U.S. Department of the Interior

U.S. Geological Survey

Professional Paper 1817

GROUNDWATER RESOURCES PROGRAM

Groundwater Availability of the 

Columbia Plateau Regional Aquifer System, 

Washington, Oregon, and Idaho

Columbia

River

Snake River

Groundwater Availability of the 

Columbia Plateau Regional Aquifer System, 

Washington, Oregon, and Idaho

By J.J. Vaccaro, S.C. Kahle, D.M. Ely, E.R. Burns, D.T. Snyder, J.V. Haynes, 

T.D. Olsen, W.B. Welch, and D.S. Morgan

GROUNDWATER RESOURCES PROGRAM

Professional Paper 1817

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2015

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Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Vaccaro, J.J., Kahle, S.C., Ely, D.M., Burns, E.R., Snyder, D.T., Haynes, J.V., Olsen, T.D., Welch, W.B., and Morgan, 

D.S., 2015, Groundwater availability of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and 

Idaho: U.S. Geological Survey Professional Paper 1817, 87 p., http://dx.doi.org/10.3133/pp1817.

Library of Congress Cataloging-in-Publication Data

Vaccaro, J.J., author.

Groundwater availability of the Columbia Plateau regional aquifer system, Washington, Oregon, and Idaho / by J.J. 

Vaccaro, S.C. Kahle, D.M. Ely, E.R. Burns, D.T. Snyder, J.V. Haynes, T.D. Olsen, W.B. Welch, and D.S. Morgan.

 pages cm. -- (Professional paper ; 1817)

Includes bibliographical references and index.

ISBN 978-1-4113-3928-6 (pbk.)

1. Groundwater–Northwest, Pacific. 2. Groundwater–Columbia Plateau. 3. Water table–Northwest, Pacific. 4. 

Water table–Columbia Plateau. 5. Aquifers–Northwest, Pacific. 6. Aquifers–Columbia Plateau. I. Title. II. Series: 

U.S. Geological Survey professional paper ; 1817.

 GB1020.V33 2015

 333.91’0409795--dc23

 2015030112

ISSN 1044-9612 (print)

ISSN 2330-7102 (online)
iii

Foreword

Although often overlooked, groundwater is increasingly important to all our lives. Groundwater 

is the Nation’s principal reserve of freshwater. It provides one-half of our drinking water and is 

essential to U.S. food production while facilitating business and industry in promoting economic 

wellbeing. Groundwater also is an important source of water for sustaining the ecosystem 

health of rivers, wetlands, and estuaries throughout the country.

Large-scale development of groundwater resources with accompanying declines in groundwater 

levels and other effects of pumping have led to concerns about the future availability of 

groundwater to meet all our Nation’s needs. The depletion of groundwater to satisfy the 

country’s thirst and the compounding effects of recent droughts emphasize the need for an 

updated status of the Nation’s groundwater resources. Assessments of groundwater resources 

provide the science and information needed by the public and decision makers to evaluate 

water availability and its effects on the water supply, as well as, to manage and use the water 

resources responsibly. Adding to this already complex task of resource assessment is the 

analysis of potential future effects due to climate variability, which can further exacerbate an 

already challenging situation.

The U.S. Geological Survey (USGS) is conducting large-scale multidisciplinary regional studies 

of groundwater availability, such as this study of the Columbia Plateau Regional Aquifer 

System. These regional studies are intended to provide citizens, communities, and natural 

resource managers with (1) improved information and knowledge of the status of the Nation’s 

groundwater resources, (2) how changes in land use, water use, and climate have affected 

those resources, and (3) tools to forecast how these resources may change in the future. Over 

time, the findings from these individual regional groundwater assessments of principal aquifers 

can be scaled up to a national synthesis and scaled down to provide information relevant to 

issues of local concern. This national scale groundwater assessment directly supports the USGS 

National Water Census.

William H. Werkheiser,

Associate Director for Water
iv

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v

Contents

Foreword ........................................................................................................................................................iii

Executive Summary.......................................................................................................................................1

Water Issues..........................................................................................................................................1

Development of the Hydrogeologic Units.........................................................................................2

Development of a Hydrologic Toolbox...............................................................................................2

Groundwater-Flow Model ...................................................................................................................2

Evapotranspiration and Groundwater Recharge ............................................................................3

Water Withdrawals ..............................................................................................................................3

Groundwater Budget and Changes from Predevelopment Conditions .......................................4

Groundwater Availability and Sustainability ....................................................................................4

Introduction.....................................................................................................................................................6

Purpose and Scope ..............................................................................................................................8

Background.....................................................................................................................................................9

Description of Study Area ...........................................................................................................................9

Physical Setting.....................................................................................................................................9

Physiography.......................................................................................................................................10

Climatic Setting ...................................................................................................................................10

Cultural Setting ...................................................................................................................................12

Water Resources and Economic Development.............................................................................14

Geologic Setting..................................................................................................................................15

Sediment Stratigraphy ..............................................................................................................17

Interbed Stratigraphy................................................................................................................18

Basalt Stratigraphy....................................................................................................................18

Hydrogeologic Units....................................................................................................................................18

Definition of Units................................................................................................................................18

Mapping of Hydrogeologic Units Using Geospatial Modeling....................................................22

Overburden Unit.........................................................................................................................22

Saddle Mountains Unit .............................................................................................................22

Mabton Interbed Unit................................................................................................................22

Wanapum Unit............................................................................................................................22

Vantage Interbed Unit...............................................................................................................22

Grande Ronde Unit ....................................................................................................................22

Older Bedrock Unit ....................................................................................................................22

Hydraulic Characteristics...........................................................................................................................25

Hydrologic Toolbox ......................................................................................................................................27

Groundwater-Flow Model ..........................................................................................................................27

Model Attributes .................................................................................................................................27

Model Fit...............................................................................................................................................29

Model Simulations .............................................................................................................................34
vi

Predevelopment Water Budget.................................................................................................................34

Precipitation.........................................................................................................................................34

Snowpack.............................................................................................................................................34

Surface Water.....................................................................................................................................37

Evapotranspiration..............................................................................................................................40

Recharge..............................................................................................................................................40

Aquifer Storage...................................................................................................................................40

Existing-Conditions Water Budget............................................................................................................43

Columbia Plateau Regional Aquifer System...................................................................................43

Surface Water............................................................................................................................45

Evapotranspiration ....................................................................................................................45

Recharge.....................................................................................................................................45

Surface-Water Use and Groundwater Pumpage.................................................................45

Aquifer Storage..........................................................................................................................49

Groundwater Model Domain ............................................................................................................49

Precipitation................................................................................................................................50

Evapotranspiration ....................................................................................................................50

Recharge.....................................................................................................................................50

Surface-Water Use and Groundwater Pumpage.................................................................50

Groundwater...............................................................................................................................51

Changes to the Water Budget between Predevelopment and Existing Conditions .........................54

Surface Water.....................................................................................................................................54

Evapotranspiration..............................................................................................................................54

Recharge..............................................................................................................................................54

Groundwater........................................................................................................................................54

Climate Variability and Climate Change ...................................................................................................58

Future Climate Projections................................................................................................................63

Groundwater Availability and Sustainability ...........................................................................................64

Implications for Groundwater Management..................................................................................69

Monitoring the Hydrologic System ...........................................................................................................71

Surface Water.....................................................................................................................................71

Groundwater........................................................................................................................................71

Use of Water-Level Data...........................................................................................................75

Future Uses of Model and Challenges to Assessing Groundwater Availability................................77

Acknowledgments.......................................................................................................................................77

References Cited..........................................................................................................................................77

Appendix 1. An Overview of Water Law in Washington, Oregon, and Idaho ....................................85

Appendix 2. Sources of Information for “Dry” Season Water-Availability Map...............................87

Contents—Continued
vii

Figures

1. Map showing Columbia Plateau Regional Aquifer System study area and

structural regions, Washington, Oregon, and Idaho ..............................................................7

2. Map showing mean annual precipitation, Columbia Plateau Regional Aquifer

System study area, Washington, Oregon, and Idaho, 1895–2007.......................................11

3. Graph showing mean monthly precipitation at selected weather stations,

Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho,

1951-2010......................................................................................................................................12

4. Map showing generalized land cover and land use in the Columbia Plateau 

Regional Aquifer System study area, Washington, Oregon, and Idaho ............................13

5. Graph showing population of the Columbia Plateau Regional Aquifer System

study area, Washington, Oregon, and Idaho, 1900–2010 .....................................................14

6. Map showing generalized surficial hydrogeologic units, and hydrogeologic

sections in the Columbia Plateau Regional Aquifer System study area,

Washington, Oregon, and Idaho...............................................................................................20

7. Map showing combined thickness of the Wanapum and Vantage Interbed units,

in the Columbia Plateau Regional Aquifer System study area, Washington,

Oregon, and Idaho ......................................................................................................................23

8. Map showing altitude of the top of the Older Bedrock unit, Columbia Plateau

Regional Aquifer System, Washington, Oregon, and Idaho ................................................24

9. Map showing lateral boundary of the groundwater-flow model, Columbia Plateau

Regional Aquifer System, Washington, Oregon, and Idaho ................................................28

10. Map showing locations of the river and drain cells, and the groundwater-flow

model boundary, Columbia Plateau Regional Aquifer System, Washington,

Oregon, and Idaho ......................................................................................................................30

11. Map showing locations of model geologic structures simulated with flow

barriers, Columbia Plateau Regional Aquifer System, Washington, Oregon, and

Idaho .............................................................................................................................................31

12. Graph showing relation between measured and simulated hydraulic heads,

Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho................32

13. Box plot showing difference between measured and simulated water levels,

Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho................33

14. Graphs showing snow water equivalent for April 1 values for 1981–2009, and

daily values for a wet water year, a dry water year, and the 30-year average,

Touchet Snow Telemetry site, Blue Mountains, Washington .............................................36

15. Graphs showing monthly percentage of average annual discharge accumulated

over a water year, and average monthly discharge for five rivers in different

climatic regimes of the Columbia Plateau Regional Aquifer System study area,

Washington, Oregon, and Idaho, period of record 1911–2011 ............................................38

16. Hydrograph showing monthly average discharge for the Columbia River at The

Dalles, Washington, for selected years during 1879–1997 ..................................................39

17. Map showing distribution of average annual evapotranspiration estimated for

predevelopment conditions in the Columbia Plateau Regional Aquifer System

study area, Washington, Oregon, and Idaho, 1985–2007 .....................................................41
viii

18. Map showing distribution of average annual recharge from infiltration of

precipitation in the Columbia Plateau Regional Aquifer System study area,

Washington, Oregon, and Idaho, 1985–2007 ..........................................................................42

19. Graph showing annual average values estimated for selected water-budget

components, Columbia Plateau Regional Aquifer System, Washington, Oregon,

and Idaho, 1985–2007 .................................................................................................................43

20. Graphs showing annual average discharge for the Columbia and Snake Rivers,

and selected rivers in the study area, Columbia Plateau Regional Aquifer System,

Washington, Oregon, and Idaho, 1985–2007 ..........................................................................44

21. Map showing distribution of average annual evapotranspiration estimated for

existing conditions, Columbia Plateau Regional Aquifer System, Washington,

Oregon, and Idaho, 1985–2007..................................................................................................46

22. Map showing distribution of annual recharge estimated for existing conditions,

Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho, 2007......47

23. Graphs showing estimated annual groundwater pumpage for irrigation; and

public, domestic, industrial, and other water supplies, Columbia Plateau Regional

Aquifer System, Washington, Oregon, and Idaho, 1985–2007.............................................48

24. Graph showing average annual values for selected water-budget components,

Columbia Plateau Regional Aquifer System groundwater-model domain,

Washington, Oregon, and Idaho, 1985–2007 ..........................................................................49

25. Graph showing estimated surface-water and groundwater irrigation, Columbia

Plateau Regional Aquifer System, Washington, Oregon, and Idaho, 1985–2007 .............51

26. Graph showing estimated annual groundwater pumpage for public, domestic,

industrial and other water supplies in the groundwater-model domain, Columbia

Plateau Regional Aquifer System, Washington, Oregon, and Idaho, 1985–2007 .............51

27. Graph showing simulated annual water-budget fluxes for the groundwater-model

domain, Columbia Plateau Regional Aquifer System, Washington, Oregon, and

Idaho, 1920–2007.........................................................................................................................52

28. Schematic diagram showing simulated water budget for existing conditions,

groundwater-model domain, Columbia Plateau Regional Aquifer System,

Washington, Oregon, and Idaho, 1985–2007 ..........................................................................53

29. Map showing change in simulated surface-water fluxes from predevelopment to

model-base conditions, Columbia Plateau Regional Aquifer System Washington,

Oregon, and Idaho ......................................................................................................................55

30. Schematic diagram showing simulated water budget change from

predevelopment to existing conditions, groundwater-model domain, Columbia

Plateau Regional Aquifer System, Washington, Oregon, and Idaho .................................56

31. Map showing simulated changes in groundwater levels in the Wanapum unit

from predevelopment to model-base conditions, groundwater-model domain,

Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho................57

32. Graph showing calculated annual values of selected water-budget components

for predevelopment and existing conditions, lower Naches River Basin,

Washington, water years 1950–2003.......................................................................................59

33. Hydrograph showing water levels for the Davenport monitoring well, Lincoln

County, Washington, 1971–2009 ...............................................................................................59

34. Graph showing annual precipitation and cumulative departure from average

annual precipitation for the Davenport weather station, National Weather Service

cooperative station number 45-2007, Lincoln County, Washington, 1971–2009 ...............60

Figures—Continued
ix

35. Graphs showing cumulative departure of annual streamflow from average annual

streamflow for the Columbia River at The Dalles, Washington, 1879–2009, and

1920–2009 .....................................................................................................................................61

36. Graph showing storage control dates for reservoirs in the Yakima River Basin,

Washington, 1926–2010..............................................................................................................62

37. Graph showing cumulative departure from 1926–2010 average of standardized

values of the storage control date; streamflow at the Yakima River at Parker,

Washington; the Pacific Decadal Oscillation; and the Southern Oscillation Index,

1926–2010 .....................................................................................................................................62

38. Map showing existing “dry”-season water-availability constraints for parts of

Washington, Oregon, and Idaho...............................................................................................65

39. Map showing simulated long-term water-level declines in the Wanapum unit

given average 2000–2007 recharge, and their relation to existing water-availability

constraints, Columbia Plateau Regional Aquifer System, Washington, Oregon, and

Idaho .............................................................................................................................................68

40. Map showing distribution of points of withdrawal for groundwater and

surface-water rights and claims in Washington and Oregon in or near the

Columbia Plateau Regional Aquifer System study area, Washington, Oregon, and

Idaho .............................................................................................................................................70

41. Map showing location of U.S. Geological Survey streamflow measurement sites,

Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho................72

42. Graphs showing number of water levels in the National Water Inventory System

by year, and by decade, Columbia Plateau Regional Aquifer System, Washington,

Oregon, and Idaho ......................................................................................................................73

43. Map showing distribution of wells with water levels in the National Water

Inventory System, Columbia Plateau Regional Aquifer System, Washington,

Oregon, and Idaho ......................................................................................................................74

44. Graphs showing water levels for selected wells in the Toppenish Creek Basin,

Cold Creek drainage, Umatilla Basin, and Toppenish Basin, Columbia Plateau

Regional Aquifer System, Washington, Oregon, and Idaho ................................................75

Figures—Continued

Tables

1. Names, descriptions, and ages of simplified geologic units in the Columbia

Plateau Regional Aquifer System, Washington, Oregon, and Idaho .................................16

2. Correlation chart showing relation between stratigraphy and generalized

hydrogeologic units, Columbia Plateau Regional Aquifer System, Washington,

Oregon, and Idaho ......................................................................................................................19

3. Hydraulic characteristics of hydrogeologic units, derived from regional

groundwater-flow models, of the Columbia Plateau Regional Aquifer System,

Washington, Oregon, and Idaho...............................................................................................26

4. Subdivision of hydrogeologic units by model layers, Columbia Plateau Regional

Aquifer System, Washington, Oregon, and Idaho.................................................................29

5. Generalized average annual water-budget components for predevelopment and

existing conditions, Columbia Plateau Regional Aquifer System, Washington,

Oregon, and Idaho, for selected years, 1895–2007 ...............................................................35
x

Conversion Factors

Inch/Pound to International System of Units

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 4,047 square meter (m2

)

square mile (mi2

) 2.590 square kilometer (km2) 

Volume

cubic mile (mi3

) 4.168 cubic kilometer (km3) 

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

)

Flow rate

acre-foot per year (acre-ft/yr) 1,233 cubic meter per year (m3

/yr)

cubic foot per second (ft3

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

gallon per day (gal/d) 0.003785 cubic meter per day (m3

/d)

inch per year (in/yr) 25.4 millimeter per year (mm/yr)

Hydraulic conductivity

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

International System of Units to Inch/Pound

Multiply By To obtain

Length

kilometer (km) 0.6214 mile (mi)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as 

°F = (1.8 × °C) + 32.

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as 

°C = (°F – 32) / 1.8.

Datums

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

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

Abbreviations

CBP Columbia Basin Project 

CP-RASA Columbia Plateau Regional Aquifer-System Analysis study

CRBG Columbia River Basalt Group 

CPRAS Columbia Plateau Regional Aquifer System

DPM deep-percolation model

ESA Endangered Species Act

ET evapotranspiration

FDR F.D. Roosevelt Lake

GCM Global Climate Model

IPCC Intergovernmental Panel on Climate Change

MAF million acre-feet

MODIS Moderate Resolution Spectroradiometer

NWIS National Water Information System

PDO Pacific Decadal Oscillation

PWS public water supply

SCD Storage Control Date

SNOTEL Snow Telemetry

SSEB Simplified Surface Energy Balance

SOWAT spatially distributed soil-water balance model

SWE snow water equivalent

3D three-dimensional

USGS U.S. Geological Survey

WRIA Water Resource Inventory Area

YRBAS Yakima River Basin Aquifer System

Executive Summary

The Columbia Plateau Regional Aquifer System 

(CPRAS) covers about 44,000 square miles of southeastern 

Washington, northeastern Oregon, and western Idaho. The 

area supports a $6-billion per year agricultural industry, 

leading the Nation in production of apples, hops, and eight 

other commodities. Groundwater pumpage and surface-water 

diversions supply water to croplands that account for about 

5 percent of the Nation’s irrigated lands. Groundwater also 

is the primary source of drinking water for the more than 

1.3 million people in the study area. Increasing competitive 

demands for water for municipal, fisheries/ecosystems, 

agricultural, domestic, hydropower, and recreational uses 

must be met by additional groundwater withdrawals and 

(or) by changes in the way water resources are allocated and 

used throughout the hydrologic system. As of 2014, most 

surface-water resources in the study area were either over 

allocated or fully appropriated, especially during the dry 

summer season. In response to continued competition for 

water, numerous water-management activities and concerns 

have gained prominence: water conservation, conjunctive 

use, artificial recharge, hydrologic implications of land-use 

change, pumpage effects on streamflow, and effects of climate 

variability and change. An integrated understanding of the 

hydrologic system is important in order to implement effective 

water-resource management strategies that address these 

concerns.

To provide information to stakeholders involved in watermanagement activities, the U.S. Geological Survey (USGS) 

Groundwater Resources Program assessed the groundwater 

availability as part of a national study of regional systems 

(U.S. Geological Survey, 2008). The CPRAS assessment 

includes: 

1. The present status of groundwater resources, 

2. How these resources have changed over time, and

3. Development and application of tools to estimate system 

responses to stresses from future uses and climate 

variability and change.

This effort builds on previous investigations, especially the 

USGS Columbia Plateau Regional Aquifer-System Analysis 

study (CP-RASA). A major product of this new assessment 

is a numerical groundwater-flow model of the system. The 

model was used to estimate water-budget components of 

the hydrogeologic units composing the groundwater system, 

and to evaluate groundwater availability under existing 

land- and water-use conditions and a possible future climate 

scenario representing an increase in pumpage demand due to a 

warming climate. Information from this study also allowed for 

analysis of:

1. The CPRAS for predevelopment times (pre-1920),

2. Variations from 1920 through 2007,

3. Conditions during 1985–2007 (referred to as “existing 

conditions”), and

4. Changes in the system from predevelopment times.

The model also is a useful tool for investigating water supply, 

water demand, management strategies, groundwater-surface 

water exchanges, and potential effects of changing climate on 

the hydrologic system. 

Water Issues

Groundwater availability is critical to managing water 

resources in the CPRAS because of the high water demand 

for agriculture, economic development, and ecological needs 

and the great competition for the limited resource. Waterresource issues that have implications for future groundwater 

availability include:

1. Widespread water-level declines associated with 

development of groundwater resources for irrigation and 

other uses;

2. Reduction in base flow to rivers and associated effects 

on water temperature and quality;

3. Limited availability of non-appropriated surface water;

Groundwater Availability of the Columbia Plateau 

Regional Aquifer System, Washington, Oregon, and Idaho

By J.J. Vaccaro, S.C. Kahle, D.M. Ely, E.R. Burns, D.T. Snyder, J.V. Haynes, T.D. Olsen, W.B. Welch, and 

D.S. Morgan
2 Groundwater Availability of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

4. Potential capture of surface water appropriated through 

senior water rights by pumpage of groundwater 

appropriated through junior water rights; and

5. Current (2014) and projected effects of climate change 

and variability on increasing pumping demand, 

groundwater recharge, base flow in rivers, and 

ultimately, sustainable groundwater yields. 

Ongoing activities in the region for enhancement of fisheries 

and obtaining additional water for agricultural, municipal, and 

domestic use may be affected by groundwater withdrawals 

and rules implemented under the Endangered Species Act for 

numerous stocks of salmonids.

The study addresses some of these groundwater 

availability issues by improving the understanding of 

the hydrogeologic system, the status and trends of the 

groundwater system, the general relation between groundwater 

and surface waters, current water use, and the water budget for 

the CPRAS for both predevelopment and existing conditions.

