accounting

surface, in feet.

# Reclamation streamflow-gaging station

# USGS streamflow-gaging station

Accounting-surface contour—Shows

equal elevation of accounting surface.

Interval is 2 feet.

E River mile

Flood plain

315

458

M o h

a

v

e

V

a l l

e

y

114°30' W 114°

35°30' N

35°

34°30'
Figure 6. Map showing the accounting surface in Parker, Palo Verde, and Cibola Valleys and adjacent tributary areas in Arizona and 

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Blythe

0 5 

Base from U.S. Geological Surveydigital data, 1:100,000. 1982,

Universal Transverse Mercatorprojection, Zone 11, Datum NAD27,

National Vertical Geodetic Datum of

1929.

between 238 ft

and 240 ft

between 212 ft

and 214 ft

Parker

Valley

Palo Verde Valley

Headgate Rock Dam

Palo Verde Dam

Colorado River

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240

306

244

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352

344

246

308

228

312

218

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304

314

242

324

216

302

258

340

230

322

316

320

278

290

214

292

232

260 288

262

210

286

280

244

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342

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330

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348

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332

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240

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240

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220

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344

246

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228

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348

234

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242

324

216

302

258

340

230

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316

320

278

290

214

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232

260

288

262

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286

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342

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330

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348

258

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222

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358

95

90

85

80

185

175

170

165

160

155

150

145

140

135

130

125

120

115

110

105

100

Area of river aquifer along rive

csurface, in feet.

ontours and number define ac

Explanation

E River mile

Reclamation streamflow-gagin

# USGS streamflow-gaging stati

Accounting-surface contour – equal elevation of accounting s

Interval is 2 feet.

!( Location of drainage ditch wate

elevation measurement

! Location of river water-surface 

measurement

Drainage ditchFlood plain

125

#

#

#

#

#

#

#
Summary 15

Figure 7. Map showing the accounting surface in the Yuma area upstream and downstream from Laguna Dam and adjacent 

tributary areas in Arizona and California.

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E

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E

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E

115° W 114°30'

33° N

32°30'

Base from U.S. Geological Survey

digital data, 1:100,000. 1982,

Universal Transverse Mercator

projection, Zone 11, Datum NAD27,

National Vertical Geodetic Datum of

1929.

0 5 10 15 Miles

Arizona

Mexico

Mexico

California

Northerly international

boundary

Southerly international

boundary

between 168 ft

and 166 ft

G i l a R i v e r

Imperial

Dam

Laguna Dam

Morelos

Dam

Explanation

E River Mile

Location of river water-surface elevation

measurement

IBWC streamflow-gaging stations

# USGS streamflow-gaging stations

Accounting surface contour

Flood plain

Area of river aquifer along river reaches—

c

surface, in feet

ontours and numbers define accounting

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16 Update of the Accounting Surface Along the Lower Colorado River

References Cited

Bureau of Reclamation, 2001, River miles chart: Boulder Canyon Operations Office, Lower Colorado Region, Bureau of 

Reclamation [http://www.usbr.gov/lc/region/pao/rvrmiles.

pdf].

Dickinson, J.E., Land, Michael, Faunt, C.C., Leake, S.A., 

Reichard, E.G., Fleming, J.B., and Pool, D.R., 2006, 

Hydrogeologic framework refinement, ground-water flow 

and storage, water-chemistry analyses, and water-budget 

components of the Yuma area, southwestern Arizona and 

southeastern California: U.S. Geological Survey Scientific 

Investigations Report 2006-5135, 88 p. [http://pubs.usgs.

gov/sir/2006/5135/pdf/sir20065135.pdf].

Harbaugh, A.W., Banta, E.R., Hill, M.C., and McDonald, 

M.G., 2000, MODFLOW-2000, the U.S. Geological Survey 

modular ground-water model—User guide to modularization concepts and the ground-water flow process: U.S. 

Geological Survey Open-File Report 00–0092, 121 p. 

 Leake, S.A., and Claar, D.V., 1999, Procedures and computer 

programs for telescopic mesh refinement using MODFLOW: U.S. Geological Survey Open-File Report 99-238, 

53 p.

Metzger, D.G., 1965, A Miocene(?) aquifer in the ParkerBlythe-Cibola area Arizona and California: U.S. Geological 

Survey Professional Paper 525–C, p. C203–C205.

Metzger, D.G.,1968, The Bouse Formation (Pliocene) of the 

Parker-Blythe-Cibola area, Arizona and California: U.S. 

Geological Survey Professional Paper 600–D, p. D126–

D136.

Metzger, D.G., and Loeltz, O.J., 1973, Geohydrology of the 

Needles area, Arizona, California, and Nevada: U.S. Geological Survey Professional Paper 486–J, 54 p.

Metzger, D.G., Loeltz, O.J., and Irelan, Burdge, 1973, Geohydrology of the Parker-Blythe-Cibola area, Arizona and 

California: U.S. Geological Survey Professional Paper 

486–G, 130 p. http://pubs.er.usgs.gov/usgspubs/pp/pp486G, 

accessed March 3, 2008.

Olmsted, F.H., Loeltz, O.J., and Irelan, Burdge, 1973, Geohydrology of the Yuma area, Arizona and California: U.S. 

Geological Survey Professional Paper 486–H, 227 p.

Owen-Joyce, S.J., and Wilson, R.P., 1994, Accounting for 

consumptive use of lower Colorado River water in Arizona, 

California, Nevada, and Utah: U.S. Geological Survey Fact 

Sheet 94–074, 2 p.

Owen-Joyce, S.J., Wilson, R.P., Carpenter, M.C., and Fink, 

J.B., 2000, 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: U.S. 

Geological Survey Water-Resources Investigations Report 

00-4085, 3 plates, 31 p. [http://az.water.usgs.gov/pubs/pdfs/

WRIR00-4085WEB.pdf].

Remondi, B.W., 1985, Global Positioning System carrier 

phase—Description and use: Rockville, Maryland, National 

Oceanic and Atmospheric Administration Technical Memorandum NOS NGS–42, 21 p.

Spangler, L.E., Angeroth, C.E., and Walton, S.J., 2007, 

Application of geographic information system methods 

to identify areas yielding water that will be replaced by 

water from the Colorado River in the Vidal and Chemehuevi areas, California, and the Mohave Mesa area, Arizona: 

U.S. Geological Survey Scientific Investigations Report 

2007–5284, 37 p.

U.S. Congress, 1948, The Hoover Dam documents: U.S. Congress, 80th, 2d session, House Document No. 717, 936 p.

U.S. Supreme Court, 2006, State of Arizona, plaintiff v. State 

of California, et al., defendants: U.S. Supreme Court Consolidated Decree—March 27, 2006, no. 8, original, 34 p.

Wilson, R.P., and Owen-Joyce, S.J., 1994, Method to identify 

wells that yield water that will be replaced by Colorado 

River water in Arizona, California, Nevada, and Utah: U.S. 

Geological Survey Water-Resources Investigations Report 

94–4005, 19 plates, 36 p. [http://pubs.er.usgs.gov/usgspubs/

wri/wri944005].
Produced in the Western Region, Menlo Park, California

Manuscript approved for publication, June 20, 2008

Text edited by James W. Hendley II

Layout by David R. Jones
Wiele and others—Update of the Accounting Surface Along the Lower Colorado River—Scientific Investigations Report 2008–5113

Printed on recycled paper