Development of the Hydrogeologic Units

Hydrogeologic units were described for the CPRAS by 

Kahle and others (2009) on the basis of the CP-RASA; they 

include the Overburden, Saddle Mountains, Mabton Interbed, 

Wanapum, Vantage Interbed, Grande Ronde, and Older 

Bedrock units. A three-dimensional (3D) geologic model was 

then constructed to refine the geometry of these units (Burns 

and others, 2011). The process yielded (1) maps that represent 

upper and lower buried unit boundaries in this complex 

terrain, (2) improved estimates of unit volumes, (3) refined 

locations of large structural features, and (4) features that 

may be important for ongoing groundwater studies. The range 

in extents, thicknesses, altitudes, and volumes of the units 

indicate the extraordinary complexity of this aquifer system.

The sedimentary Overburden unit mainly occurs in 

structural basins and is an important water-bearing unit. The 

3D-model derived a thickness of the Overburden unit ranging 

from 0 to about 2,000 feet and a volume of about 244 cubic 

miles. The Saddle Mountains unit, a Columbia River Basalt 

Group (CRBG) unit, covers about 11,700 square miles and 

its thickness ranges from 0 to about 2,000 feet. The volume 

of this basalt unit was calculated at 665 cubic miles. The 

thickness of the Mabton Interbed unit, which underlies the 

Saddle Mountains unit, ranges from 0 to about 205 feet, and 

averages 39 feet. The Wanapum unit (CRBG) is the surficial 

unit covering more than 30 percent of the area and extending 

over about 24,400 square miles. The unit is an important 

source of water on the Columbia Plateau and its thickness 

ranges from 0 to about 3,250 feet. Its calculated volume is 

2,160 cubic miles. The thickness of the Vantage Interbed unit, 

which underlies the Wanapum unit, ranges from 0 to about 

80 feet. The Grande Ronde unit (CRBG) is the most extensive 

unit of the CPRAS—it underlies 96 percent of the study area, 

extends over about 41,900 square miles, its thickness ranges 

from 0 to about 16,000 feet (average of about 3,950 feet), and 

its calculated volume is 31,300 cubic miles. The altitude of 

the top of the Grande Ronde unit ranges from about -2,200 to 

7,900 feet, or a total difference of about 10,000 feet. 

The Older Bedrock unit that borders and underlies the 

CPRAS is composed of various rock types older than the 

CRBG. The Older Bedrock unit is considered the base of the 

regional flow system. The Older Bedrock unit is the surficial 

unit covering only small parts of the study area. A map of the 

altitude of the top of this unit was constructed to estimate the 

thickness of the immense Grande Ronde unit.

Development of a Hydrologic Toolbox

As part of a hydrologic toolbox, new applications 

were developed for estimating evapotranspiration (ET), 

surface-water use, groundwater pumpage, and recharge from 

irrigation. The applications can be used over large regional 

areas and were summarized in Kahle and others (2011). ET 

is a large component of the water budget; it can account for 

100 percent of the annual precipitation in the arid areas and 

45–70 percent in the more humid uplands, and historically, has 

not been estimated. A new Simplified Surface Energy Balance 

(SSEB) method that uses satellite data was developed to 

estimate monthly ET. The spatial distribution of landscape ET 

estimated using SSEB provides an estimate of consumptive 

use in irrigated areas. The SSEB has allowed for a new 

understanding of the interrelationship between hydrologic 

components. A spatially distributed soil-water balance model 

(SOWAT) was developed to use relations among climatic, 

soils, land-cover, and irrigation data to compute monthly 

irrigation requirements and surplus moisture available for 

recharge in irrigated areas. Estimates of groundwater pumpage 

and surface-water diversions for irrigation and recharge 

associated with irrigation were then calculated with ET from 

the SSEB driving the calculations. Estimates of monthly 

recharge from infiltration of precipitation were made using 

regression equations. This combination of methods was used 

because a major control on groundwater availability (and 

streamflow) is the quantity and timing of water entering 

the aquifer system—groundwater recharge. The quantity 

and timing of recharge also influences how the flow system 

functions and responds to pumpage. 

A web interface tool that allows users to explore the 3D 

geologic framework also was developed for the project. This 

tool allows one to view the subsurface geology by drawing 

diagrams of "well logs" at any site, or building complete 

geologic cross sections between multiple sites.

Groundwater-Flow Model

A numerical model that simulates groundwater flow 

was constructed and documented as part of this study (Ely 

and others, 2014). Compared to previous models, the new 

model includes a finer vertical spatial discretization, uses a 
Executive Summary 3

more detailed depiction of the subsurface hydrogeology, and 

simulates annual water budgets for 1920–2007. The simulation 

results were used to help assess groundwater availability and 

sustainability.

The model includes 74 percent of the study area. Its 

boundaries are nearly identical to the previous CP-RASA 

model constructed in the 1980s, thus allowing for a consistent 

assessment of the response of the system to stresses over time. 

The model domain was represented by a grid of 3-kilometer 

(9,842.5-foot) cells. Except for the thick Grande Ronde unit, 

each of the hydrogeologic units was approximately subdivided 

into 100-foot model layers, yielding 100 model layers. The 

detailed vertical discretization improved the understanding 

of the important vertical flow component within the CPRAS. 

The no-flow lower boundary is the top of the Older Bedrock 

unit. Internal boundary conditions simulate flow to and from 

the major rivers, and groundwater discharge to small upland 

streams, groundwater-controlled surface-water features such 

as lake complexes, and drains in agricultural areas. Major 

geologic structures, which are an important control on the 

flow system and locally limit groundwater availability, were 

simulated with what are called flow barriers for 38,308 model 

cells.

The model was calibrated to conditions representing the 

flow system during two periods—estimates of predevelopment 

conditions (1920) and annual conditions during 1920–2007. 

The differences between measured and simulated water levels 

were small, indicating the model can represent the complex 

hydrologic system, both laterally and vertically.

The model was used to estimate annual water-budgets 

for the system, flow between hydrogeologic units, changes in 

the system from predevelopment conditions, changes in the 

system if the conditions in 2007 existed through 2050, and 

potential effects in 2050 of increased irrigation demand owing 

to climate warming. The model results are used to show the 

potential usefulness of this tool in providing information for 

short- to long-term management planning and decisions by 

numerous stakeholders.

Evapotranspiration and Groundwater Recharge

The estimate of average annual ET for predevelopment 

conditions (native vegetation with no irrigation) was about 

24 million acre-feet (MAF), and spatially ranged from about 

1 to 38 inches. As a percentage of precipitation, ET ranged 

from a low of 10 percent to a high of nearly 100 percent, 

and averaged 59 percent. The estimate of average annual 

ET for existing conditions was 28 MAF (an increase from 

predevelopment conditions of about 3.5 MAF—a quantity 

of water that could irrigate more than 1 million acres). The 

complex spatial distribution of this important water-budget 

component differs from the predevelopment distribution in 

areas affected by human activities, particularly irrigation. 

The differences are especially pronounced in the surfacewater irrigated areas such as in the Yakima River Basin and 

Columbia Basin Project (CBP), where increases in ET ranged 

from about 20 to 36 inches due to consumptive water use by 

irrigated crops.

Estimated predevelopment groundwater recharge (same 

conditions as for ET) averaged 11 MAF and annually ranged 

from 9.6 to 25.6 MAF. The large spatial-temporal variability 

of this important water-budget component indicates not only 

the precipitation distribution but also the ET distribution. 

Average annual recharge for existing conditions was 14 MAF 

(an increase of 3.6 MAF from predevelopment conditions) 

and locally ranged from nearly zero to more than 40 inches. 

Low recharge values throughout much of the central Columbia 

Plateau indicate a potential limitation on regional groundwater 

availability. Total recharge for existing conditions includes 

additional recharge from surface-water and groundwater 

irrigation that was calculated using the SOWAT model. 

Estimated average annual recharge in the study area from the 

delivery and use of irrigation water was 4.2 MAF, with 50 

percent occurring in the predominately surface-water irrigated 

regions—the Yakima and Umatilla River Basins, and the CBP. 

Water Withdrawals

Surface-water use and groundwater pumpage were 

estimated on an annual basis for 1985–2007 for five categories 

of use:

1. Irrigation;

2. Public water supply (PWS);

3. Domestic (also referred to as exempt wells in the Western 

United States, as these wells are exempt from State 

permitting processes); 

4. Industrial; and

5. “Other,” which includes mining, livestock, and fisheries 

(Kahle and others, 2011).

Irrigation water use was estimated using SOWAT for both 

surface water and groundwater. For the groundwater model, 

water use for 1920–1984 was estimated based on previously 

published data (in particular from the CP-RASA study), 

increase in the number wells over time on the Columbia 

Plateau, and the start of surface-water irrigation by the 

irrigation districts. Only withdrawals under existing conditions 

are summarized here.

Irrigation water use averaged 5.3 MAF during 1985– 2007, 

with 1.4 MAF (26 percent) supplied from groundwater and 

3.9 MAF supplied from surface water. Surface-water use was 

largest in the CBP, followed by the Yakima River Basin and 

then the Umatilla River Basin. Annually, irrigation water use 

for the entire study area ranged from about 4.1 to 7.1 MAF, and 

the amount supplied by groundwater ranged from about 25 to 

35 percent of the total. Thus, if the increases in both ET and 

recharge are considered, more than 12 MAF of water fluxes are 

associated with irrigation.
4 Groundwater Availability of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

Excluding irrigation pumpage, annual pumpage averaged 

0.4 MAF during 1985–2007, and including irrigation pumpage, 

averaged 1.8 MAF. For the pumpage categories, annual PWS 

averaged 0.23 MAF, and increased from 0.2 MAF in 1985 to 

0.27 MAF in 2007. Domestic self-supplied pumpage increased 

from 0.06 MAF in 1985 to 0.07 MAF in 2007 (average annual 

pumpage of 0.06 MAF). Industrial annual pumpage averaged 

0.05 MAF, and decreased from 0.05 to 0.04 MAF during 

1985–2007. Other annual pumpage averaged 0.03 MAF, and 

increased from 0.02 to 0.04 MAF over the 23-year period.

Groundwater Budget and Changes from 

Predevelopment Conditions

Recharge is the primary water-budget component of 

the groundwater system, closely followed by discharge to 

streams; this relation represents typical Pacific Northwest 

hydrology, with much of the recharge discharging along shortto-intermediate flow paths to surface-water features. Storage 

changes are much smaller than recharge and, as with discharge 

to streams, annual variations in storage changes closely follow 

recharge variations. Pumpage is about 25 percent as large 

as recharge but, in major pumping centers, it is the largest 

water-budget component. The water budget for the individual 

units follows the same general trend as the total water budget, 

with recharge and discharge to streams dominating. Excluding 

recharge and discharge to streams, the water budget for the 

CRBG units indicates that pumpage is the largest component, 

highlighting the importance of pumpage and its historical, 

current (2014), and potential future effects on the system. 

Downward flow from overlying to underlying units is the next 

largest component and is nearly three times larger than upward 

flow. Changes in storage fluxes within the units are about as 

large as the upward fluxes between units. 

Human activities have substantially altered the 

predevelopment hydrologic system. The construction of 

hundreds of small-to-large dams and diversion structures 

for supplying irrigation water and hydropower, coupled 

with thousands of instream diversions, has resulted in an 

intensively modified streamflow system throughout the study 

area. Streamflow characteristics (from hourly to annual) 

for most streams have changed from the natural streamflow 

regimen. These changes have affected the aquatic ecosystem 

and have cascaded through the groundwater system. Indeed, 

diversions and the need for instream flows have limited the 

future availability of groundwater withdrawals in some areas. 

The delivery and application of surface water for irrigation 

of more than 1.2 million acres has resulted in groundwaterlevel rises occurring over about 2,400 square miles (with local 

rises of more than 200 feet) and a net increase in groundwater 

storage in these areas of more than 11 MAF. The area of rises 

encompasses about 5 percent of the study area. This increase 

in storage is the result of the increase in recharge that primarily 

occurs in the surface-water irrigated areas. Many of these rises 

have occurred in the Overburden unit where irrigation occurs, 

but rises also have occurred in the underlying basalt units. For 

example, water-level rises in the Wanapum unit were more than 

200 feet in part of the Quincy Basin, and the overlying Saddle 

Mountains unit (the largest basalt-unit benefactor of the excess 

irrigation water) has increased importance as a transmitter 

and supplier of water because of the increase in the area and 

volume of its saturated material. The return of excess irrigation 

water through surface-water drains and groundwater discharge 

also has contributed to the alteration of the streamflow 

characteristics and availability of water in the area. As a result, 

surface-water demands in some locations are met by these 

return flows; that is, the return flows are relied upon to meet 

downstream uses.

The groundwater system has been substantially affected 

by the extraction of groundwater. Groundwater levels have 

declined over more than 10,000 square miles (about 23 percent 

of the study area) because of historical and current pumpage. 

Declines have exceeded 300 feet in parts of Washington 

and 200 feet in Oregon. The declines due to pumpage have 

decreased groundwater storage by more than 10 MAF. As 

documented in this study, these declines are continuing to 

occur in the areas with large irrigation withdrawals and have 

expanded to new areas since the CP-RASA study. Concurrently, 

groundwater quantities moving between units have been altered 

because of pumpage and surface-water irrigation, resulting 

in total increased inflow to units from other units exceeding 

1,500 cubic feet per second. However, net outflows (discharge 

to surface-water features, pumpage, and storage changes) for 

all but the Overburden unit have exceeded these increased 

inflows since predevelopment times (partly because declining 

groundwater levels in some areas have resulted in streams 

losing water).

Groundwater Availability and Sustainability

Groundwater availability is a function of not only the 

physical processes that govern the quantity and quality of water, 

but the laws, rules, regulations, and socioeconomic factors 

that control its demand and uses (Reilly and others, 2008). 

Groundwater availability and, ultimately, its sustainability in 

the study area, are issues that are interrelated to groundwater 

management and the availability and use of surface water. 

Water management, in turn, generally is intricately related to 

environmental protection and public health; thus, groundwater 

availability is related to conflicts between consumptive use 

and instream uses. Important institutional factors (such as 

use restrictions, basin adjudication, and surface-water rights/

instream flows) limit the availability of water because storage 

changes due to groundwater pumpage can affect streamflow. 

Aquifer-system hydraulic characteristics, well yields, the cost 

of drilling/deepening wells, the cost of energy for pumping 

water, and the design of the well and pump also can limit the 

availability of groundwater, and these hydrologic limitations are 

amplified by the institutional factors.
Executive Summary 5

An overarching control is the laws, rules, and regulations 

codified in State law that ultimately affect water availability 

and, thus, sustainability. Entering into this mix are a mosaic of 

rulings from court cases and Federal government intervention 

in State water policy in response to a State request or when 

State law may encroach on the U.S. Constitution. There is 

limited surface-water and groundwater availability during 

the “dry” season (when water demand is greatest) in or near 

large parts of the study area. Limited surface water indicates 

that groundwater necessarily would need to be part of the 

future water-availability mix. However, most of the senior 

(and junior) surface-water rights are senior to groundwater 

rights and, thus, “dry” season groundwater availability may be 

further limited in the future. 

Water availability and sustainability were addressed by 

(1) estimating the water in storage in the system, fluxes within 

and through the system, uses of water, and changes due to 

human activities; and (2) describing the legal framework. 

For example, the comparison of the predevelopment system 

to existing conditions defined the overall effect of human 

activities and their relation to groundwater availability. 

Availability was further addressed by analysis of the 

groundwater system changes simulated by the CPRAS 

model for long-term (2050) conditions under (1) existing 

water-management strategies, and (2) a set of conditions 

estimated to occur with a changing climate that represents 

average increased groundwater irrigation demand due to 

climate warming. Increased surface-water demand was not 

addressed, but that increase would have large repercussions on 

groundwater availability.

Although pumpage is only about 10 percent of recharge, 

depletion of aquifer-system storage by pumpage has been 

substantial in parts of the study area (more than 10 MAF for 

the Wanapum unit alone). This depletion has occurred because 

much of the pumpage occurs in areas with minimal recharge. 

Water-level records and previous studies confirm that large 

amounts of water were removed from storage in the basalt 

units prior to 1985 (Vaccaro, 1999). Measured water levels 

from this study verified continual declines in the basalt units 

over large areas (Snyder and Haynes, 2010; Burns and others, 

2011), and groundwater levels have declined significantly 

in some areas (Snyder and Haynes, 2010). Between 1985 

and 2007, CPRAS simulations indicate that the total storage 

(associated with recharge variations and groundwater-level 

declines) has been depleted by an additional 7.7 MAF (Ely 

and others, 2014).

Model simulation results show even larger areas of 

water-level declines by 2050. Declines from 2007 to 2050 

in the Wanapum unit extend over more than 20,000 square 

miles, with a concomitant large loss in storage. In these areas 

of water-level declines, groundwater availability may become 

limited hydrologically or by regulations, or may be already 

limited. The water-level declines may result in changes in 

surface-water quantity and quality, and (or) a decrease in 

the economic viability of groundwater irrigation because 

of increased energy costs with the increased pumping lifts. 

Water-level declines, in turn, can factor into environmental 

issues by changing and (or) degrading habitats if groundwater 

discharge to surface-water bodies decreases. These related 

effects may constitute the primary constraint to groundwater 

development. For example, under long-term 2050 conditions 

with 2007 pumpage, groundwater discharge to streams in 

the model area was simulated to decrease by more than 600 

cubic feet per second. Thus, during the summer low-flow 

period, some streams may not have enough groundwater input 

to sustain environmental flows, which include codified or 

“target” instream flows. 

In the future, population growth and irrigation demand 

may increase conflicts over water (Washington State 

Department of Ecology, 2011). Factoring in projected climate 

change, without increased storage, the projected changing 

surface-water regime with earlier runoff because of less 

snowpack will lead to larger water demands, especially 

during the summer low-flow season (Climate Impacts Group, 

2009; Washington State Department of Ecology, 2011). If 

the amount of surface water available during the dry season 

decreases, the amount and timing of groundwater pumpage 

ultimately may be affected. This effect would be in addition 

to increased crop-water demand. That is, the crop water 

demand can be expected to increase in the future and add 

more competitive demands on the available water supply. The 

large quantity of irrigated lands in the study area dedicated to 

perennial crops (such as apples, cherries, asparagus, grapes, 

hops, timothy hay, and alfalfa) makes changing crop types 

difficult, but water limitations may result in a changing crop 

mix. Effects of historical usage indicate that in many areas 

the groundwater system cannot meet competing demands 

indefinitely, and any increased groundwater use under a 

changing climate will increase the competitive demand for 

the limited resource and (or) result in the loss of agricultural 

production. In particular, projected climate effects may 

decrease the number of years in which water demands for 

agricultural uses are met. Assuming these climate projections 

are correct, decreases in water storage likely will continue in 

some areas, perhaps at an accelerated rate.

Since the late 1970s, State and Federal water projects 

have not expanded with growing urbanization and the 

increased agricultural and environmental uses of water on the 

Columbia Plateau. The study area faces ongoing and projected 

increases in competing demands for water resources, few of 

which are likely to be met by improvements in agricultural 

efficiencies. Indeed, except for a few areas, most water 

available from agricultural efficiencies is being directed 

to environmental uses. The demand for water resources 

by people directly competes with environmental uses such 

as maintaining minimum streamflows and preserving fish 

habitat, and this competition is expected to increase. The 

interrelationship between these competing demands and legal 

constraints likely will strengthen.
6 Groundwater Availability of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

Introduction

Groundwater availability in the Columbia Plateau 

Regional Aquifer System (CPRAS), in northeastern Oregon, 

southeastern Washington, and western Idaho (fig. 1), is a 

critical water-resource management issue because of the 

high water demand for agriculture, economic development, 

and ecological needs, creating competing interests for the 

limited resource. These demands generally must be met 

by groundwater withdrawals and (or) by changes in the 

way water resources are allocated and used throughout the 

hydrologic system because surface water in many of the basins 

in the study area is fully appropriated or over-appropriated; 

more water has been legally distributed through surfacewater rights than actually exists in some rivers and streams. 

Ongoing activities in the region for enhancement of fisheries 

and obtaining additional water for agriculture and municipal 

use may be affected by groundwater withdrawals. Listing of 

numerous stocks of salmonids under the Endangered Species 

Act (ESA) has introduced new constraints on the management 

of water resources in the region.

The development of the groundwater resources and 

nearly full development of surface-water resources has set the 

stage for conflict over any remaining water and existing water 

uses. Water-resources issues that have implications for future 

groundwater availability in the region include:

1. Widespread water-level declines associated with 

development of groundwater for irrigation and other 

uses,

2. Reduction in base flow to rivers and associated effects 

on water temperature and quality,

3. Limited availability of non-appropriated surface water 

and constraints on groundwater uses in some areas,

4. Potential capture of surface water appropriated through 

senior water rights by pumpage of groundwater 

appropriated through more junior water rights, and

5. Current (2014) and anticipated effects of climate change 

and variability on groundwater recharge, base flow in 

rivers, and, ultimately, sustainable groundwater yields.

The economic viability of the area and its ecosystems, 

especially as related to the iconic Pacific Northwest salmon, 

depend directly or indirectly on groundwater. Because of 

intensified competition for available water, there was a need 

to quantify water resources of the Columbia Plateau and to 

better understand the spatial-temporal variations in the system 

to help with future water-use considerations. In response, the 

U.S. Geological Survey (USGS) assessed the groundwater 

availability of the CPRAS as part of a National Groundwater 

Resources Program (U.S. Geological Survey, 2008). The 

USGS national assessment of groundwater availability was 

initiated because of the need for improved understanding 

of these regional resources throughout the United States. 

The framework for the national assessments is provided in 

U.S. Geological Survey (2002) and National Science and 

Technology Council (2004). The framework defines a waterresource assessment of status and trends of water in storage, 

flow rates, and water use and withdrawals.

The broad goals of the national assessment were to:

1. Characterize the status of the hydrologic system,

2. Identify trends in groundwater storage and use,

3. Assess the relation of groundwater availability to the 

overall hydrologic system,

4. Estimate components of landscape and groundwaterflow system water-budget, and

5. Quantify groundwater availability. 

To achieve these goals, data were collected and analyzed, 

the hydrogeologic framework was updated, a transferable 

hydrologic toolbox that incorporates new techniques was 

developed as part of the CPRAS study, and a regional 

groundwater-flow model was developed.

The availability and sustainability of groundwater as a 

source of water supply is a function of many factors—both 

natural and human—that control its use. Natural factors 

include the quantity and quality of water, climate, and 

environment. Human factors include laws, regulations, 

and economics (U.S. Geological Survey, 2002). As with 

groundwater availability of the Central Valley Aquifer in 

California that was assessed as part of the USGS National 

Groundwater Resources Program (Faunt and others, 2009), 

water concerns and conflicts can be categorized under three 

general categories:

1. Natural distribution (both spatial and temporal);

2. Technical-hydrologic; and

3. Political, legal, and social. 

Political, legal, and social concerns include fully or overappropriated waters in some basins, streamflows needed 

to support ESA listed species, and potential impairment of 

senior surface-water rights. This study addresses some of 

these problems and conflicts by contributing to the improved 

understanding of the hydrologic system and providing an 

updated description of groundwater availability in the CPRAS. 

To help improve the understanding of the hydrologic system, 

a regional groundwater-flow model was constructed for an 

area referred to as the “model domain.” The model domain 

includes 74 percent of the study area (fig. 1). Nearly all the 

agriculture (irrigated and dryland), population, and human 

activities are within the model domain. 
Introduction 7

Figure 1. Columbia Plateau Regional Aquifer System study area and structural regions, Washington, Oregon, and Idaho (from 

Kahle and others, 2011).

watac11-0648_fig01 Structural regions

Grand Coulee Dam

B L U E M O U N T A I N S

CASCADE RANGE

OKANOGAN HIGHLANDS

CASCADE RANGE

WALLOWA

MOUNTAINS

R O C K Y M O U N T A I N S 

R O C K Y M O U N T A I N S

HORSE HEAVEN HILLS

BEEZLEY HILLS

Hanford 

site

Snohomish

Skagit

King

Pierce

Lewis

Chelan

Okanogan

Kittitas

Yakima

Skamania

Klickitat

Hood

River

Clackamas

Marion

Linn

Ferry

Stevens

Pend

Orielle

Benton

Grant

Lincoln

Spokane

Adams

Franklin

Walla

Walla

Whitman

Columbia

Garfield

Asotin

Sherman

Jefferson

Crook

Gilliam

Morrow

Umatilla

Wheeler

Malheur

Baker

Wallowa

Bonner

Kootenai

Benewah

Latah

Lincoln

Sanders

Clearwater

Nez Perce

Washington Valley

Adams

Idaho

Douglas

Wasco

Grant

Union

Shoshone

Lewis

Snake

Tucannon River

Touchet River

River

Crab Creek

Columbia

River

COL

UMBIA

Naches River

Yakima River

Umatilla River

Columbia

River

RIVER

N orth Fork John Day River

Snake River

Imnaha River

Banks

Lake Coeur

D'Alene

Lake

Grande Ronde River

Salmon River

John D ay River

Moses Coulee

Walla Walla River

Deschutes River

Klickitat River

White Salmon R 

Clearwater

River

Hangman Cr

 F.D Ro osevelt La ke

Hood R

Wenatch e e R.

Palouse River

Satus

T

Cr

oppenish Creek

Spokane

River

Potholes 

Reservoir

Moses

Lake

Rocky Ford Cr.

Ellensburg

Umatilla

Walla Walla

WASHINGTON

OREGON

IDAHO

Yakima

Heppner

MiltonFreewater

Pasco

Lewiston

Moscow

Pullman

Spokane

Odessa

The

Dalles

La Grande

0 20 40 60 80 MILES

0 20 40 60 80 KILOMETERS

122° 120° 118° 116°

46°

48°

47°

45°

Study

area

WASHINGTON

OREGON IDAHO

EXPLANATION

Structural regions of Columbia 

Plateau Regional Aquifer System

Yakima Fold Belt

Palouse Slope

Blue Mountains

Clearwater Embayment

Study area boundary

Groundwater-flow model boundary

Base modified from U.S. Geological Survey digital data, various scales. Coordinate system: 

State Plane, Washington South, FIPS 4602: Projection: Lambert Conformal Conic. Horizontal 

datum: North American Datum 1983, Vertical datum: North American Vertical Datum of 1988.
8 Groundwater Availability of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

Ultimately, groundwater availability and water 

availability generally have been, are, and will be controlled 

by State water law and its association with political and 

social complexities. Water law on the Columbia Plateau, as 

in most of the Western United States, is the doctrine of “prior 

appropriation” (principle of appropriative rights—the first in 

time is the first in right), especially for surface water. A brief 

overview of the water codes for Washington, Oregon, and 

Idaho is provided in appendix 1 because of the importance of 

State water law and its effect on groundwater availability.

Purpose and Scope

This report summarizes a series of reports published as 

part of the groundwater availability analysis of the CPRAS, 

and incorporates, to the extent possible, the nomenclature 

and wording of each published report to provide consistency. 

Figures from these reports also are incorporated when 

possible. Detailed descriptions of these reports and results are 

available in the following previously published reports:

• Ground-water availability assessment for the 

Columbia Plateau Regional Aquifer System, 

Washington, Oregon, and Idaho (U.S. Geological 

Survey, 2008)

• Geologic setting and hydrogeologic units of the 

Columbia Plateau Regional Aquifer System, 

Washington, Oregon, and Idaho (Kahle and others, 

2009)

• Groundwater conditions during 2009 and changes 

in groundwater levels from 1984 to 2009, Columbia 

Plateau Regional Aquifer System, Washington, 

Oregon, and Idaho (Snyder and Haynes, 2010)

• Three-dimensional model of the geologic framework 

for the Columbia Plateau Regional Aquifer System, 

Washington, Oregon, and Idaho (Burns and others, 

2011)

• Groundwater status and trends for the Columbia 

Plateau Regional Aquifer System, Washington, 

Oregon, and Idaho (Burns and others, 2012)

• A coupled remote sensing and simplified surface 

energy balance approach to estimate actual 

evapotranspiration from irrigated fields (Senay and 

others, 2007)

• Global reference evapotranspiration modeling and 

evaluation (Senay and others, 2008) 

• Characterizing landscape evapotranspiration dynamics 

in the Columbia Plateau using remotely sensed data and 

global weather datasets (Senay and others, 2009)

• Hydrogeologic framework and hydrologic budget 

components of the Columbia Plateau Regional Aquifer 

System, Washington, Oregon, and Idaho (Kahle and 

others, 2011) 

• Numerical simulation of the groundwater flow in the 

Columbia Plateau Regional Aquifer System, Idaho, 

Oregon, and Washington (Ely and others, 2014)

This report is one in a series of reports summarizing the 

findings of studies conducted as part of the USGS National 

Groundwater Resources Program assessment of groundwater 

availability throughout the Nation. Additionally, to be 

consistent with the other national summaries, selected section 

headings and wording also have been incorporated from the 

following national summary reports: 

• Groundwater availability of the Central Valley Aquifer, 

California (Faunt, 2009) 

• Groundwater availability in the Atlantic Coastal Plain 

of North and South Carolina (Campbell and Coes, 

2010) 

• Water availability and use pilot—A multiscale 

assessment in the U.S. Great Lakes Basin (Reeves, 

2010) 

• Groundwater availability of the Denver Basin aquifer 

system, Colorado (Paschke, 2011)

• Groundwater availability of the Mississippi embayment 

(Clark and others, 2011)

The report includes a description of the physical setting, 

physiography, climatic setting, cultural setting, and water 

resource and economic development, and the geologic setting 

to provide an understanding of the generalized background for 

the CPRAS area. Each of these items is intricately related to 

water availability. This report includes descriptions of:

• The hydrogeologic units and their characteristics,

• The model of groundwater flow,

• Generalized water budgets for the complete study area,
Description of Study Area 9

• Detailed water budgets for the hydrogeologic units in the 

groundwater-model domain, and

• Changes in the water budget within the groundwatermodel domain due to human activities.

When appropriate, the generalized water budgets for the study 

area are presented for predevelopment conditions and existing 

conditions. Predevelopment conditions represent the hydrologic 

system before appreciable land-uses changes, especially 

agriculture. Existing conditions represent the developed 

system during 1985–2007, as defined by Kahle and others 

(2011). The influence and importance of climate variability 

and projected climate change on the groundwater flow system 

also is presented. The information described in this paragraph 

provides for the assessment and presentation of groundwater 

availability of the CPRAS. The monitoring of the hydrologic 

system is described because of its importance in understanding 

groundwater sustainability and, thus, availability. 

Background

The CPRAS is in the Columbia Plateau in northeastern 

Oregon, southeastern Washington, and northwestern Idaho 

(fig. 1); an overview of the aquifer system is presented in 

Whitehead (1994). The aquifer system covers about 44,000 

mi2

. The area supports a $6 billion per year agricultural 

industry, and leads the Nation in production of apples and nine 

other commodities (State of Washington Office of Financial 

Management, 2007; U.S. Department of Agriculture, 2007a). 

Surface-water diversions and groundwater pumpage supply 

water to the irrigated croplands that primarily occur in the 

arid and semi-arid parts of the study area. These croplands 

account for about 5 percent of the Nation’s irrigated croplands. 

Groundwater is the primary source of drinking water to the more 

than 1.3 million people in the study area. 

To study such a large and important aquifer system, an 

approach was developed that included:

• Updating the regional hydrogeologic framework,

• Documenting changes in the status of the hydrologic 

system,

• Quantifying the predevelopment and existing hydrologic 

budget (including water withdrawals) for the complete 

system, and

• Developing a numerical regional groundwater-flow 

model for part of the system.

The model was used to estimate water-budget components and 

to evaluate groundwater availability under existing land- and 

water-use conditions and a possible future climate scenario 

representing an increase in pumpage demand because of 

changing climate. The model is based on the USGS CP-RASA 

groundwater-flow model developed by Hansen and others 

(1994), and its boundary is nearly identical to the CP-RASA 

model boundary. The new model incorporates information on 

the surface-water and groundwater systems that has become 

available since the original model was developed, includes 

a higher degree of spatial resolution than previous models, 

and makes use of the advancements in numerical hydrologic 

models and computer capabilities that have occurred since the 

original model was developed.

Description of Study Area 

Much of the information presented in this section is 

based on the presentation in the CP-RASA report of Whiteman 

and others (1994). This information provides generalized but 

important information for understanding the study area and its 

hydrologic system. 

Physical Setting

The Columbia Plateau lies within the Columbia 

intermontane physiographic province (Freeman and others, 

1945). It is bordered by the Cascade Range on the west, the 

Okanogan Highlands on the north, and the Rocky Mountains 

on the east; its southern boundary is defined more by the 

extent of the Columbia River Basalt Group (CRBG) than 

by any physiographic feature. Altitudes in the study area 

range from less than 300 ft to more than 9,000 ft. About 

58 percent of the study area is in Washington, 34 percent is 

in Oregon, and 8 percent is in Idaho. The Columbia Plateau 

is drained by the Columbia River and its major tributaries in 

the study area—the Yakima, Spokane, Clearwater, Salmon, 

Imnaha, Grande Ronde, Tucannon, Snake, Touchet, Walla 

Walla, Umatilla, John Day, Deschutes, Klickitat, Hood, and 

White Salmon Rivers (fig. 1). The headwater areas for most 

of the rivers are outside of the study area, and nearly all the 

streamflow entering, flowing through, and exiting the study 

area is derived from the snowpack. Continuous discharge 

measurements are available for many streams in the study 

area and have been measured at more than 208 active and 

discontinued USGS stream-gaging stations.
10 Groundwater Availability of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

Physiography

The Columbia Plateau was divided into four structural 

regions—the Yakima Fold Belt, Palouse Slope, Blue 

Mountains, and Clearwater Embayment subprovinces 

(fig. 1; Myers and Price, 1979; Reidel and others, 2002). 

The subprovince delineation is important for describing 

groundwater availability. The Yakima Fold Belt includes 

most of the western half of the Columbia Plateau north of 

the crest of the Blue Mountains and is characterized by a 

series of anticlinal ridges and synclinal basins. The Palouse 

Slope occupies the northeast quarter of the Columbia Plateau 

in Washington, north of the Blue Mountains, and extends 

eastward into Idaho. It consists of nearly undeformed basalt 

with a gentle southwestern slope. The Blue Mountains 

subprovince includes the Blue Mountains, a composite 

anticlinal structure, and surrounding areas. This subprovince 

is characterized by high plateaus that are deeply dissected 

by many streams. The Clearwater Embayment marks the 

eastward extent of the CPRAS along the foothills of the 

Rocky Mountains. The easternmost extent of this subprovince 

extends into the Rocky Mountains along the Clearwater 

River drainage; most of its rugged terrain is forested and 

relatively unpopulated.

The topography in central Washington, commonly 

referred to as the "channeled scablands," was produced by 

catastrophic floods during Pleistocene time (Bretz, 1923; 

Bretz and others, 1956). Referred to as the “Missoula Floods,” 

these floods resulted from the breakup of glacial ice dams that 

impounded immense lakes in western Montana and northern 

Idaho, and carved spectacular erosional features into the basalt 

plateau. Floodwaters stripped away overlying sediments and 

left behind deep canyons and coulees, rugged cliffs and buttes, 

large gravel bars, and giant ripple marks that measure 20–30 ft 

in height. The scablands cover about 15,000 mi2 between the 

Columbia, Snake, and Spokane Rivers.

Climatic Setting

The climatic setting of the CPRAS controls the natural 

water fluxes across the landscape. For example, water 

demand by vegetation is most directly related to precipitation, 

temperature, growing-season length, and hours of daylight. 

Much of the area is semiarid, with 57 percent of the area 

receiving less than 15 in. of annual precipitation (fig. 2). 

Average annual precipitation ranges from about 7 in. in the 

center of the study area to more than 45 in. in the surrounding 

mountains. Precipitation is greater in the Cascade Range to 

the west than at similar altitudes in the east and southeast. 

The average annual precipitation from 1895 to 2007 (fig. 2; 

PRISM Climate Group, 2010) was about 17 in. or 55,000 ft3

/s 

(about 40 million acre-ft [MAF]). The interannual variability 

of precipitation generally is similar throughout the study area, 

but varies in some years. The mean monthly precipitation 

distribution at selected weather sites in the study area (fig. 3; 

National Oceanic and Atmospheric Administration, 2013) 

shows that precipitation generally is greatest in December 

and January and least in July. Typically, about 75 percent of 

the annual precipitation occurs during October through April, 

and 42 percent falls from November through January. Much 

of the precipitation from October through April falls as snow. 

Winter snowfall (snow water equivalent) ranges from about 8 

to 20 in. in the central Columbia Plateau to more than 70 in. 

at higher altitudes in the Cascade Range and Blue Mountains. 

Spatial and interannual variations in mountain snowpack can 

be large and can indicate the large temporal-spatial variability 

of climate across the study area that greatly influences 

water availability.

Minimum air temperatures range from about 3 °C in the 

low-lying areas to about -3 °C in higher-altitude areas, and 

maximum air temperatures range from about 30 °C in the 

central Columbia Plateau to about 12 °C in the mountainous 

areas. Average daily temperature fluctuations typically 

range from 8 °C in winter to as much as 20 °C in summer; 

temperatures vary by about 40 °C between winter lows and 

summer highs. The percentage of sunshine ranges from about 

10 percent in the winter to 75 percent in the summer; it is 

about 50 percent in the spring and autumn. In response to 

the climate variations, the growing season ranges from about 

220 days near Walla Walla, Washington, to less than 100 days 

in the Cascade Range and Blue Mountains. 

The types of natural vegetation present on the Columbia 

Plateau are largely dependent on the climatic setting and 

land-surface altitudes, and this vegetation is defined by the 14 

Level IV ecoregions of the Columbia Plateau (Omernik, 1987; 

http://www.epa.gov/wed/pages/ecoregions/level_iii_iv.htm). 

In the central part of the Columbia Plateau, where landsurface altitude ranges from 350 to 2,000 ft and precipitation 

ranges from 7 to 15 in/yr, vegetation primarily is sagebrush 

and grasslands, and there are few perennial streams. At 

altitudes ranging from 2,000 to 3,000 ft, vegetation is 

typical of semiarid-to-transitional humid zones and includes 

grasslands and forest. These areas generally receive from 

15 to 25 in. of annual precipitation, but in the Cascade 

Range, precipitation can be much greater. Where altitudes 

generally are greater than 3,000 ft, forests predominate and 

small perennial streams in deep canyons are common. The 

generalized land cover and land use in the study area shows 

the regional pattern of vegetation (fig. 4). The locations of 

Bureau of Reclamation (Reclamation) projects that provide 

most of the surface water for irrigation on the Columbia 

Plateau also are shown in figure 4.
Description of Study Area 11

Figure 2. Mean annual precipitation, Columbia Plateau Regional Aquifer System study area, Washington, Oregon, and Idaho, 

1895–2007 (from PRISM Climate Group, 2010; and Kahle and others, 2011).

watac11-0648_fig02

Snake

River

Crab Creek

Columbia

River

COL

UMBIA

Naches River

Yakima River 

Umatilla River

Columbia

River

RIVER

N orth Fork John Day River

Snake River

Imnaha River

Banks

Lake Coeur

D'Alene

Lake

Grande Ronde River

Salmon River

John D ay River

Moses Coulee

Walla Walla River

Deschutes River

Klickitat River

White Salmon R 

Clearwater

River

Hangman Cr

 F.D Ro osevelt La ke

Hood R

Wenatch e e R.

Palouse River

Satus

T

Cr

oppenish Creek

Spokane

River

Snohomish

Skagit

King

Pierce

Lewis

Chelan

Okanogan

Kittitas

Yakima

Skamania

Klickitat

Hood

River

Clackamas

Marion

Linn

Ferry

Stevens

Pend

Orielle

Benton

Grant

Lincoln

Spokane

Adams

Franklin

Walla Walla

Whitman

Columbia

Garfield

Asotin

Sherman

Jefferson

Crook

Gilliam

Morrow

Umatilla

Wheeler

Malheur

Baker

Wallowa

Bonner

Kootenai

Benewah

Latah

Lincoln

Sanders

Clearwater

Nez Perce

Washington Valley

Adams

Idaho

Douglas

Wasco

Grant

Union

Shoshone

Lewis

Potholes 

Reservoir

Moses

Lake

Ro ck y Ford Cr.

Ellensburg

Umatilla

Walla Walla

WASHINGTON

OREGON

IDAHO

Yakima

Heppner

MiltonFreewater

Pasco

Lewiston

Moscow

Spokane

The

Dalles

La Grande

Grand Coulee Dam

B L U E M O U N T A I N S

CASCADE RANGE

OKANOGAN HIGHLANDS

CASCADE RANGE

WALLOWA

MOUNTAINS

R O C K Y M O U N T A I N S 

R O C K Y M O U N T A I N S

HORSE HEAVEN HILLS

BEEZLEY HILLS

Cle Elum Odessa

Sunnyside

The

Dalles

Walla Walla

Pullman 2 NW

Hermiston 2 S

Davenport

EXPLANATION

Mean annual precipitation, in inches

Less than 8

8.1 to 15

15.1 to 25

25.1 to 40

40.1 to 60

60.1 to 80

Greater than 80

Study area boundary

Weather station

Base modified from U.S. Geological Survey digital data, various scales. Coordinate system: 

State Plane, Washington South, FIPS 4602: Projection: Lambert Conformal Conic. Horizontal 

datum: North American Datum 1983, Vertical datum: North American Vertical Datum of 1988.

Sunnyside

0 20 40 60 80 MILES

0 20 40 60 80 KILOMETERS

Study

area

WASHINGTON

OREGON IDAHO

122° 120° 118° 116°

46°

48°

47°

45°
12 Groundwater Availability of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

Figure 3. Mean monthly precipitation at selected weather stations, Columbia Plateau Regional Aquifer System, 

Washington, Oregon, and Idaho, 1951-2010 (from National Oceanic and Atmospheric Administration, 2013). Weather station 

locations are shown in figure 2.

tac11-0648_fig03

0 January Febuary March April May June July August September October November December

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Mean monthly precipitation, in inches

Calendar year

EXPLANATION

Odessa

Walla Walla

Pullman 2 NW

Sunnyside

The Dalles

Hermiston 2 S

Cle Elum

Cultural Setting 

The cultural setting of the CPRAS is intricately linked to 

the use and development of the water resources in the region. 

Although there are numerous sources of historical information 

for the CPRAS, the following summary, mostly from the 

Washington State Historical Society (2014), provides the 

context and history of exploration and development. 

Lewis and Clark explored parts of southern Washington 

and northern Oregon along the Snake and Columbia Rivers in 

1805. What early explorers found was semiarid land, home to 

an estimated 14,000 aboriginal people, including about 6,000 

Nez Perce. With the arrival of fur traders and the resulting 

development of the fur trade in the late 1790s, steamboats first 

appeared along the Columbia River in 1836. Fur traders were 

followed by missionaries and the establishment of forts, and 

then by the migration of miners and cattlemen. The Oregon 

Treaty of 1846 established the 49th parallel as the boundary 

between British and American lands and the creation of the 

Oregon Country, which in 1848 became the Oregon Territory. 

Settlers petitioned for the Oregon Territory to be separated, 

with a new territory north of the Columbia River. Congress 

established the Washington Territory in 1853; present-day 

Idaho was included in both the Oregon and Washington 

Territories. When Oregon became the 33rd State in 1859, 

Idaho became part of the Washington Territory, and in 1863, 

Idaho was made into a separate territory. Washington was 

admitted to the Union in 1889 as the 42nd State, and was 

followed the next year by Idaho as the 43rd State.

During the 1850s, the U.S. Government signed numerous 

treaties with various Tribes in eastern Washington, eastern 

Oregon, and western Idaho establishing reservations; in 13 

months in the 1850s, Governor Stevens signed 10 treaties. 

Intertwined with the treaties were the Cayuse, Yakama, 

and Palouse Indian Wars from 1855 through 1858. In 

1879, Chief Moses of the Columbia Tribe signed a treaty 

creating the Moses Columbia Reservation (modified in 

1883, creating the Colville Reservation); it was during this 

period that the Spokane Indian Reservation was established. 

Lands opened up for settlement as a result of Tribal ceding 

and relinquishment of tens of millions of acres outside 

reservations and Federal land grants associated with the 

railroads. In exchange for ceding and relinquishment of 

lands, however, the treaties provided Tribes reserved rights, 

including reserved rights to water in sufficient quantities under 

the terms of the establishment of reservations; the priority 

date of each reserved water right is the date of the treaty 

(appendix 1). These rights, some of which have not been 

quantified, generally have much earlier priority dates than 

non-Tribal rights. 

The railroads and Columbia River were the main conduits 

for commerce. The railroads provided access to markets in the 

rapidly developing parts of western Washington and Oregon. 

The building of the Cascade Locks and Canal in 1896 and the 

Celilo Canal in 1915 on the Columbia River provided easy 

passage for boats to move farther upstream, and provided 

the means to transport agricultural and logging products to 

markets. Large dams and surface-water storage reservoirs later 
Description of Study Area 13

Figure 4. Generalized land cover and land use in the Columbia Plateau Regional Aquifer System study area, Washington, Oregon, 

and Idaho (modified from Kahle and others, 2011).

watac11-0648_fig04 Land cover

Grand Coulee Dam

B L U E M O U N T A I N S

CASCADE RANGE

OKANOGAN HIGHLANDS

CASCADE RANGE

WALLOWA

MOUNTAINS

R O C K Y M O U N T A I N S 

R O C K Y M O U N T A I N S

HORSE HEAVEN HILLS

BEEZLEY HILLS

WASHINGTON

Snake

River

Crab Creek

Columbia

River

COL

UMBIA

Naches River

Yakima River 

Umatilla River

Columbia

River

RIVER

N orth Fork John Day River

Snake River

Imnaha River

Banks

Lake Coeur

D'Alene

Lake

Grande Ronde River

Salmon River

John D ay River

Moses Coulee

Walla Walla River

Deschutes River

Klickitat River

White Salmon R 

Clearwater

River

Hangman Cr

 F.D Ro osevelt La ke

Hood R

Wenatc hee R

Palouse River

Satus

T

Cr

oppenish Creek

Spokane

River

Potholes 

Reservoir

Moses

Lake

Rocky Ford Cr.

Ellensburg

Umatilla

Walla Walla

OREGON

IDAHO

Yakima

Heppner

MiltonFreewater

Pasco

Lewiston

Moscow

Spokane

The

Dalles

La Grande

122° 120° 118° 116°

46°

48°

47°

45°

0 20 40 60 80 MILES

0 20 40 60 80 KILOMETERS

Base modified from U.S. Geological Survey digital data, various scales. Coordinate system: 

State Plane, Washington South, FIPS 4602: Projection: Lambert Conformal Conic. Horizontal 

datum: North American Datum 1983, Vertical datum: North American Vertical Datum of 1988.

Study

area

WASHINGTON

OREGON IDAHO

EXPLANATION

Land-cover classification

Developed, barren, or other

Dryland agriculture

Bureau of Reclamation

Projects

Columbia Basin Project

All other projects

Irrigated agriculture

Shrub and grassland

Forest

Water

Study area boundary

USGS well and identifier

USGS

473442118162201

USGS

473442118162201
14 Groundwater Availability of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

were developed on the Columbia River system beginning in 

the early 1900s, and ended with the construction of Lower 

Granite Dam on the Snake River in 1975. The reservoirs were 

built to meet the water and energy demands of this expanding 

agricultural region. The reservoirs on the Columbia and 

lower Snake Rivers alone have the potential to store more 

than 43 MAF of water. The stored water is used primarily for 

hydroelectric power production, irrigation, navigation, and 

flood control. 

Population trends of the Columbia Plateau (fig. 5; U.S. 

Census, 2009, 2011) correspond to changes in economic 

conditions and the development of water resources. Population 

increases from 1900 to 1910 indicate the influx of settlers 

attracted by the availability of land made accessible by 

the newly completed railroads. Population leveled out in 

response to slower economic growth between 1910 and 1930. 

Construction of the Reclamation Columbia Basin Project 

(CBP) (see fig. 4), water-power projects, and the Hanford 

Nuclear Reservation contributed to the steep rise in population 

in the 1940s. Irrigation water made available by CBP and 

through drilling of deep wells in the basalts spurred growth 

from the 1950s through the 1970s. 

Water Resources and Economic Development

The cultural and economic development of the Columbia 

Plateau has depended heavily on the availability of water and 

the ability to store and redistribute water from the Columbia 

River and its major tributaries. Irrigation began as early as the 

1840s and 1850s at missions in Walla Walla, Lewiston, and 

the Yakima River Valley. The first known irrigation system 

was constructed in 1818 by Donald McKenzie of the North 

West Company at Fort Nez Perces, near the confluence of 

the Walla Walla and Columbia Rivers (Northwest Council, 

2013). Transportation was greatly improved in the late 1800s, 

when three railroad systems were built across the Columbia 

Plateau. Large land grants given to the railroads served to 

attract settlers, to encourage export of crops, and to promote 

investments in irrigation and agriculture-related businesses. In 

the early 1900s, the lumber and agricultural industries grew 

steadily. Small-grain production on dryland farms and dairy 

and poultry farming were especially profitable because these 

enterprises did not need large quantities of water. 

The dry summer climate of the region forced the early 

settlers to develop water supplies for irrigation wherever 

possible. Most early attempts to form and finance irrigation 

districts and water-user associations failed. By the 1930s, 

economic growth was relatively slow because of the 1929 

depression and severe droughts that occurred after 1919. 

Those who survived these hardships lived along surface-water 

bodies or in areas where groundwater was available at shallow 

depths. A notable exception to this pattern was the Yakima 

River Basin, where reservoirs, diversion dams, and canals 

were constructed during 1892–1933. By 1902, there were 

about 120,000 acres under mostly surface-water irrigation in 

the Yakima River Basin (Parker and Storey, 1916; Bureau of 

Reclamation, 1999). Reclamation projects between 1910 and 

1933 allowed the irrigated acreage to increase to more than 

500,000 acres. These water projects ensured that irrigation 

water was available. 

The start of the CBP with the construction of Grand 

Coulee Dam creating F.D. Roosevelt Lake (FDR) (fig. 1) in 

1933 and construction of the Hanford Nuclear Reservation 

(located in northeastern Benton County, Washington) in the 

1940s brought a large influx of workers and associated service 

industries. By 1946, about 850,000 acres were irrigated in 

the study area (Simons, 1953). Much of these lands were 

irrigated with water supplied by Reclamation projects created 

under the Federal Reclamation Act of 1902 that enabled the 

construction of Federal water projects in the Western United 

States to expand development of the West. The Reclamation 

Act also allowed settlers to own 160 acres (a quarter section) 

for irrigating croplands. Irrigation water from FDR became 

available in 1952, and by 1972 more than 0.5 million acres 

were being irrigated by this project in Washington, producing 

more than 60 types of crops. As of 2010, about 2 million 

acres of croplands were irrigated with surface water and 

groundwater, with much of the surface water supplied by 

Reclamation projects (fig. 4). The surface-water withdrawals 

account for about 75 percent of the total irrigation water 

(Kahle and others, 2011). 

Starting with the advent of new technology in the early 

1950s, a rapid and intensive expansion of deep-well irrigation 

practices took place in areas not served by surface-water 

irrigation projects. These areas included parts of the Yakima, 

Pasco, Umatilla, The Dalles, and Walla Walla Basins and 

the Odessa area in western Adams County, Washington. By 

1984, about 0.5 million acres were irrigated with groundwater 

(Whiteman and others, 1994). Some areas receive sufficient 

precipitation to support a dryland farming economy primarily 

based on wheat and barley, but an estimated 1.5 million acres 

of these lands could be cropped profitably if water supplies 

Figure 5. Population of the Columbia Plateau Regional 

Aquifer System study area, Washington, Oregon, and Idaho, 

1900–2010 (U.S. Census, 2009, 2011).

tac11-0648_fig05

20001990198019701960195019401930

Year

192019101900

0

0.5

1.0

2.0

1.5

Population, in millions

2010
Description of Study Area 15

were available (Pacific Northwest River Basins Commission, 

1971). The dryland agricultural areas on the Columbia Plateau, 

however, are some of the most productive and profitable in the 

world, with wheat yields in some areas exceeding 90 bushels 

per acre.

Earlier groundwater development occurred in areas 

scattered throughout the Columbia Plateau. Some of the 

most intensive early development of groundwater was in the 

Yakima River Basin and near Walla Walla, Washington. As 

groundwater pumpage increased throughout the Columbia 

Plateau, groundwater levels declined. In central Washington, 

declines locally exceeded 150 ft by 1981 (Cline, 1981). 

By 1984, declines had been identified in the Pullman, 

Washington-Moscow, Idaho area; the Umatilla area; The 

Dalles area; parts of the Yakima River Basin; the Walla Walla 

area; and the south slope of Horse Heaven Hills (Hansen and 

others, 1994). Data collected as part of this study show that 

groundwater-level declines have continued to occur from 1984 

through 2009 (Snyder and Haynes, 2010). The large declines 

in the Odessa area in Washington have had economic effects 

on farmers and the local economy in two ways. First, the 

declining groundwater levels have led to increased pumpingpower costs and, thus, decreased profits; this also has occurred 

in some of the major groundwater pumping centers in Oregon. 

Second, to save costs, some farmers who may have been 

irrigating four quarter-sections now only irrigate two, reducing 

their irrigated lands by one-half. The economic effects of 

reduced croplands (and fewer profits), in turn, ripple through 

the allied sales and service sectors that support the agricultural 

industry.

Whiteman and others (1994) indicated that, during 

1949–82, there had been a steady increase in irrigated acreage 

but a trend toward fewer but larger farms. The trend in 

increasing farm size generally continued from 1992 through 

2002, with some stabilization and decrease in farm size from 

2002 to 2007, and the total number of irrigated acres generally 

has stabilized (U.S. Department of Agriculture, 2007b). 

These trends indicate financial hardships that are affecting 

the owners of small- to intermediate-sized farms. Largefarm operators are better able to survive difficult financial 

periods, and owners of small farms often work outside jobs to 

supplement farm income. The primary occupation of a farm 

operator may be farming or it may be an off-farm occupation; 

a trend toward more off-farm work is a national trend (U.S. 

Department of Agriculture, 2007b). In the study area, most 

farms are owned by individuals or families.

Geologic Setting

The Columbia Plateau is underlain by massive basalt 

flows of the CRBG, with an estimated composite thickness 

of at least 14,000 ft at one of the lowest points on the plateau 

near Pasco, Washington (Drost and Whiteman, 1986; Drost 

and others, 1990; Reidel and others, 2002). The characteristics 

of the older rocks overlain by the CRBG are distinguishable 

only at exposures in the uplands. Sedimentary deposits overlie 

the basalt over about 33 percent of the study area, exceeding 

2,100 ft in thickness in the Yakima River Valley (Jones and 

others, 2006) and 2,000 ft in the Grande Ronde Valley near La 

Grande, Oregon (Drost and others, 1990).

The CPRAS includes, from youngest to oldest:

1. The Overburden, a collective term used in this study for 

all materials (from Miocene to Holocene age) overlying 

the CRBG;

2. A minor amount of Miocene sediment interlayered with 

the basalt; and

3. A large thickness of CRBG rocks.

Pre-CRBG rocks underlie the Columbia Plateau and primarily 

consist of less permeable rocks that form the lower boundary 

of the CPRAS (table 1).These are sedimentary, igneous, and 

metamorphic rocks that range from Precambrian through early 

Tertiary age. 

The CRBG underlies nearly all of the study area and 

it is the thickest, most extensive, and hydrologically most 

important geologic unit in the CPRAS (Whiteman and others, 

1994; Vaccaro, 1999; Kahle and others, 2011). The CRBG 

consists of a series of flows erupted during various stages 

during the Miocene age, 17 to 6 million years ago. The basalt 

lava flowed from fissures and vents in eastern Washington, 

northeastern Oregon, and western Idaho (Swanson and others, 

1975; Hooper, 1982). Flows covered about 63,200 mi2 of 

eastern Washington, parts of western Washington, northern 

parts of western Oregon, and western Idaho to an average 

total thickness of 3,300 ft. About 95 percent of the basalt was 

extruded in episodic eruptions during the first 3 million years. 

Warping and folding of the Columbia Plateau increased late 

in the eruptive cycle, forming numerous geologic structures 

associated with faults or folds with large anticlines. More than 

300 flows have been identified and individual flows range in 

thickness from 10 to more than 300 ft.

The physical characteristics of the basalt flows govern the 

movement of groundwater in basalts and, thus, its availability. 

Upper zones of the flows were exposed to weathering 

processes and were broken and fractured by subsequent flows, 

resulting in the formation of conductive “flow tops.” These 

flow tops, when combined with the base of the overlying 

basalt flow, form interflow zones that readily transmit water 

(Lindolm and Vaccaro, 1988). The interflow zones commonly 

make up 5–10 percent of the thickness of a flow. The interflow 

zones are separated by the less transmissive entablature and 

colonnade in which fractures are typically oriented vertically 

(Tomkeieff, 1940; Waters, 1960; MacDonald, 1967; Swanson 

and Wright, 1978; Sublette, 1986; Hansen and others, 1994). 

These fractures are a result of contraction during cooling of 

basalt flows (MacDonald, 1967; Long and Wood, 1986) and 

of later folding and faulting. The greatest density of fractures 

generally occurs in the entablature (Wood and Fernandez, 

1988; Reidel and others, 2002). 
16 Groundwater Availability of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

Geologic 

unit symbol

and color

Age

CPRAS 

simplified 

geologic map 

unit

Idaho

[310; MU_SYMBOL]

Oregon

[729; MAP_UNIT_NAME]

Washington

[746; GUNIT_TXT]

Qs Quaternary Sediment Alluvial, eolian, glacial, 

glacial outburst flood, 

glaciolacustrine, 

landslide, and peat 

deposits 

Alluvial, colluvial, eolian, 

glacial, glacial outburst flood, 

lacustrine, landslide, terrace, 

and peat deposits; artificial 

fill, ash, debris-avalanche and 

debris-flow deposits, mine 

tailings, talus; sedimentary 

deposits or rocks

Alluvial, eolian, glacial, 

glacial outburst flood, 

glaciolacustrine, 

lacustrine, landslide, 

and peat deposits; talus; 

artificial fill

QTi QuaternaryTertiary

Intrusives Not a map unit in Idaho Not a map unit in Oregon Mostly Mount Rainier/

Adams intrusives

QTsdr QuaternaryTertiary

Sedimentary 

deposits or 

rocks

Alluvial gravel, terrace; 

Conglomerate near 

Round Mountain; 

consolidated alluvial 

and (or) glacial deposits

Alkali Canyon, Dalles, 

Deschutes, Ellensburg, 

Madras, McKay, Rattlesnake, 

and Simtustus Formations; 

alluvial, fluvial, lacustrine, and 

terrace deposits; sedimentary 

rocks; tuffaceous sedimentary 

deposits and rocks; 

volcaniclastic deposits

Continental sedimentary 

deposits or rocks, 

alluvium, landslides

QTv QuaternaryTertiary

Non-CRBG 

volcanics

Basalt of Cuddy 

Mountain

Numerous non-CRBG 

volcanics

Mostly Mount Rainier/

Adams flows, lahars, 

intrusives, volcaniclastic 

deposits or rocks

Msdr Miocene Sedimentary 

deposits or 

rocks

Alluvial gravel, Latah 

Formation, Deer Creek 

Beds, Payette Formation

Not a map unit in Oregon Continental sedimentary 

deposits or rocks, 

landslide deposits

Mv(SMB) Miocene Saddle 

Mountains 

Basalt

Includes Asotin, 

Grangeville, Onaway, 

Wilbur Creek, and 

Weissenfels Ridge 

Members; and basalt of 

Cragmont, Lapwai, and 

Weippe

Buford, Elephant Mountain, 

and Umatilla members; basalt 

of Eden, Ferguson Spring, and 

Umatilla; Pomona basalt

Includes Asotin, Buford, 

Elephant Mountain, 

Esquatzel,Grangeville, 

Ice Harbor, Lower 

Monumental, Onaway, 

Pomona, Umatilla, 

Wilbur Creek, and 

Weissenfels Ridge 

Members 

Mv(WB) Miocene Wanapum 

Basalt

Includes Eckler 

Mountain, Priest Rapids, 

and Roza Members; and 

basalt of Ferry Creek

Frenchman Springs and Roza 

members; Basalt of Ginkgo, 

Lookingglass, Lyons Ferry, 

Palouse Falls, Powatka, 

Robinette Mountain, Sand 

Hollow, and Sentinel Gap

Includes Eckler 

Mountain, Frenchman 

Springs, Priest Rapids, 

and Roza Members

Table 1. Names, descriptions, and ages of simplified geologic units in the Columbia Plateau Regional Aquifer System, Washington, 

Oregon, and Idaho.

[From Kahle and others (2009). CPRAS, Columbia Plateau Regional Aquifer System. CRBG, Columbia River Basalt Group unit. Notes are summarized from 

State digital compilations (number of original map units; unit type)]
Description of Study Area 17

When the hiatus between flows was sufficiently long, 

soil developed or sediments were deposited on the surface 

of a flow. If these sediments were preserved, a sedimentary 

interbed occurred between flows. Sedimentary interbeds are 

more common in younger than older basalt units. During the 

Pleistocene, the surface expression of the basalt was modified 

greatly during the repeated catastrophic outburst flooding of 

the Missoula Floods, which caused erosion of vast channels 

and ancient waterfalls in places, as well as removal and (or) 

deposition of overlying sediment (Bretz, 1923; Bretz and 

others, 1956).

Sediment Stratigraphy

After the eruption of basalt ceased, folding, erosion, and 

deposition of sediments continued. The Cascade Range has 

been the primary source of sediments and, thus, the greater 

sediment thicknesses are in the western part of the study 

area. The sediments include consolidated–to-unconsolidated 

deposits of fluvial, lacustrine, volcanic, and eolian origin 

ranging from Miocene to Holocene age. 

Within the Yakima Fold Belt, Miocene sediment of the 

Ellensburg Formation underlies, intercalates, and overlies the 

CRBG and composes most of the thickness of the deposits 

in the basinal areas (Jones and others, 2006). In eastern 

Washington and west-central Idaho, sediment of the finegrained Latah Formation underlies, intercalates, and overlies 

the CRBG (Pardee and others, 1926; Leek, 2006), but occurs 

mostly beyond the extent of the CRBG. In the Walla Walla 

River, Quincy, and Pasco Basins in Washington, the overlying 

sediments are part of the Miocene-Pliocene Ringold Formation, 

which is at least 500 ft thick in these basins. Eolian deposits of 

fine Pleistocene loess cover much of the Columbia Plateau and 

reach thicknesses of as much as 250 ft, but generally are much 

thinner. The thickest deposits are present in the Palouse Slope 

subprovince and are part of the Palouse Formation.

Geologic 

unit symbol

and color

Age

CPRAS 

simplified 

geologic map 

unit

Idaho

[310; MU_SYMBOL]

Oregon

[729; MAP_UNIT_NAME]

Washington

[746; GUNIT_TXT]

Mv(GRB) Miocene Grande 

Ronde Basalt

Includes upper and 

lower flows of normal 

magnetic polarity (N2 

and N1) and upper and 

lower flows of reverse 

magnetic polarity (R2, 

R1)

Includes upper and lower flows 

of normal magnetic polarity 

(N2 and N1) and upper 

and lower flows of reverse 

magnetic polarity (R2, R1)

Includes upper and lower 

flows of normal magnetic 

polarity (N2 and N1) and 

upper and lower flows of 

reverse magnetic polarity 

(R2, R1)

Mv(PB) Miocene Prineville 

Basalt

Not a map unit in Idaho CRBG, Prineville chemical 

type

Not a map unit in 

Washington

Mv(PGB) Miocene Picture Gorge 

Basalt

Not a map unit in Idaho Basalt intrusives and flows 

(normal and reversed magnetic 

polarity); Dayville, Monument 

Mountain, and Twickenham 

Basalt

Not a map unit in 

Washington

Mv(IB) Miocene Imnaha 

Basalt

Basalt Basalt with minor tuff breccia Basalt

Mv(CRBG) Miocene CRBG, 

undivided

CRBG, undivided CRBG, undivided; basalt and 

dikes

Not a map unit in 

Washington

preM preMiocene Pre-CRBG 

rocks, 

undivided

Intrusive, 

metamorphosed 

intrusive, sedimentary, 

and metamorphosed 

sedimentary rocks

Volcanic, metavolcanic, 

intrusive, sedimentary, and 

metamorphosed sedimentary 

rocks; sedimentary deposits 

(pre-CRBG)

Intrusive, metamorphosed 

intrusive, sedimentary, 

metamorphosed 

sedimentary, and volcanic 

rocks (pre-CRBG)

Table 1. Names, descriptions, and ages of simplified geologic units in the Columbia Plateau Regional Aquifer System, Washington, 

Oregon, and Idaho.—Continued

[From Kahle and others (2009). CPRAS, Columbia Plateau Regional Aquifer System. CRBG, Columbia River Basalt Group unit. Notes are summarized from 

State digital compilations (number of original map units; unit type)]
18 Groundwater Availability of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

The Cordilleran ice sheet that covered the northern and 

northwestern parts of the Columbia Plateau left extensive 

deposits of till, stratified drift, and ice-contact materials. 

Alpine glaciers from the Cascade Range and Wallowa 

Mountains also reached the plateau margin and left deposits.

Pleistocene deposits from the Missoula Floods were 

deposited by high-energy flood currents in and along scabland 

channels, in depositional basins, and in the Columbia River 

east of The Dalles. Finer-grained slackwater deposits in 

peripheral areas of basins in the central part of the Columbia 

Plateau are known as the Touchet Beds (Flint, 1938).

Interbed Stratigraphy

The two interbeds of hydrologic importance are the 

interbed between the Saddle Mountains Basalt and the 

Wanapum Basalt (informally called the Mabton Member 

of the Ellensburg Formation) and the interbed between the 

Wanapum Basalt and the Grande Ronde Basalt (informally 

called the Vantage Member of the Ellensburg Formation). 

In the northeastern part of the study area, the sedimentary 

interbed in the same position as the Vantage Interbed is 

assigned to the Latah Formation (Swanson and others, 1979). 

The interbed units are fairly extensive laterally, but are thin 

when compared with the thickness of the basalts. The Mabton 

Interbed generally consists of clay, shale, claystone, clay with 

basalt, clay with sand, and sandstone, but also may contain 

small amounts of sand and sand-and-gravel. The Vantage 

Interbed consists of clay, shale, sandstone, tuff with claystone, 

and clay and basalt, but also may contain small amounts of 

sand and sand-and-gravel.

Basalt Stratigraphy

The CRBG has been divided into six geologic 

formations: Imnaha Basalt, Picture Gorge Basalt, Prineville 

Basalt, Grande Ronde Basalt, Wanapum Basalt, and Saddle 

Mountains Basalt (Swanson and others, 1979; U.S. Geological 

Survey, 2009; table 1). Flows belonging to the Imnaha Basalt, 

the oldest known in the CRBG, are present in western Idaho 

and eastern Washington and Oregon. The Picture Gorge and 

Prineville Basalt formations are limited to areas in central 

Oregon at the southern extent of the CRBG. The Grande 

Ronde Basalt and older basalts constitute about 90 percent 

of the CRBG by volume and cover most of the area where 

the CRBG is present (Reidel, 1982; Tolan and others, 1987; 

Bjornstad and others, 2007; U.S. Geological Survey, 2009). 

Flows of the widely distributed Wanapum Basalt commonly 

overlie the Grande Ronde Basalt. Flows of the Saddle 

Mountains Basalt are less widely distributed and generally are 

confined to the central part of the study area. 

Hydrogeologic Units

The regional hydrogeologic framework was updated in a 

two-step process. In the first step, a digital surficial geologic 

map was compiled and simplified, well data were compiled, 

hydrogeologic units were defined, hydrogeologic sections 

were constructed, and generalized extents of hydrogeologic 

units were defined (Kahle and others, 2009). The second 

step involved building a digital three-dimensional (3D) 

hydrogeologic framework using the information from step one 

and data from numerous databases and detailed studies that 

were completed during the past 3 decades. The 3D framework 

was developed using geospatial statistical modeling techniques 

(Burns and others, 2011).

Definition of Units

Generalized hydrogeologic units defined for the 

CPRAS include the Overburden, Saddle Mountains, Mabton 

Interbed, Wanapum, Vantage Interbed, Grande Ronde, and 

Older Bedrock units (table 2; Kahle and others, 2009). The 

approximate surficial distribution of these units is shown in 

figure 6A; note that the Mabton Interbed does not outcrop 

and the Vantage Interbed outcrops only in isolated areas too 

small to show at the scale of this figure. The sedimentary 

Overburden unit is a composite unit, and is an important 

source of groundwater. The unit covers about 33 percent of 

the area and consists of unconsolidated (clay to gravel) to 

consolidated material (sandstone to shale) that form important 

water-bearing units, which include water table, semiconfining, 

and confining units. The unit contains numerous types of 

sedimentary deposits (table 2). The three basalt units are the 

Saddle Mountains, Wanapum, and Grande Ronde Basalts and 

their intercalated sediments. In the southeastern part of the 

study area, the Imnaha Basalt and any intercalated sediments 

are included with the Grande Ronde unit. The interbed 

units are equivalent to the Saddle Mountains-Wanapum and 

Wanapum-Grande Ronde interbeds, referred to in this study 

as the Mabton and Vantage Interbeds, respectively (Kahle and 

others, 2009; Burns and others, 2011). The Older Bedrock 

unit consists of pre-CRBG rocks that generally have much 

lower permeabilities than the basalts and is considered the 

base of the regional flow system—the basement confining 

unit (Vaccaro, 1999; Kahle and others, 2009). Hydrogeologic 

sections in figure 6B indicate the relative placement of the 

basalt units; the interbed units are too thin to show at the 

scale used. Detailed descriptions of the sediment, basalt, 

and interbed units are available in Drost and others (1990), 

Whiteman and others (1994), Jones and others (2006), Jones 

and Vaccaro (2008), Kahle and others (2009), and Burns and 

others (2011).
Hydrogeologic Units 19

Table 2. Correlation chart showing relation between stratigraphy and generalized hydrogeologic units, Columbia Plateau Regional 

Aquifer System, Washington, Oregon, and Idaho.

[From Kahle and others (2009)]

Generalized 

hydrogeologic unit ERA PERIOD EPOCH Sediment stratigraphy Basalt stratigraphy

Overburden

Cenozoic

Quaternary

Holocene Alluvial, colluvial, eolian, 

glacial, glacial outburst 

flood, lacustrine, landslide, 

terrace, and peat deposits; 

ash, debris-avalanche and 

debris-flow deposits, talus; 

Touchet Beds, Palouse 

Formation Quaternary and Pliocene Basalts

Pleistocene

Tertiary

Pliocene

Alluvial fan deposits; 

Alkali Canyon, Chenoweth, 

Deschutes, Madras and 

Ringold Formations; Dalles 

Group; Thorpe Gravel; 

and unknown continental 

sedimentary deposits

Miocene

Ellensburg, Deschutes, 

Latah, Madras, Payette, and 

Ringold Formations; Dalles 

Group; Snipes Mountain 

deposits; Deer Creek Beds; 

and unknown continental 

sedimentary deposits

Columbia 

River 

Basalt 

Group

Saddle Mountains 

Basalt flow 

members and 

interbeds

Saddle Mountains

Mabton Interbed

Mabton interbed 

(Mabton Member of the 

Ellensburg Formation)

Wanapum

Wanapum Basalt 

flow members and 

interbeds

Vantage Interbed

Vantage interbed 

(Vantage Member of the 

Ellensburg Formation)

Grande Ronde

Grande 

Ronde 

Basalt flow 

members 

and 

interbeds

Prineville 

Basalt

Picture 

Gorge 

Basalt

Imnaha Basalt

Older Bedrock pre-Columbia River Basalt Group rocks, undivided
20 Groundwater Availability of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

Figure 6. Generalized (A) surficial hydrogeologic units (from Kahle and others, 2009), and (B) hydrogeologic sections (from Burns 

and others, 2011) in the Columbia Plateau Regional Aquifer System study area, Washington, Oregon, and Idaho. 

Snake

River

Crab Creek

Columbia

River

COL

UMBIA

Naches River

Yakima River

Umatilla River

Columbia

River

RIVER

N orth Fork John Day River

Snake River

Imnaha River

Banks

Lake Coeur

D'Alene

Lake

Grande Ronde River

Salmon River

John D ay River

Rattlesna ke Creek

Walla Walla River

Deschutes River

Klickitat River

White Salmon R 

Clearwater

River

Hangman Cr

 F.D Ro osevelt La ke

Hood R

Wenatc hee R

Palouse River

Satus

T

Cr

oppenish Creek

Spokane

River

Potholes 

Reservoir

Moses

Lake

Rocky Ford Cr.

Ellensburg

Umatilla

Walla Walla

WASHINGTON

OREGON

IDAHO

Yakima

Heppner

MiltonFreewater

Pasco

Lewiston

Moscow

Spokane

The

Dalles

La Grande

Grand Coulee Dam

B L U E M O U N T A I N S

CASCADE RANGE

OKANOGAN HIGHLANDS

CASCADE RANGE

WALLOWA

MOUNTAINS

R O C K Y M O U N T A I N S 

R O C K Y M O U N T A I N S

HORSE HEAVEN HILLS

BEEZLEY HILLS

B

B'

A

A'

watac11-0648_fig06a Hydrogeologic units

EXPLANATION

Generalized hydrogeologic units

Overburden

Saddle Mountains

Wanapum

Grande Ronde

Older Bedrock

Water

Study area boundary

A'A Hydrogeologic section

122° 120° 118° 116°

46°

48°

47°

45°

0 20 40 60 80 MILES

0 20 40 60 80 KILOMETERS

Base modified from U.S. Geological Survey digital data, various scales. Coordinate system: 

State Plane, Washington South, FIPS 4602: Projection: Lambert Conformal Conic. Horizontal 

datum: North American Datum 1983, Vertical datum: North American Vertical Datum of 1988.

Study

area

WASHINGTON

OREGON IDAHO
Hydrogeologic Units 21

watac11-0648_fig06b sections

WA

OROR

WAWA

ID

Lower Crab Creek

Rattlesnake Creek

COLUMBIA RIVERCOLUMBIA RIVER

John Day River

Willow Creek

COLUMBIA RIVER

Yakima River

Little Klickitat River

East Fork Major Creek

COLUMBIA RIVERLake Wallula

North ForkWenaha River

Cottonwood Creek

Butte Creek

Little Klickitat River

Klickitat River

Rock Creek

Harrison Creek

Alder Creek

Walla Walla River

Mill Creek

Snake River

SECTION

B–B'

Umtanum Ridge

Saddle Mountains

Frenchman Hills

Beezley Hills

Moses Coulee

Dyer Hill

Horse Heaven Hills

Collins Butte

Jordan Butte

Rattlesnake Hills

Blue Ridge

Monument Ridge

Saddle Butte

Craig Mountain

SECTION

A–A'

Paterson Ridge

Vansvale Canyon Warm Springs Canyon

Klickitat Valley

Walla Walla Valley

Putter Butte

Blue Mountains

VERTICAL SCALE GREATLY EXAGGERATED

14,000

16,000

18,000

12,000

10,000

8,000

6,000

4,000

2,000

0

2,000

4,000

6,000

FEET

A'

NAVD 1988

METERS

1,000

1,000

2,000

2,000

3,000

4,000

5,000

14,000

16,000

18,000

12,000

10,000

8,000

6,000

4,000

2,000

0

2,000

4,000

6,000

FEET

A

14,000

16,000

18,000

12,000

10,000

8,000

6,000

4,000

2,000

0

2,000

4,000

6,000

8,000

10,000

FEET

B'

EAST

NAVD 1988

1,000

1,000

2,000

2,000

3,000

4,000

METERS

3,000

5,000

14,000

16,000

18,000

12,000

10,000

8,000

6,000

4,000

2,000

0

2,000

4,000

6,000

8,000

10,000

FEET

B

WEST

SOUTH NORTH

Altitude, in feet above North American Vertical Datum of 1988

Contact

Fault

Section trace intersection

EXPLANATION

Generalized hydrogeologic units

Overburden

Saddle Mountains

Wanapum

Grande Ronde

Older Bedrock

Note: Datum is North American 

Vertical Datum of 1988.

0 20 40 60 80 Miles

0 20 40 60 80 Kilometers

Figure 6.—Continued.
22 Groundwater Availability of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

Mapping of Hydrogeologic Units Using 

Geospatial Modeling

A 3D-geologic model was constructed for the CPRAS to 

define the general aquifer system geometry (Burns and others, 

2011) for use in the regional numerical groundwater-flow model. 

Simplifications and assumptions were made that were consistent 

with the uncertainty in the available data and the intended use of 

the results. The final hydrogeologic units constructed are for the 

44,000-mi2

 study area. Data compiled for the model were simplified 

and used to construct piecewise-smooth trend surfaces that represent 

upper and lower subsurface unit boundaries in this complex terrain. 

The modeling process yielded improved estimates of unit volumes, 

refinement of location of large structural features, and identification 

of features that may be important for ongoing groundwater studies. 

Two hydrogeologic sections show the complexity of the aquifer 

system (fig. 6B). Summary descriptions (from youngest to oldest) of 

the units based on the work of Kahle and others (2009) and Burns 

and others (2011) are provided in the following sections.

Overburden Unit

For the 3D framework, Burns and others (2011) used the 

lateral extents of most of the sedimentary deposits in the study area, 

including those between 0 and 100 ft thick (excluded in the mapping 

of Kahle and others, 2009). The final distribution of the Overburden 

unit covers 14,644 mi2

, which is larger than the 6,134 mi2 estimated 

by Kahle and others (2009) because the areas with less than 100 ft 

of thickness were included when constructing the 3D model. The 

thickness of the unit ranges from 0 to about 2,000 ft (average and 

median of 88 and 860 ft, respectively), and the unit has a volume of 

about 244 mi3

. The thickest deposits occur in structural basins and 

areas adjacent to the Cascade Range.

Saddle Mountains Unit

The Saddle Mountains unit has an extent of about 11,700 mi2

compared to the 8,000-mi2

 estimate of Kahle and others (2009); the 

model estimated that this unit is present in additional parts of the 

Palouse Slope, and other information seems to support this larger 

extent (Burns and others, 2011). The estimated thickness of the unit 

(which for the 3D modeling included the Mabton Interbed unit) 

ranges from 0 to about 2,000 ft. The average thickness of the unit 

is about 300 ft and the median thickness is 1,154 ft. Where buried, 

the constructed top-of-unit map indicates that altitudes range from 

about 500 to 3,000 ft. The total volume of the unit was calculated at 

663 mi3

.

Mabton Interbed Unit

The Mabton Interbed unit is located mostly in the westcentral part of the study area, and no surficial outcrops of the unit 

are present within the study area. For the 3D model, the extent 

was assumed to be the extent of the Saddle Mountains unit. The 

thickness of the unit was estimated by Burns and others (2011) as a 

function of distance between the Saddle Mountains and Wanapum 

unit tops. Calculated thickness ranged from 0 to about 205 ft 

(average thickness of 39 ft).

Wanapum Unit

The Wanapum unit is a surficial unit that covers more than 

30 percent of the study area (fig. 6A). Much of the unit lies beneath 

the Overburden and Saddle Mountain units. The computed area 

of the Wanapum unit is about 24,400 mi2

, and the calculated unit 

thickness (which included the Vantage Interbed unit thickness) 

ranges from 0 to about 3,250 ft (average and median thickness of 

476 and 1,115 ft, respectively). Where buried, the altitude of the 

top of the unit ranges from more than 3,500 ft in the eastern part 

of the Blue Mountains to less than -1,000 ft at the Hanford Site. 

The calculated volume for the unit is 2,156 mi3

. The thickness 

map for the unit and the hydrogeologic sections (Burns and others, 

2011) indicate structural basins, effects of geologic structure on 

the distribution of the unit, and the overall thickening of the unit 

towards the center of the Columbia Plateau (figs. 6B and 7).

Vantage Interbed Unit

Limited surficial outcrops of the Vantage Interbed unit occur 

in the study area, and for the 3D model, the extent was assumed 

to be the extent of the Wanapum unit. Generalized estimates of 

the thickness of the unit were generated as a function of distance 

between the Wanapum and Grande Ronde unit tops. Calculated 

thickness values for the unit resulted in an average of about 20 ft 

and a median of 15 ft, and thickness ranged from 0 to about 30 ft. 

Drost and others (1990) and Whiteman and others (1994) reported a 

similar average thickness of 25 ft. 

Grande Ronde Unit

The Grande Ronde unit underlies most of the study area 

(fig 6A), and occurs at land surface over more than 30 percent of 

the study area. The 3D model produced a 41,900-mi2

 areal extent 

for the Grande Ronde unit. The thickness of the unit ranged from 

0 to about 16,000 ft under the assumption that the paleosurface 

(top of the Older Bedrock unit) was relatively smooth. The average 

thickness was calculated as 3,950 ft. The altitude of the top ranges 

from about -2,200 to 7,888 ft, or a relief difference of about 

10,000 ft. The total calculated volume for the unit was 31,273 mi3. 

A map of the top of the Older Bedrock unit (fig. 8), analogous to 

the bottom of the CRBG, indicates the complex shape of the basalt 

surfaces within the CPRAS.

Older Bedrock Unit

The Older Bedrock unit that borders and underlies the CPRAS 

is composed of various rock types older than the CRBG (Kahle and 

others, 2009). The Older Bedrock unit outcrops only over a small 

part of the study area. Burns and others (2011) constructed a map of 

the top of this unit as part of developing the 3D framework in order 

to estimate the thickness of the Grande Ronde unit.
Hydrogeologic Units 23

Figure 7. Combined thickness of the Wanapum and Vantage Interbed units, in the Columbia Plateau Regional Aquifer System study 

area, Washington, Oregon, and Idaho (from Burns and others, 2011). 

tac11-0648_fig07

SNAKE

RIVER

Crab

Spokane

Creek

River

Palouse

River

COLUMBIA

RIVER

Toppenish Creek

Yakima

Naches

Klickitat R

Umatilla

Banks

Lake

Lake

Chelan Lake

Pend

Oreille

Moses

Lake

Priest

Rapids

Lake

Coulee

Moses

Walla Walla River

Ford Cr

Clearwater

River

Satus Creek

Hangman Creek

River River

River

River

River

River

River

RIVER

Imnaha

Salmon

Grande

SNAKE

Ronde

John Day River

John Day River

John

Day River Fork

North

Deschutes

Lake

Billy

Chinook

Lower Crab Cr

Pend Oreill e River

Coeur

d'Alene

Lake

F. D. Roosevelt Lake

COLUMBIA RIVER

2,000-feet thick

3,000-feet 

 thick

Ellensburg

Walla

Walla

Yakima

Pasco

Lewiston

Moscow

Spokane

The

Dalles

La Grande

Pullman

Wenatchee

Moses

Lake

Coeur

d'Alene

Pendleton

Hermiston

OREGON

IDAHO

MONT.

WASHINGTON

0 20 40 60 80 MILES

0 20 40 60 80 KILOMETERS

Study

area

WASHINGTON

OREGON IDAHO

Base modified from U.S. Geological Survey digital data, various scales. Coordinate system: 

State Plane, Washington South, FIPS 4602: Projection: Lambert Conformal Conic. Horizontal 

datum: North American Datum 1983, Vertical datum: North American Vertical Datum of 1988.

EXPLANATION

Combined thickness of Wanapum and 

Vantage Interbed units, in feet

0

400

800

1,200

1,600

2,000

Fault affecting distribution of the 

Wanapum unit

Extent of the Columbia River Basalt 

Group used for mapping

122° 121° 120° 119° 118° 117° 116°

48°

47°

46°

45°
24 Groundwater Availability of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

Figure 8. Altitude of the top of the Older Bedrock unit, Columbia Plateau Regional Aquifer System, Washington, Oregon, and 

Idaho (from Burns and others, 2011).

tac11-0648_fig08

0 20 40 60 80 100 MILES

0 20 40 60 80 100 KILOMETERS

-16,000

-14,000

-14,000

-12,000

-12,000

-10,000

-10,000

-9,000

-8,000

-8,000

-7,000

-6,000

-6,000

-5,000

-5,000

-4,000

-4,000

-3,000

-3,000

-2,000

-1,000

0

1,000

-2,000

-1,000

0 1,000

2,000

3,000

-2,000

-1,000

0

0 0

0 0 0

1,500

-1,000

2,000

2,000

2,000

2,000

2,000

2,000

2,000

2,000

2,000

2,000

2,000

2,000

2,000

2,000

2,000

2,000

2,000

1,000

1,000

2,000

2,000

2,000

2,000

3,000

3,000

3,000

3,000

3,000

3,000

3,000

3,000

3,000

4,000

4,000

4,000

4,000

4,000

4,000

4,000

4,000

4,000

4,000

3,000

3,000

3,000

4,000

4,000 4,000

4,000

3,000

4,000

Base modified from U.S. Geological Survey digital data, various scales. Coordinate system: 

State Plane, Washington South, FIPS 4602: Projection: Lambert Conformal Conic. Horizontal 

datum: North American Datum 1983, Vertical datum: North American Vertical Datum of 1988.

122° 120° 118° 116°

48°

47°

46°

45°

Study

area

WASHINGTON

OREGON IDAHO

Older Bedrock unit—top of unit buried 

under land surface

Older Bedrock unit—top of unit at land 

surface

2,000 Contour of top of Older Bedrock unit—

units in feet relative to datum. Contour 

line bold at 2,000-foot interval.

Fault affecting distribution of the Older 

Bedrock unit

Extent of the Columbia River Basalt 

Group used for mapping

EXPLANATION
Hydraulic Characteristics 25

Hydraulic Characteristics

The ability of a groundwater-flow system to store 

and transmit groundwater (its hydraulic characteristics) 

determines how it functions and is a major control on water 

availability. Knowledge of the hydraulic characteristics is 

necessary to evaluate how the flow system responds to stresses 

such as pumpage and to climatic variations as represented 

by variations in recharge, irrigation, and streamflow. 

These characteristics include lateral and vertical hydraulic 

conductivity and the storage coefficient. Compilations of 

previous estimates of hydraulic characteristics provided 

an overview of the range and median/average for the 

hydrogeologic units, including the Older Bedrock unit 

(Vaccaro and others, 2009; Kahle and others, 2011).

Information presented in this section and summarized 

in table 3 is based on values derived from the calibration of 

the CPRAS model (Ely and others, 2014), a groundwaterflow model for the Yakima River Basin Aquifer System 

(YRBAS) (Ely and others, 2011), and the CP-RASA model 

(Hansen and others, 1994). A detailed discussion of hydraulic 

characteristics is presented in Ely and others (2014). Hydraulic 

characteristics of the Older Bedrock unit are not discussed 

because it typically has as much as five orders of magnitude 

smaller values of porosity and permeability than the other 

units (Vaccaro and others, 2009; Kahle and others, 2011).

Lateral hydraulic conductivity is a measure of the ability 

of a hydrogeologic unit to transmit water laterally. Overburden 

deposits are diverse in lithology and, thus, so are their 

hydraulic characteristics. The estimates of this characteristic 

for the Overburden unit as a whole or its subdivisions indicate 

that, where saturated, the unit generally is a productive 

aquifer. However, some subdivisions of the Overburden unit 

in the YRBAS function as confining units because of the 

small values of hydraulic conductivity. Values for the Mabton 

and Vantage Interbeds indicate that they generally function 

as semiconfining to confining units, which affects water 

availability by limiting flow into and out of the overlying/

underlying basalt unit. The values for the basalt units vary 

widely (table 3), indicating the heterogeneous nature of these 

units. The estimated values show that most of the groundwater 

supply to wells is from interflow zones and that the basalt 

flow interiors transmit much less water laterally with flow 

predominantly vertical through fractures. Small values 

derived for anticlines and other low-permeability flow barriers 

indicate their ability to compartmentalize the flow system 

and, thus, limit groundwater availability in some areas. These 

features are major controls on the flow system, especially 

in the Yakima Fold Belt (fig. 1), and have been documented 

on the Columbia Plateau in early studies (Newcomb, 1969). 

Faults in low-lying areas can impede lateral flow and increase 

vertical flow.

Vertical hydraulic conductivity is a measure of the 

ability of a geologic material to transmit water vertically 

(the impedance to downward or upward flow). It is a major 

control on the movement of water in the flow system by 

affecting vertical hydraulic gradients, and, therefore, flow 

rates into, within, and out of units—the water budget for the 

aquifer system. The range in values for the Overburden unit 

(table 3) shows that, in some locations, water will readily be 

transmitted vertically and, in other locations, it can impede 

vertical flow. Where flow is impeded, the amount of water 

that can move into the deep basalt part of the flow system 

is limited, which, in turn, limits groundwater availability. 

Vertical hydraulic conductivity values for the Mabton and 

Vantage Interbeds indicate that the interbeds not only limit 

lateral movement of water but also vertical movement, adding 

to the confining nature of these units. Values for the basalt 

units show that lateral flow dominates over vertical flow, and 

that flow interiors impede flow. Even so, for the CPRAS as 

a whole, the net vertical movement would be large because 

the quantity moving vertically equals the area, which is very 

large, multiplied by the vertical hydraulic conductivity and 

the vertical gradient. Locally, vertical hydraulic conductivity 

can be large, as shown by the rapid and large water-level rises 

in the Wanapum unit in response to recharge from irrigation 

supplied by the CBP.

The storage coefficient is a measure of the ability of a 

unit to store and release water, and is defined as the volume of 

water that a unit will absorb or release from storage per unit 

surface area per unit change in head. The storage properties 

of the CPRAS materials are important for understanding 

groundwater availability and, ultimately, sustainability. The 

water table to confined aquifers in the unconsolidated to 

consolidated materials (the Overburden) has values that range 

from 0.01 to 0.000025 per foot (ft-1) (Ely and others, 2011; 

Ely and others, 2014). Where the water table is present in 

the basalt units, values averaged 0.025 ft-1. Otherwise, values 

ranged from about 0.0007 to 0.000003 ft-1. Ely and others 

(2011) also estimated that the flow interiors of basalt flows 

had values that were one order of magnitude less than the 

interflow zones.
26 Groundwater Availability of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

Table 3. Hydraulic characteristics of hydrogeologic units, derived from regional groundwater-flow models, of the Columbia Plateau 

Regional Aquifer System, Washington, Oregon, and Idaho.

[Median: Only median values for vertical hydraulic conductivity were reported for the CP-RASA model. Abbreviations: CPRAS, Columbia Plateau Regional 

Aquifer System (Ely and Burns, 2014); YRBAS, Yakima River Basin Aquifer System Analysis (Ely and others, 2011); CP-RASA, Columbia Plateau Regional 

Aquifer System Analysis study (Hansen and others, 1994); –, not applicable]

Model YRBAS CPRAS CP-RASA

Hydrogeologic unit Lateral hydraulic conductivity, in feet per day

Minimum Mean Maximum Constant Minimum Mean Maximum Minimum Mean Maximum

Overburden1 1.46 203 1,672 – 3.05 96.0 200 0.04 98.00 1,100

Saddle Mountains, interflow 

zones

35.8 119 261 – – – – – – –

Saddle Mountains, flow interiors 0.0005 0.0017 0.0036 – – – – – – –

Saddle Mountains, effective 

value2

– 10 – – 1.07 14.4 27.9 0.20 1.30 2.60

Mabton Interbed 0.36 0.88 1.57 – 1.07 14.1 27.9 – – –

Wanapum, interflow zones 25.7 130 278 – – – – – – –

Wanapum, flow interiors 0.00007 0.00035 0.00075 – – – – – – –

Wanapum, effective value2 – 13 – – 1.07 13.0 27.9 0.10 3.20 8.00

Vantage Interbed – – – 9.31 1.07 12.6 27.9 – – –

Grande Ronde, interflow zones 4.28 23.0 91.0 – – – – – – –

Grande Ronde, flow interiors 0.00008 0.0004 0.0015 – – – – – – –

Grande Ronde, effective value2 – 2.3 – – 1.07 10.5 27.9 0.10 2.30 9.00

Hydrogeologic unit Vertical hydraulic conductivity, in feet per day

Minimum Mean Maximum Constant Minimum Mean Maximum Minimum Median Maximum

Overburden1 0.001 1.81 15.1 – 0.031 0.96 2.0 1.40E-07 2.00 1,400

Saddle Mountains, interflow 

zones

0.008 0.03 0.06 – – – – – – –

Saddle Mountains, flow interiors 0.003 0.009 0.02 – – – – – – –

Saddle Mountains, effective 

value2

– 0.01 – – 0.0001 0.003 0.005 – – –

Mabton Interbed 0.002 0.006 0.01 – 0.0001 0.0001 0.0001 – – –

Wanapum, interflow zones 0.05 0.25 0.5 – – – – – – –

Wanapum, flow interiors 0.00004 0.0002 0.0005 – – – – – – –

Wanapum, effective value2 – 0.025 – – 0.0001 0.001 0.005 – – –

Vantage Interbed – – – 0.02 0.0001 0.0001 0.0001 – – –

Grande Ronde, interflow zones 0.002 0.01 0.04 – – – – – – –

Grande Ronde, flow interiors 0.0002 0.001 0.005 – – – – – – –

Grande Ronde, effective value2 – 0.002 – – 0.0001 0.0002 0.005 – – –

All basalt units3 – – – – 0.00005 0.0013 7.17

1

Overburden unit modeled as one unit with spatially varying values in the CPRAS and CP-RASA models; unit was subdivided into 20 subunits in the YRBAS 

model.

2

Effective values based on thickness weighted values for interflow zones and flow interiors.

3

Range in the vertical hydraulic conductivity was reported for all the basalt units and not individual units for the CP-RASA model.
Groundwater-Flow Model 27

Hydrologic Toolbox

New tools were developed as part of the study and 

integrated into a hydrologic toolbox. These tools allow for 

viewing the hydrogeologic setting of the CPRAS, and include 

the development of methods for estimating evapotranspiration 

(ET), surface-water use, groundwater pumpage, and recharge 

from irrigation practices. The methods are applicable over 

large regional areas, and are briefly described in this section 

and summarized in Kahle and others (2011).

A web interface tool was developed that allows users 

to explore the hydrogeologic framework of the CPRAS in 

3D by one of two methods: (1) Drawing diagrams of "well 

logs" at any site, or (2) building complete hydrogeologic 

cross sections between multiple sites (Burns and others, 

2011). Senay and others (2007) developed a new Simplified 

Surface Energy Balance (SSEB) method that uses satellite 

data for land-surface temperature to estimate monthly ET. 

The spatial distribution of landscape ET can be used as an 

indicator of vegetation growth in terms of vegetative biomass 

accumulation, which is directly associated with water use 

by plants. Furthermore, ET can be used to estimate the 

spatial-temporal dynamics of the rates and total amounts 

of withdrawal from aquifer systems in irrigated areas and, 

in conjunction with water-balance models, can be used to 

estimate groundwater recharge from both groundwater and 

surface-water irrigation. SSEB uses elevation-corrected 

land-surface temperature data from the Moderate Resolution 

Spectroradiometer (MODIS) sensor to identify “cold pixels” 

in heavily irrigated areas and “hot pixels” in dry barren or 

fallow areas. The ratio of the temperature of each pixel to the 

cold pixel temperature is used to compute ET as a fraction 

of reference ET. Monthly reference ET is computed using 

weather data from the Global Data Assimilation System 

(GDAS), as described in Senay and others (2008). Senay and 

others (2009) applied the SSEB method to the study area for 

1989–2007 (Kahle and others, 2011). This method is described 

in detail in Senay and others (2008) and Senay and others 

(2007, 2008). 

A spatially distributed soil-water balance model 

(SOWAT) that uses simple relations among climatic, soils, 

land-cover, and irrigation data, was developed to compute 

monthly irrigation requirements and surplus moisture available 

for recharge during the irrigation season (Kahle and others, 

2011). Estimates of groundwater pumpage and surface-water 

diversions for irrigation and recharge associated with irrigation 

were then calculated using SOWAT. The SOWAT model 

includes the concepts of climatic water supply (precipitation) 

and climatic water demand (ET), seasonality in climatic water 

supply and demand, soil moisture storage, and irrigation 

practices. The primary purpose of the model is to estimate 

(1) the irrigation demand when climatic water demand 

exceeds climatic water supply and available soil moisture; 

and (2) the surplus moisture available for deep percolation 

below the root zone, which is an estimate of recharge to the 

groundwater system. SOWAT estimates recharge for irrigated 

lands only and that irrigation use is assumed to be the amount 

of water needed for plants and soil moisture; for surface 

water, it is the irrigation use, not the diversion quantity. The 

model is written in Python and reads all spatial data directly 

from raster files (L.L. Orzol, U.S. Geological Survey, written 

commun., April 2010). Data required are land cover and 

soil properties, spatial-temporal dynamics of climate, and 

irrigation practices.

Although not strictly part of the toolbox, the 

groundwater-flow model constructed as part of this study is 

an important hydrologic tool. 

Groundwater-Flow Model

Groundwater flow for part of the CPRAS was simulated 

using a 3D finite-difference numerical model (USGS 

MODFLOW-NWT; Niswonger and others, 2011). The 

model was constructed, calibrated, and documented as part 

of this study (Ely and others, 2014). Summarized here are 

the (1) attributes of the model, (2) model calibration process 

and results, and (3) model scenarios simulated. Results of 

model simulations for the scenarios are integrated in several 

subsequent sections of this report. In comparison to the 

CP-RASA model, the CPRAS model has a finer vertical 

spatial discretization, uses a more detailed depiction of 

the subsurface hydrogeology, and, unlike the steady-state 

CP-RASA model, it simulates annual water budgets on an 

annual time step for 1920 (approximate representation of 

predevelopment conditions) through 2007.

Model Attributes

The CPRAS model includes about 32,796 mi2

(74 percent) of the 44,070 mi2 study area. A large range 

in land-surface altitudes is included in the model (fig. 9). 

The boundaries of the CPRAS model are nearly identical 

to the boundaries of the CP-RASA model, which included 

32,688 mi2 (less than one-half of one percent difference 

in area), thus allowing for a consistent assessment of the 

response of the system to stresses over time. The boundaries 

differ slightly because a more detailed representation of 

no-flow boundaries of the system was completed during 
28 Groundwater Availability of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

Figure 9. Lateral boundary of the groundwater-flow model, Columbia Plateau Regional Aquifer System, Washington, Oregon, 

and Idaho.

watac11-0648_fig09

14,410

20

Snake

River

Crab Creek

Columbia

River

COLUMBIA

Naches River

Yakima River Umatilla River

Columbia

River

RIVER N Fk John Day River

Snake River

Imnaha River

Banks

Lake

Coeur

D'Alene

Lake

Grande Ronde River

Sa ml on River

John D ay River

Moses Coulee

Walla Walla River

Deschutes River

Klickitat River

Clearwater

River

Hangman Cr

F.D. Roos evelt Lake

Palouse River

Satus Cr

Toppenish Creek

Spokane

River

Potholes 

Reservoir

Moses

Lake Ellensburg

Umatilla

Yakima

Heppner

MiltonFreewater

Pasco

Lewiston

Moscow

Spokane

The

Dalles

La Grande

Walla

Walla

B L U E M O U N T A I N S

CASCADE RANGE

OKANOGAN HIGHLANDS

CASCADE RANGE

WALLOWA

MOUNTAINS

R O C K Y M O U N T A I N S 

R O C K Y M O U N T A I N S

HORSE HEAVEN HILLS

BEEZLEY HILLS

W A S H I N G T O N

O R E G O N

I D A H O

EXPLANATION

Groundwater-flow model boundary

Elevation, in feet. Datum is

North American Vertical 

Datum of 1988

122° 120° 118° 116°

48°

47°

45°

0 20 40 60 80 MILES

0 20 40 60 80 KILOMETERS

Base modified from U.S. Geological Survey digital data, 1:2,000,000, 1972. Coordinate system: 

State Plane, Washington South, FIPS 4602: Projection: Lambert Conformal Conic. Horizontal 

datum: North American Datum 1983, Vertical datum: North American Vertical Datum of 1988.

Model

area

WASHINGTON

OREGON IDAHO
Groundwater-Flow Model 29

this study. The model domain was represented by a grid of 

126 rows and 131 columns made up of 3-km (9,842.5-ft) cells. 

Each of the hydrogeologic units was subdivided (table 4), 

resulting in a total of 100 model layers generally 100 ft thick. 

The no-flow lower model boundary at the bottom of model 

layer 100 simulates the top of the Older Bedrock unit. All 

lateral boundaries were designated as no-flow boundaries 

based on the low permeability of the Older Bedrock unit and 

major mountain divides. 

Internal boundary conditions (fig. 10) include flow to 

and from the major rivers and groundwater discharge to small 

upland streams, groundwater-controlled surface-water features 

such as lake complexes, and drains in agricultural areas. 

Although not strictly a boundary, flow barriers were assigned 

along the major rivers to prevent flow across the rivers from 

model layers that were above the river. Major geologic 

structures (particularly anticlines), which are an important 

control on the flow system, were simulated with flow barriers 

comprising 38,308 model cells (fig. 11). 

Model Fit

The model was calibrated to conditions representing the 

flow system during two periods—estimates of predevelopment 

conditions (steady state approximated for climatic conditions 

in 1920) and existing conditions (1920–2007). The initial 

steady-state model was calibrated primarily by using 

automated parameter estimation techniques (Doherty, 2010; 

Doherty and Hunt, 2010). The steady-state model included 

10,525 observed groundwater levels. The transient model was 

calibrated manually using 46,460 water-level measurements. 

Base flow at selected sites also was analyzed during the 

transient calibration. The relation between measured and 

simulated hydraulic heads (water levels) (fig. 12) indicates 

that the model captures the large variation and range (more 

than 4,000 ft) in water levels throughout the system. An 

analysis of the differences between measured and simulated 

water levels (fig. 13) indicates that the average difference was 

Table 4. Subdivision of hydrogeologic units by model layers, Columbia Plateau Regional Aquifer System, Washington, Oregon, and 

Idaho.

[From Ely and Burns (2014). ft, foot]

Geologic model units 

(Burns and others, 2011)

Groundwater-flow 

model layers Description

Overburden 1–45 Approximately 100-ft layers between the uppermost rock unit and land surface.

Saddle Mountains 11–18 Approximately 100-ft layers between the geologic model top of Mabton Interbed and the

geologic model top of Saddle Mountains Basalt. Cells are constructed from the bottom up,

with the lowest layer being the most laterally extensive.

Mabton Interbed 19–35 Equally spaced layers between the geologic model bottom of Saddle Mountains Basalt and

the next unit rock unit below.

Wanapum Basalt 20–34 Approximately 100-ft layers between the geologic model top of Vantage Interbed and the

geologic model top of Wanapum Basalt. Cells are constructed from the bottom up, with the

lowest layer being the most laterally extensive.

Vantage Interbed 35 Equally spaced layers between the geologic model bottom of Wanapum Basalt and the next

rock unit below.

Grande Ronde Basalt 36–100 Approximately 100-ft layers between the geologic model top of Older Bedrock and the

geologic model top of Grande Ronde Basalt. Cells were constructed from the top down,

using a trend surface for top of Grande Ronde Basalt as a guide surface, allowing the

representation of river and stream incision exceeding model cell thickness.

Older Bedrock No flow
30 Groundwater Availability of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

Figure 10. Locations of the river and drain cells, and the groundwater-flow model boundary, Columbia Plateau Regional 

Aquifer System, Washington, Oregon, and Idaho (from Ely and others, 2014).

tac11-0648_fig10

Pasco

Yakima

Moscow

Pullman

Spokane

Lewiston

Wenatchee

The

Dalles

Ellensburg

Walla

Walla

Moses

Lake

EXPLANATION

River cell locations

Drain cell locations

Groundwater-flow model boundary

121° 120° 119° 118° 117°

48°

47°

46°

45°

John Day

River Deschutes River

Palouse River

Yakima River

Klickitat River

COLUMBIA

RIVER

Spokane

River

Snake River

Lake

Coeur

d'Alene

COLUMBIA RIVER

Umatilla River

Walla Walla R

W A S H I N G T O N

O R E G O N

I D A H O

Model

area

WASHINGTON

OREGON IDAHO

Base modified from U.S. Geological Survey digital data, 1:2,000,000, 1972. Coordinate system: 

State Plane, Washington South, FIPS 4602: Projection: Lambert Conformal Conic. Horizontal 

datum: North American Datum 1983, Vertical datum: North American Vertical Datum of 1988.

0 20 40 60 80 MILES

0 20 40 60 80 KILOMETERS
Groundwater-Flow Model 31

watac11-0648_fig11

Ellensburg

Umatilla

Walla

Walla

Yakima

Heppner

MiltonFreewater

Pasco

The

Dalles

La Grande

Lewiston

Moscow

Spokane

B L U E M O U N T A I N S

CASCADE RANGE

OKANOGAN HIGHLANDS

CASCADE RANGE

WALLOWA

MOUNTAINS

R O C K Y M O U N T A I N S 

R O C K Y M O U N T A I N S

HORSE HEAVEN HILLS

BEEZLEY HILLS

W A S H I N G T O N

O R E G O N

I D A H O

0 20 40 60 80 MILES

0 20 40 60 80 KILOMETERS

122° 120° 118° 116°

48°

47°

46°

45°

Base modified from U.S. Geological Survey digital data, 1:2,000,000, 1972. Coordinate system: 

State Plane, Washington South, FIPS 4602: Projection: Lambert Conformal Conic. Horizontal 

datum: North American Datum 1983, Vertical datum: North American Vertical Datum of 1988.

Model

area

WASHINGTON

OREGON IDAHO

EXPLANATION

Groundwater-flow model boundary

Modeled geologic structures

Figure 11. Locations of model geologic structures simulated with flow barriers, Columbia Plateau Regional Aquifer System, 

Washington, Oregon, and Idaho (modified from Ely and others, 2014).
32 Groundwater Availability of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

Figure 12. Relation between measured and simulated hydraulic heads (water levels), Columbia Plateau Regional Aquifer 

System, Washington, Oregon, and Idaho (from Ely and others, 2014).

tac11-0648_fig12

1,000

2,000

3,000

4,000

5,000

6,000

7,000

1,000 2,000 3,000 4,000 5,000 6,000 7,000

Measured hydraulic head, in feet

Simulated hydraulic head, in feet

0

0

1:1 line

EXPLANATION

Grande Ronde

Wanapum

Saddle Mountains

Overburden
Groundwater-Flow Model 33

small. Simulated levels generally were equally distributed 

between over- and under-prediction. The spatial distribution 

of the differences indicated some areas with a bias toward 

over- or under-simulation. The measurements compared to 

simulated values generally fall along a straight line with a 

slope of nearly 1. As shown in figure 13, the magnitude of 

the differences increases with the deeper units; part of these 

differences is due to the large variations in water levels in the 

uplands over the spatial dimension of the model cells. For 

example, water levels may vary by more than 1,000 ft across a 

cell in the steep uplands, and the model calculates an average 

value for a cell.

Figure 13. Difference between measured and simulated water levels, Columbia Plateau Regional 

Aquifer System, Washington, Oregon, and Idaho (after Ely and others, 2014).

-1,000 

-500 

0 

500 

1,000 

1,500 

Overburden Saddle Mountains Wanapum Grande Ronde 

Unit 

Difference, in feet

EXPLANATION 

Vertical line—Line from

box extends to the limit 

of data

75th percentile

Median

25th percentile

Semiquartile range
34 Groundwater Availability of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

Model Simulations 

Four scenarios were simulated with the model. The first 

scenario simulated predevelopment conditions (steady state 

for 1920) using estimates of recharge without the influence 

of human development, and without groundwater pumpage. 

The second scenario simulated the period from 1920 to 2007 

using an annual time step. The water budget and groundwaterlevel differences between the predevelopment and 1985–2007 

(existing conditions) simulations were used to represent the 

best estimate of the cumulative effects of human development 

on the CPRAS. The third scenario simulated annual values 

from 2008 through 2050, assuming that the pumpage in 2007 

continued through 2050, but used average recharge from 2000 

to 2007 in order to best represent the potential future recharge 

that accounted for the most current estimates of recharge 

with their attendant large interannual variability. Thus, this 

simulation represents a long-term equilibrium condition under 

2007 management practices with 2007 average recharge. 

The fourth scenario modified the equilibrium conditions 

(third) scenario to account for potential increased pumpage 

(13 percent) under projected temperature increases with 

climate change. 

Predevelopment Water Budget

An estimate of the predevelopment water budget 

is needed to understand the changes in the hydrologic 

system owing to human activities and, thus, to assess water 

availability in general and groundwater availability in 

particular. Predevelopment conditions represent the hydrologic 

system before appreciable land-use changes, especially 

agriculture. The predevelopment hydrologic system for 

the complete study area has six interrelated water-budget 

components that describe the landscape hydrologic budget. 

These components are:

1. Precipitation,

2. Snowpack,

3. Surface water,

4. ET,

5. Recharge, and

6. Groundwater.

Lack of information precludes such estimates for snowpack; 

therefore, snowpack for an average year (2006) is used as 

a surrogate for comparison with the other components. The 

magnitude and spatial and temporal variability of the six 

water-budget components govern the flux and storage of water 

and affect its availability throughout the study area. Except 

for snowpack and groundwater storage, the predevelopment 

water-budget components estimated in this study are for 

climatic conditions during 1895–2007 and 1985–2007, 

assuming that the climate is the same for areas with native 

vegetation as for areas of human activities, such as agricultural 

lands and cities. The 1895–2007 results highlight the close 

relation between long-term conditions and the 1985–2007 

period. For both periods, the predevelopment water budget 

represents climatic conditions during those periods but without 

human influences and allows for a comparison with human 

effects on the budget. The predevelopment water-budget 

components for the CPRAS (table 5) are generalized estimates 

because of the lack of information for the predevelopment 

period. A detailed accounting of the water-budget components 

for existing (developed) conditions (1985–2007, the waterbudget accounting period used for much of this study) is 

presented in the section, “Existing-Conditions Water Budget.”

Precipitation

Precipitation is a major component of the water budget 

(table 5); ultimately, all water in the study area is derived from 

precipitation, either as rainfall or snowmelt. As noted in the 

section, “Climatic Setting,” and figure 2, the average annual 

precipitation in the study area during 1895–2007 was about 

17 in. or 55,000 ft3/s (about 40 MAF). During 1985–2007, 

average annual precipitation was about 17.2 in. or 55,800 ft3/s 

(about 40 MAF). In recent decades, the water-year (October 

1 through September 30) precipitation ranged from 12.3 in. 

(in 2001) to 24.4 in. (in 1997), indicating the large interannual 

variability that can potentially translate to a large effect on 

water availability. Precipitation in the study area also shows 

extensive spatial variability, as indicated in figure 2. For 

example, during the drought of 2005, precipitation ranged 

from 3.5 in/yr near Richland, Washington, to 89 in/yr in the 

Cascade Range in the northwestern part of the study area.

Snowpack

Water that is stored as snow is considered the important 

“natural” storage reservoir for water availability, in contrast 

to water stored in reservoirs formed by dams. The snowpack 

generally accumulates during the late autumn through early 

spring, and melts during late winter to early summer. The 

dates for snow accumulation and snowmelt vary spatially 

and by type of climatic year. The larger streams that traverse 

the study area (such as the Columbia, Snake, Clearwater, 

Salmon, Spokane, John Day, Deschutes, and Yakima Rivers) 

emanate from outside the study area and are fed mostly by 
Predevelopment Water Budget 35

snowmelt. These streams provide water for multiple uses in 

the study area, and those uses generally have senior rights to 

groundwater uses. The snowpack throughout the Columbia 

River Basin (not just the study area), therefore, is an important 

source and store of water, especially because most of the study 

area receives little precipitation during the summer.

The spatial distribution of the snow water equivalent 

(SWE, the amount of water in the snowpack) is for April 1 

(National Operational Hydrologic Remote Sensing Center, 

2004), which generally is considered the date of near 

maximum snow accumulation, and is the primary date for 

making the important water-supply forecasts for the runoff 

season. April 1 typically is the start date when most surfacewater and groundwater irrigators can use irrigation water 

based on their water rights. In an average water year such as 

2006, the April 1 SWE in the study area was only about 0.9 in. 

(2 MAF, about 5 percent of the average precipitation during 

1985–2007; table 5), and spatially ranged from 0.04 to 52.4 in. 

The 2006 SWE averaged 11 in. for the 9 percent of the study 

area that was covered in snow, primarily the Blue Mountains, 

indicating that the snowpack in the study area mainly feeds 

streams emanating from the Blue Mountains. Most other 

streams in the study area are supported by snowpack from 

outside the study area, and the quantity of streamflow 

traversing the study area is orders of magnitude greater than 

the SWE stored within it. For example, the annual average 

discharge in water year 2006 for the Yakima River at Horlick 

(where the Yakima River enters the study area) was about 

26 in. (Bureau of Reclamation, 2012). 

The large interannual variability in the quantity and 

spatial distribution of SWE in the study area is shown in the 

Blue Mountains at the Touchet Snow Telemetry (SNOTEL) 

site at an altitude of 5,530 ft (National Resources Conservation 

Service, 2012) (fig. 14A). The daily SWE for the wettest (large 

snow accumulation) and driest (small snow accumulation) 

years in comparison to the average year shows the inherent 

large interannual variability, from less than 10 in. in 2005 to 

more than 60 in. in 1997 (fig. 14B). In turn, the amount of 

streamflow generated, water availability, and groundwater 

recharge is directly related to the SWE and varies greatly 

between wet and dry years, as shown by the annual average 

discharge measured at USGS streamgages at three streams that 

drain the Blue Mountains:

Stream

Discharge, in cubic 

feet per second 

Water year 

1997

Water year 

2005

Walla Walla River near Touchet 1,062 283

Tucannon River near Starbuck 300 84

Mill Creek near Walla Walla 180 63

Table 5. Generalized average annual water-budget components for predevelopment and existing conditions, Columbia Plateau 

Regional Aquifer System, Washington, Oregon, and Idaho, for selected years, 1895–2007.

[All values in millions of acre-feet; values rounded; Snowpack is April 1 snowpack for average water year; Recharge is water leaving the roots zone. 

Streamflow is discharge leaving the study area as measured by the Columbia River at The Dalles. –, not available]

Predevelopment conditions 

averaging period Prior to human 

activities

Existing conditions 

averaging period After human 

activities

1895–2007 1985–2007 2006 1985–2007 2006

Precipitation 40 40 40

Snowpack1 – – 2 – 2

Streamflow2 144 137 125

Evapotranspiration 22 24 28

Recharge 11 11 14

Water use 0 0 6

Groundwater storage3

Overburden unit 84 94

Basalt units 800 800

1

Snowpack is shown for an average representative year; spatially distributed values are not available for full period.

2

Streamflow is represented by the discharge for the Columbia River at The Dalles, which accounts for most of the streamflow leaving the study area.

3

Groundwater storage for predevelopment conditions is approximated based on the total volume of the Overburden unit, and for the basalt units the 

values are estimated based on the upper 500 feet of saturation. Values provide information on the relative magnitude of this “storage reservoir” compared to 

the other water-budget components.
36 Groundwater Availability of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

tac11-0648_fig14a-b

0 October November December January Febuary March April May June July August September

Water year

10

20

30

40

50

60

70

0

10

20

30

40

50

60

70

1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010

Calendar year

Snow water equivalent, in inches Snow water equivalent, in inches

B

A

EXPLANATION

1997

2005

1971–2000 climate normal

April 1 snow water equivalent

Average

EXPLANATION

April 1 snow water 

equivalent—dashed 

where data are missing

Figure 14. Snow water equivalent for (A) April 1 values for 1981–2009, and (B) daily values for a wet water year (1997), 

a dry water year (2005), and the 30-year average (1971–2000), Touchet Snow Telemetry (SNOTEL) site, Blue Mountains, 

Washington.
Predevelopment Water Budget 37

Surface Water

The study area contains an extensive system of rivers, 

creeks, and lakes (figs. 1 and 10), almost all of which 

are fed by snowmelt. Many of the streams support some 

species or stock of salmonids, including ESA-listed species. 

Streamflow magnitude and its interannual variability have 

a direct effect on groundwater availability and use. More 

than 7,000 ft3/s are diverted annually for irrigation in the 

study area, and groundwater withdrawals can affect these 

more senior diversions. A brief overview of surface-water 

resources is provided in this section because of its importance 

in understanding the regional hydrologic budget; a more 

detailed assessment was published for the CP-RASA (Nelson, 

1991). The emphasis of this section is on providing readers 

with an overview of the quantities of surface water entering, 

passing through, and leaving the study area, and the relation 

of these quantities to the spatial and temporal variations in 

climate across the area. The surface-water information is 

presented with respect to water-year values because they are 

the commonly used and referenced values. For the streams 

in the study area, annual differences between water year and 

calendar year values typically are less than 3 percent.

Streamflow generated in the study originates from three 

areas. The first area is the Blue Mountains, and the mean 

annual discharge from streams emanating from this area is 

about 1.24 MAF (1,715 ft3

/s).These streams discharge to both 

the Snake and the Columbia Rivers. Additionally, two streams 

(the Grande Ronde and Imnaha Rivers) originate from outside 

the area in the Wallowa Mountains and from inside the study 

area in the Blue Mountains. These two streams provide about 

0.74 MAF (1,023 ft3/s) to the Snake River. The second area is 

downstream of The Dalles, and three rivers contribute a mean 

annual discharge of 2.6 MAF (3,650 ft3/s ), but, excluding the 

Klickitat River, most of that discharge originates from outside 

the study area. The third area where streamflow is generated 

in the study is the Rocky Mountains in the eastern part of 

the study area. The Clearwater River is the largest stream, 

with a mean annual flow of about 10.8 MAF (14,862 ft3/s ), 

and its discharge to the Snake River is used by Snake River 

water users. The Salmon River contributes about 30 percent 

of the flow of the Snake River, but most of its drainage area is 

outside the study area. Thus, excluding the Clearwater River, 

about 4.58 MAF of streamflow is partly generated in the study 

area (about 20 to 30 percent of this quantity is derived from 

outside the area).

The different climatic regimes in the study area produce 

a runoff season with different monthly relations (fig. 15A, B), 

but indicates that about 70–80 percent of the total tributary 

runoff to the Columbia and Snake Rivers occurs from January 

through June. The long-term average percentage of total 

runoff, accumulated by month, for three of the four largest 

tributary inflows (Spokane, Salmon, and Yakima Rivers) in the 

study area and for the Klickitat and John Day Rivers is shown 

in figure 15A. Runoff for these five rivers is representative 

of different climatic regimes in the study area, with the John 

Day and Klickitat Rivers representative of the central Blue 

Mountains and the western extent of the CPRAS, respectively. 

The average monthly discharge for these rivers (fig. 15B) 

also indicates the variations in streamflow owing to different 

climatic regimes.

Streamflow entering and leaving the study area can be 

approximated by the streamflow for the Columbia River 

at Grand Coulee Dam and at The Dalles, respectively. 

Streamflow at the USGS Grande Coulee Dam streamgage 

includes the contribution from the Spokane River (average 

annual discharge of about 6,600 ft3

/s). Streamflow at The 

Dalles accounts for all the Columbia River tributary flows 

in the study area except the contribution from the Hood, 

Klickitat, and White Salmon Rivers, which have a total 

average annual discharge of about 3,650 ft3/s (about 2 percent 

of the discharge at The Dalles). The Columbia River at 

The Dalles accounts for about 82 percent of the river’s 

total drainage area of 258,000 mi2. During 1879–2007, the 

average annual discharge in the Columbia River at The 

Dalles, based on water years and calendar years, differed by 

only 120 ft3/s. During 1895–2007 and 1985–2007, Columbia 

River streamflow at The Dalles averaged 186,700 ft3/s 

(135 MAF) and 172,700 ft3/s (125 MAF), respectively. In 

Washington State, water in the Columbia River in the study 

area is considered fully allocated during the dry season 

(July– September), and as of 2011, additional dry-season water 

(88,000 acre-ft for non-interruptible diversionary uses and 

44,500 acre-ft for instream uses) is obtained from increased 

drawdown from FDR (Washington State Department of 

Ecology, 2012a). Future ‘new’ water in quantities greater 

than the drawdown quantities would need to be derived 

from new storage facilities (Washington State Department of 

Ecology, 2012b).
38 Groundwater Availability of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

For water years 1914–2002 (the longest period for which 

data were available for both sites), average annual streamflow 

at Grand Coulee Dam and The Dalles was 107,000 and 

181,000 ft3/s, respectively. Thus, streamflow at Grand Coulee 

Dam accounts for about 59 percent of the streamflow at The 

Dalles. The Snake River accounts for about 29 percent of the 

streamflow at The Dalles, and the remaining contributions 

(about 12 percent) are primarily accounted for by the larger 

tributaries, such as the Yakima River. Streamflow generated in 

the study area from smaller rivers and creeks contributes little 

to the total outflow. However, this streamflow is extensively 

used and relied upon for multiple uses. These percentage 

Figure 15. (A) Monthly percentage of average annual discharge accumulated over a water year, and (B) average 

monthly discharge for five rivers in different climatic regimes of the Columbia Plateau Regional Aquifer System study area, 

Washington, Oregon, and Idaho, period of record 1911–2011. Source: U.S. Geological Survey National Water Information 

System, accessed September 2, 2012, at http://waterdata.usgs.gov/nwis.

tac11-0648_fig15a-b

October November December January Febuary March April May June July August September

Water year

October November December January Febuary March April May June July August September

Water year

Average monthly discharge, in cubic feet per second

B

A

EXPLANATION

Spokane River

Salmon River

Yakima River

Klickitat River

John Day River

0

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Accumulated total average annual discharge, in percent 

5,000

10,000

15,000

20,000

25,000

30,000

35,000

40,000
Predevelopment Water Budget 39

contributions to streamflow at The Dalles vary spatially and 

temporally because in some years, parts of both the study area 

and the Columbia River Basin may be drier than normal and 

other parts may be wetter than normal (McDonald and Riggs, 

1948). The two largest controls on total streamflow entering 

the study area and its interannual variability are climatic 

conditions in the Columbia River drainage area upstream 

of Grand Coulee Dam and the Snake River drainage area 

upstream of Clearwater, Idaho. Reservoirs and diversions 

affect the temporal variability but do not have a large effect on 

total streamflow.

Much of the measured streamflow at The Dalles 

represents regulated conditions and water depletions that 

are primarily due to ET losses (referred to as “streamflow 

depletions”) from irrigated lands and water bodies. Most 

irrigation water that is not lost to ET returns to the streams 

as either direct surface-water return flows or groundwater 

discharge; this discharge contributes to downstream irrigation 

withdrawals. Simons (1953) estimated these depletions 

at The Dalles to be about 5 percent of the natural average 

annual streamflow in 1946, with 70 percent of the depletion 

occurring in the Snake River Basin (primarily in Idaho outside 

the study area). Simons also estimated that, in 1946, about 

850,000 irrigated acres were in the study area. After 1946, 

numerous reservoirs were built and surface-water withdrawals 

increased with a concomitant increase in irrigated croplands. 

By 2007, an estimated 2 million acres were irrigated in the 

study area (Kahle and others, 2011). This information suggests 

that irrigated lands increased by more than two-fold, further 

suggesting that 2007 streamflow at The Dalles has been 

depleted from irrigation water use by as much as 10 percent of 

the natural average annual streamflow. This estimate is similar 

to the Bonneville Power Administration estimate of about 

11 percent depletion at McNary Dam, about 120 mi upstream 

of The Dalles (Northwest Council, 2013).

Although water-management activities and use of water 

have changed the streamflow hydrograph, it is useful to 

compare the monthly average discharge for the Columbia 

River at The Dalles for different years (fig. 16). The largest 

measured annual (and monthly) discharge for the Columbia 

River was in 1894, before major human influences on the river 

system. There was a distinct runoff season in 1894, with the 

peak runoff occurring in June. In contrast, in 1926 (the second 

lowest flow year of record and also a year with minimal 

human effects on the river system), the hydrograph was much 

flatter, with a peak runoff in May. The 1926 hydrograph 

is similar to the 1993 hydrograph (another low-flow year 

but a year that includes most of the current regulation and 

water use); however, there are distinct differences with 

the 1993 hydrograph, which shows the effects of water 

storage and releases for hydropower generation. Overall, 

differences between the hydrographs indicate the complex 

relation between climate and runoff, as further shown by the 

differences between 1984 (a relatively high-flow year) and 

1986 (a median-flow year).

Figure 16. Monthly average discharge for the Columbia River at The Dalles, Washington, for selected years during 

1879–1997.

Monthly average discharge, in cubic feet per second

October November December January Febuary March April May June July August September

Water year

0

200,000

400,000

600,000

800,000

1,000,000

1,200,000

EXPLANATION

Historical yearly discharge, in cubic feet per second

1984—211,500

1986—190,100

1993—155,200

1997—263,700

1879—242,000

1894—313,600

1926—118,100

1948—235,500
40 Groundwater Availability of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

Evapotranspiration

Evapotranspiration is a large but least documented 

component of the water budget. ET can account for 100 

percent of the annual precipitation in the arid parts of the 

study area and 45–70 percent in the more humid uplands 

(Bauer and Vaccaro, 1990; Vaccaro and others, 2009). ET 

was estimated based on several new techniques developed as 

part of this study that were described previously, including 

the new SSEB method. Senay and others (2009) applied the 

SSEB method to the study area during 1989–2007. Kahle and 

others (2011) used the ET estimates for 1989 to approximate 

values for 1985–88, resulting in initial estimated values for the 

period of interest for hydrologic budgeting—1985–2007. For 

consistency, the ET values are described for this period.

The calculated average annual ET values for areas 

without human influence were regressed to average annual 

precipitation for 1985–2007. The regression estimates of ET 

were used in areas of human influence in conjunction with 

the SSEB ET estimates in other areas to derive a distribution 

of ET that was assumed to approximate predevelopment 

conditions. The resulting ET distribution (fig. 17) shows 

the large spatial variability of this water-budget component. 

Average annual ET was about 10.1 in or 33,000 ft3

/s (24 MAF) 

for the study area (table 5), and ranged from 1.7 to 38.3 in. 

(fig. 17). As a percentage of precipitation, ET values varied 

from a low of 10 percent to a high of nearly 100 percent, and 

averaged 59 percent. Using the regression method, average 

annual ET was calculated for 1895–2007 at about 9.5 in. or 

31,000 ft3/s (22 MAF) (table 5). The average ET was about 

55 percent of the precipitation during that period.

Recharge

A major control on groundwater availability (and 

streamflow) is the quantity of water entering the aquifer 

system—groundwater recharge. Importantly, the quantity of 

recharge influences how the flow system responds to pumpage. 

Recharge from infiltration of precipitation was estimated using 

a regression equation on the basis of annual precipitation 

(Kahle and others, 2011). The regression equation was 

developed by Bauer and Vaccaro (1990), who estimated 

groundwater recharge to part of the CPRAS for 1956–77 using 

a deep-percolation model (DPM; Bauer and Vaccaro, 1987) 

that computes daily values of recharge and all other waterbudget components such as soil moisture. Annual recharge 

distributions were computed for 1895–2007 and average 

annual values were calculated for 1895–2007 and 1985–2007 

for comparison with the other water-budget components 

(Kahle and others, 2011).

Estimated predevelopment annual recharge for1895–2007 

averaged 4.7 in. or 15,300 ft3

/s (11 MAF), and ranged from 

nearly 0 to more than 25 in. The recharge for 1985–2007 

(fig. 18, table 5) was nearly the same (4.6 in. or 14,900 ft3

/s—

11 MAF, about 27 percent of the average annual precipitation), 

indicating a similar amount of precipitation during the two 

periods. Recharge (1985–2007) annually ranged from 9.6 to 

25.6 MAF. The large spatial variability of estimated recharge 

represents not only the precipitation distribution but also the 

ET distribution. The low recharge values throughout much 

of the central Columbia Plateau (fig. 18) indicate a potential 

limitation on groundwater availability. Additionally, Bauer and 

Vaccaro (1990) indicated that the regression-based estimates 

in drier parts of the study area are less accurate. Drier areas 

are defined by annual precipitation typically less than 11 in., 

which includes much of the irrigated areas on the Columbia 

Plateau. A comparison of figure 18 with the mapped estimates 

from Bauer and Vaccaro (1990) indicates that, in this study, 

recharge may be overestimated in these areas by about 

5–10 percent.

Aquifer Storage

The aquifer system not only conveys groundwater but 

also stores water. The storage of water in the system directly 

affects groundwater availability. Generalized estimates of 

the water stored in the system provide information on the 

relative magnitude of this “storage reservoir” compared to 

the other water-budget components. The quantities described 

in this section represent generalized estimates of storage in 

the Overburden unit and water stored in the upper, saturated 

500 ft of the basalt units. These estimates do not represent 

extractable water because such extraction would not be 

economically viable or sustainable, and likely would be 

ecologically harmful. Additionally, the volume of groundwater 

in storage is not by itself meaningful in analyses of water 

availability (Alley, 2007); it is presented here for context. 

A simplified method was used to estimate the quantity 

of water stored in the system. For the Overburden unit, the 

estimate is for all water stored in its deposits, and the total 

volume of the deposits was multiplied by 0.1, a conservative 

estimate of the effective porosity that accounts for variations 

in overburden lithology and depth to water. About 84 MAF of 

water in storage was calculated using these assumptions. The 

storage in the CPRAS basalt aquifers was estimated based on 

the storage depth of 500 ft, and an effective porosity of 0.04 

(Hansen and others, 1994; Vaccaro, 1999). The calculated 

storage volume was 800 MAF. Throughout most of the 

study area, this shallow storage wedge of freshwater allows 

seasonal recharge to add storage to the water table and, thus, 

to supply water to streams. These two water storage estimates 

only provide a basis for comparison to the other waterbudget components and are not related to the conveyance of 

groundwater by the CPRAS over the long term, which is on 

the order of billions of acre-feet. 
Predevelopment Water Budget 41

Figure 17. Distribution of average annual evapotranspiration estimated for predevelopment conditions in the Columbia Plateau 

Regional Aquifer System study area, Washington, Oregon, and Idaho, 1985–2007.

tac11-0648_fig17

Potholes 

Res

Moses

Lake

Rocky Ford

Cr.

SNAKE

RIVER

Crab Creek

COLUMBIA

RIVER

Columbia

Naches R

Yakima River

Uma tilla River

COLUMBIA

RIVER

River

N Fk John Day R

SNAKE

RIVER

Imnah a River

Banks

Lake

River

Spokane Coeur

D'Alene

Lake

Grande Ronde R

Salmon River

John Day R

Moses Coulee

Walla Walla River

Deschutes R

Klickitat R

Clearwater

River

Hangman Cr

F.D. Roosevelt La ke

Palouse River

Toppenish Creek

Satus Creek

WASHINGTON

OREGON

IDAHO

Ellensburg

Umatilla

Walla Walla

Yakima

Heppner

MiltonFreewater

Pasco

Lewiston

Moscow

Spokane

The

Dalles

La Grande

Grand Coulee

Dam

B L U E 

 M O U N T A I N S

CASCADE RANGE

OKANOGAN HIGHLANDS

CASCADE RANGE

MOUNTAINS

WALLOWA

R O C K Y M O U N T A I N S 

R O C K Y M O U N T A I N S

HORSE HEAVEN HILLS

BEEZLEY HILLS

Seattle

38.3 (high)

1.7 (low)

Average annual evapotranspiration

 estimated for predevelopment

 conditions, 1985–2007, in inches

EXPLANATION

Study area boundary

0 20 40 60 80 MILES

0 20 40 60 80 KILOMETERS

Base modified from U.S. Geological Survey digital data, various scales. Coordinate system: 

State Plane, Washington South, FIPS 4602: Projection: Lambert Conformal Conic. Horizontal 

datum: North American Datum 1983, Vertical datum: North American Vertical Datum of 1988.

Study

area

WASHINGTON

OREGON IDAHO

122° 120° 118° 116°

46°

48°

47°

45°
42 Groundwater Availability of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

Figure 18. Distribution of average annual recharge from infiltration of precipitation in the Columbia Plateau Regional Aquifer 

System study area, Washington, Oregon, and Idaho, 1985–2007.

tac11-0648_fig18

Potholes 

Res

Moses

Lake

Rocky Ford

Cr.

SNAKE

RIVER

Crab Creek

COLUMBIA

RIVER

Columbia

Naches R

Yakima River

Uma tilla River

COLUMBIA

RIVER

River

N Fk John Day R

SNAKE

RIVER

Imnah a River

Banks

Lake

River

Spokane Coeur

D'Alene

Lake

Grande Ronde R

Salmon River

John Day R

Moses Coulee

Walla Walla River

Deschutes R

Klickitat R

Clearwater

River

Hangman Cr

F.D. Roosevelt La ke

Palouse River

Toppenish Creek

Satus Creek

CASCADE RANGE

BEEZLEY HILLS

WASHINGTON

OREGON

IDAHO

Ellensburg

Umatilla

Walla Walla

Yakima

Heppner

MiltonFreewater

Pasco

Lewiston

Moscow

Spokane

The

Dalles

La Grande

Grand Coulee

Dam

B L U E 

 M O U N T A I N S

OKANOGAN HIGHLANDS

CASCADE RANGE

MOUNTAINS

WALLOWA

R O C K Y M O U N T A I N S 

R O C K Y M O U N T A I N S

HORSE HEAVEN HILLS

Seattle

0.2 to 1 

1.1 to 2

2.1 to 5

5.1 to 10

10.1 to 15

15.1 to 20

Greater than 20

Average annual recharge from precipitation, 

1985–2007, in inches

EXPLANATION

Study area boundary

0 20 40 60 80 MILES

0 20 40 60 80 KILOMETERS

Study

area

WASHINGTON

OREGON IDAHO

Base modified from U.S. Geological Survey digital data, various scales. Coordinate system: 

State Plane, Washington South, FIPS 4602: Projection: Lambert Conformal Conic. Horizontal 

datum: North American Datum 1983, Vertical datum: North American Vertical Datum of 1988.

122° 120° 118° 116°

46°

48°

47°

45°
Existing-Conditions Water Budget 43

Existing-Conditions Water Budget

The existing-conditions hydrologic system has the same 

six interrelated water-budget components (precipitation, 

snowpack, surface water, evapotranspiration, recharge, and 

groundwater) as the predevelopment budget, but includes a 

new component—water use. The components govern the flux 

and storage of water and affect its availability throughout the 

study area. Except for precipitation and snowpack, which 

are assumed to be the same for existing conditions, these 

components are described for 1985–2007 for the complete 

CPRAS study area and the model domain. For the groundwater 

component, generalized values for aquifer storage are 

described for the study area, and for the model domain, a 

detailed water budget (including its temporal variability) of the 

aquifer system is presented on the basis of model results.

Irrigation is the largest use of water in the CPRAS. 

The major pumping centers occur where groundwater is 

the primary source of irrigation, and the areas of greatest 

recharge from irrigation return flow occur where surface 

water is the primary source of irrigation. The pumping and 

irrigation recharge budget components have implications for 

groundwater availability, both regionally and locally, because 

the use of water for irrigation has affected the system by 

increasing ET and recharge, and by decreasing storage owing 

to groundwater-level declines in pumping centers. Increased 

recharge, in turn, has affected (increased) aquifer storage, as 

has groundwater pumpage (decreased storage). The increase 

in aquifer storage has locally increased streamflow, whereas 

the decrease in storage has locally decreased streamflow—for 

example, upper Crab Creek in north-central Washington. Kahle 

and others (2011) showed that the monthly soil-water balance 

in irrigated lands is dominated by the climatic water demand 

of ET and the application of irrigation water to satisfy crop 

water requirements. Precipitation replenishes soil moisture and 

contributes to groundwater recharge in the late autumn and 

winter months (November–January). Some early spring ET 

demand is supplied by soil moisture, but beginning in April, 

irrigators apply sufficient water to maintain more soil moisture

than the maximum allowable depletion. Groundwater recharge 

in the irrigated lands peaks again in the summer with ET 

demand and irrigation delivery and applications, and tapers off 

in the autumn before precipitation increases.

Columbia Plateau Regional Aquifer System

The water-budget components within the complete 

CPRAS are based on Kahle and others (2011). These 

components are summarized in table 5, which also presents the 

predevelopment components. Unless otherwise stated, averages 

are for 1985–2007, and annual average estimates are presented 

in figure 19. Columbia River at The Dalles was scaled in 

figure 19 to show how its interannual variations compare to 

other components. Similarly, several values in figure 20 are 

scaled because the main interest is to show the relation of 

interannual variation between various major streams and the 

actual values would obscure these variations. For example, in 

1997, average discharge in the Columbia River at The Dalles 

exceeded 260,000 ft3

/s, whereas the Hood River in Oregon was 

less than 2,000 ft3

/s (fig. 20).

Figure 19. Annual average values estimated for selected water-budget components, Columbia Plateau Regional Aquifer 

System, Washington, Oregon, and Idaho, 1985–2007.

0

5

10

15

20

25

Values, in inches per year

1988198719861985 1989 199819971996199519941993199219911990 1999 2006200520042003200220012000 2007

Water year

Recharge

Evapotranspiration

Precipitation

EXPLANATION

Discharge at Columbia River at 

The Dalles (value × 0.3)
44 Groundwater Availability of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

Figure 20. Annual average discharge for (A) the Columbia and Snake Rivers, and (B) selected rivers in the study area, 

Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho, 1985–2007.

tac11-0648_fig20a-b

1988198719861985 1989 199819971996199519941993199219911990 1999 2006200520042003200220012000 2007

Water year

50,000

75,000

100,000

125,000

150,000

175,000

200,000

225,000

250,000

275,000

0

1988198719861985 1989 199819971996199519941993199219911990 1999 2006200520042003200220012000 2007

Water year

12,000

A

Annual average discharge, in cubic feet per second 

B

2,000

4,000

6,000

8,000

10,000

Annual average discharge, in cubic feet per second 

Columbia River at The Dalles

Snake River at Anatone (value × 4)

Columbia River at Grand Coulee Dam

Snake + Clearwater Rivers (value × 3)

EXPLANATION

EXPLANATION

Spokane River

John Day River

Salmon River (value × 0.5) Snake River (value × 0.18)

Clearwater River (value × 0.4)

Hood River

Klickitat River

Yakima River

Columbia River at The Dalles 

(value × 0.045)
Existing-Conditions Water Budget 45

Surface Water

Most of the streams traversing the study area are 

regulated and (or) have diversions for irrigation or water 

supply, and streamflow is representative of existing conditions, 

including streamflow depletions. However, the annual 

average discharge generally is representative of the type of 

climatic year (dry to wet—preserving the overall interannual 

variability) and, thus, water-year values (particularly 

interannual variations) are presented and discussed. 

Annual discharge on the Columbia River at The Dalles 

averaged 172,700 ft3/s (125 MAF), which was about 9 percent 

less than the long-term average. At Grand Coulee Dam, which 

had a shorter period of record, annual average streamflow was 

5 percent less. Interannual variability at The Dalles was large, 

ranging from 117,600 ft3/s in 2001 to 263,700 ft3/s in 1997, 

and this range (146,100 ft3/s) was 85 percent of the 23-year 

period average. The large range in values (indicating the 

temporal-spatial variations in SWE throughout the Columbia 

River Basin) affects water use and is correlated to groundwater 

recharge and, thus, water availability in the study area. 

There is general consistency in the temporal variations 

in annual discharge values between streams, but some 

inconsistencies occur. The shapes of the hydrographs for the 

Columbia River at Grand Coulee Dam (measure of inflow to 

the study area) and The Dalles (outflow) generally are similar, 

with annual differences in the shapes of the hydrographs 

for these two sites primarily owing to variations in Snake 

River inflows; for example, Snake River inflows affected 

these differences in 1989, 1993, 1997, and 2006 (fig. 20A). 

Hydrographs for selected rivers (fig. 20B) also show this type 

of spatial variation. The Snake and Salmon Rivers follow 

the same temporal pattern with only a few variations. The 

Salmon River accounts for about 30 percent of the flow of 

the Snake River at Anatone, and the Clearwater River (the 

largest tributary to the Snake River) accounts for an additional 

45 percent increase in the annual flow of the Snake River. 

The other stream hydrographs show that there are distinct 

interannual differences in the shape of hydrographs across 

the study area, especially during 1985–94 when all sites had 

lower average annual discharge than the 23-year average. This 

period contains two multi-year periods (1987–88, 1992–94) 

that were considered droughts for water-supply purposes.

Evapotranspiration

The estimates of ET for the study area (Senay and others, 

2009) provide the existing-conditions information for this 

water-budget component. The estimates were for 1989–2007, 

and the ET estimates for 1989 were used to approximate the 

annual values for 1985–88, producing values for the period of 

interest for hydrologic budgeting, 1985–2007.

The average annual estimate of ET was 11.7 in. or 

38,000 ft3/s (28 MAF), and ranged from about 1 to 38 in. (the 

same range as estimated for predevelopment conditions). 

The spatial distribution of ET (fig. 21) indicates a strong 

correlation to precipitation, but also indicates large differences 

from the estimated predevelopment distribution (fig. 17) 

in areas affected by irrigation. ET is a larger percentage 

of precipitation under existing conditions than under 

predevelopment conditions because of the additional ET from 

irrigated and dryland croplands. The differences are especially 

pronounced in the surface-water irrigated areas, such as in 

the Yakima River Basin and CBP, where increases in ET 

ranged from about 20 to 36 in. because of the consumptive use 

by crops. 

Recharge

Existing-conditions recharge includes the additional 

recharge from surface-water and groundwater irrigation that 

was simulated using the SOWAT model (Kahle and others, 

2011). Average annual recharge was 6.1 in. or 19,800 ft3/s 

(14 MAF), and ranged from nearly zero to more than 40 in. 

During the 23-year period, annual recharge ranged from 4.1 to 

10.9 in. (fig. 19). The increased recharge in the surface-water 

irrigated areas is clearly indicated in the spatial distribution 

of recharge for 2007 (fig. 22). The estimated average annual 

recharge (1985–2007) from irrigation return flow was 

5,800 ft3/s (4.2 MAF), with 50 percent occurring within the 

predominately surface-water irrigated regions—the Yakima 

River Basin, Umatilla River Basin, and CBP (fig. 4). 

Surface-Water Use and Groundwater Pumpage

Surface-water use and groundwater pumpage were 

estimated for the study area on an annual basis for 1985–

2007 (Kahle and others, 2011). Groundwater pumpage 

was estimated for five categories of use: (1) irrigation; 

(2) public water supply (PWS); (3) domestic (exempt wells); 

(4) industrial; and (5) “other,” which includes mining, 

livestock, thermoelectric needs, and fisheries. 

Average annual irrigation water use estimated using 

SOWAT was 5.3 MAF (7,320 ft3

/s) during 1985–2007, 

with 1.4 MAF or 1,935 ft3/s (26 percent) supplied from 

groundwater and 3.9 MAF (5,385 ft3/s) supplied from surface 

water (Kahle and others, 2011). Note that, for surface water, 

this is the irrigation use, which is less than the amount 

diverted. Surface-water use was largest in the CBP, followed 

by the Yakima River Basin and the Umatilla River Basin. 

Annually, irrigation water use ranged from about 4.1 to 

7.1 MAF, and the part supplied by groundwater ranged from 

about 25 to 35 percent. Groundwater pumpage was relatively 

stable during 1985–2007 (fig. 23), as most of the growth 

in pumpage occurred from about 1945 to 1979, when the 

irrigation pumpage estimated for the major pumping centers 

increased from 0.05 to 0.8 MAF (Cline and Collins, 1992). 

Most of the variability in irrigation pumpage during 1985–

2007 is attributable to climatic conditions and not changes in 

irrigated crop areas or crop types.
46 Groundwater Availability of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

tac110648fig21

Potholes 

Res

Moses

Lake

Rocky Ford

Cr.

BEEZLEY HILLS

WASHINGTON

OREGON

IDAHO

SNAKE

RIVER

Crab Creek

COLUMBIA

RIVER

Columbia

Naches R

Yakima River

Uma tilla River

COLUMBIA

RIVER

River

N Fk John Day R

SNAKE

RIVER

Imnah a River

Banks

Lake

River

Spokane Coeur

D'Alene

Lake

Grande Ronde R

Salmon River

John Day R

Moses Coulee

Walla Walla River

Deschutes R

Klickitat R

Clearwater

River

Hangman Cr

F.D. Roosevelt La ke

Palouse River

Toppenish Creek

Satus Creek

Ellensburg

Umatilla

Walla Walla

Yakima

Heppner

MiltonFreewater

Pasco

Lewiston

Moscow

Spokane

The

Dalles

La Grande

Grand Coulee

Dam

B L U E 

 M O U N T A I N S

CASCADE RANGE

OKANOGAN HIGHLANDS

CASCADE RANGE

MOUNTAINS

WALLOWA

R O C K Y M O U N T A I N S 

R O C K Y M O U N T A I N S

HORSE HEAVEN HILLS

Seattle

EXPLANATION

38.3 (high)

1.7 (low)

Average annual evapotranspiration

estimated for existing conditions, 

1985-2007, in inches

Study area boundary

0 20 40 60 80 MILES

0 20 40 60 80 KILOMETERS

Base modified from U.S. Geological Survey digital data, various scales. Coordinate system: 

State Plane, Washington South, FIPS 4602: Projection: Lambert Conformal Conic. Horizontal 

datum: North American Datum 1983, Vertical datum: North American Vertical Datum of 1988.

Study

area

WASHINGTON

OREGON IDAHO

122° 120° 118° 116°

46°

48°

47°

45°

Figure 21. Distribution of average annual evapotranspiration estimated for existing conditions, Columbia Plateau Regional Aquifer 

System, Washington, Oregon, and Idaho, 1985–2007.
Existing-Conditions Water Budget 47

Figure 22. Distribution of annual recharge estimated for existing conditions, Columbia Plateau Regional Aquifer System, 

Washington, Oregon, and Idaho, 2007 (from Kahle and others, 2011).

watac11-0581_fig22

Potholes 

Res

Moses

Lake

Rocky Ford

Cr. WASHINGTON

OREGON

IDAHO

SNAKE

RIVER

Crab Creek

COLUMBIA

RIVER

Columbia

Naches R

Yakima River

Uma tilla River

COLUMBIA

RIVER

River

N Fk John Day R

SNAKE

RIVER

Imnah a River

Banks

Lake

River

Spokane Coeur

D'Alene

Lake

Grande Ronde R

Salmon River

John Day R

Moses Coulee

Walla Walla River

Deschutes R

Klickitat R

Clearwater

River

Hangman Cr

F.D. Roosevelt La ke

Palouse River

Toppenish Creek

Satus Creek

Ellensburg

Umatilla

Walla Walla

Yakima

Heppner

MiltonFreewater

Pasco

Lewiston

Moscow

Spokane

The

Dalles

La Grande

Grand Coulee

Dam

B L U E 

 M O U N T A I N S

CASCADE RANGE

OKANOGAN HIGHLANDS

CASCADE RANGE

MOUNTAINS

WALLOWA

R O C K Y M O U N T A I N S 

R O C K Y M O U N T A I N S

HORSE HEAVEN HILLS

BEEZLEY HILLS

Seattle

EXPLANATION

0.2 to 5

5.1 to 10

10.1 to 15

15.1 to 20

Greater than 20

Recharge from precipitation and irrigation 

return flow in 2007, in inches

Study area boundary

0 20 40 60 80 MILES

0 20 40 60 80 KILOMETERS

Base modified from U.S. Geological Survey digital data, various scales. Coordinate system: 

State Plane, Washington South, FIPS 4602: Projection: Lambert Conformal Conic. Horizontal 

datum: North American Datum 1983, Vertical datum: North American Vertical Datum of 1988.

122° 120° 118° 116°

46°

48°

47°

45°

Study

area

WASHINGTON

OREGON IDAHO
48 Groundwater Availability of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

Excluding irrigation pumpage, average annual pumpage 

was 0.4 MAF (about 22 percent of total irrigation pumpage 

and only 8 percent of total irrigation water use). Total average 

annual pumpage was about 1.8 MAF. By pumpage category, 

the average annual PWS was 0.23 MAF, and increased from 

0.2 MAF in 1985 to 0.27 MAF in 2007 (fig.23B). Domestic 

self-supplied pumpage increased from 0.06 MAF in 1985 to 

0.07 MAF in 2007 (average annual pumpage of 0.06 MAF). 

The average annual industrial pumpage was 0.05 MAF for 

1985–2007, and decreased from 0.05 to 0.04 MAF over 

this period; this trend in decreased industrial pumpage also 

was identified by Whiteman and others (1994). The trend is 

Figure 23. Estimated annual groundwater pumpage for (A) irrigation; and (B) public, domestic, industrial, and other water 

supplies, Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho, 1985–2007.

tac11-0648_fig23

A

B

0

1988198719861985 1989 199819971996199519941993199219911990 1999 2006200520042003200220012000 2007

Calendar year

1988198719861985 1989 199819971996199519941993199219911990 1999 2006200520042003200220012000 2007

Calendar year

50,000

100,000

150,000

200,000

250,000

300,000

Groundwater pumpage, in acre-feet per year Groundwater pumpage, in acre-feet per year

1,000,000

1,200,000

1,400,000

1,600,000

1,800,000

2,000,000

2,200,000

Public water supply

EXPLANATION

Domestic water supply

Industrial water supply

Other water supply

Irrigation

EXPLANATION
Existing-Conditions Water Budget 49

likely attributable to a decrease in industrial production, the 

adoption of more water-efficient processes, and a shift in 

the type of industry from those that require large amounts of 

water (refining and manufacturing of wood products) to ones 

that use less water (computer server farms). Average annual 

“other” pumpage (mostly for livestock) was 0.03 MAF during 

1985–2007, and increased from 0.02 to 0.04 MAF over the 

23-year period. However, livestock use is the most difficult 

quantity to measure because of water law, and it may be much 

larger than these numbers.

Aquifer Storage

Hansen and others (1994) estimated that about 10 MAF 

of water was added to storage from predevelopment 

conditions through 1985 in the Overburden unit owing 

to excess irrigation water that raised groundwater levels. 

This suggests that about 94 MAF of water is in storage for 

existing conditions based on the estimate for predevelopment 

conditions described in section, “Predevelopment Water 

Budget—Aquifer Storage,” especially because groundwater 

level rises in irrigated areas had nearly stabilized by 1985. 

Irrigation practices have added about 3 MAF of storage to 

the basalts (about 1.8 MAF in the Saddle Mountains unit) by 

raising groundwater levels from predevelopment conditions 

through 1985 (Hansen and others, 1994). However, Hansen 

and others (1994), measured water levels, and model 

simulations indicate that groundwater-level declines owing to 

pumpage also have occurred, and the declines in the Wanapum 

and Grande Ronde units have been more widespread and of 

greater magnitude than the rises—resulting in a net loss in 

groundwater storage in these units. That is, the declines in the 

groundwater-irrigated areas generally negate the increases in 

water in storage from surface-water irrigation, and the basalt 

storage is estimated to approximate the predevelopment basalt 

storage of 800 MAF (table 5). 

Groundwater Model Domain

The estimates of the water-budget components 

of precipitation, ET, recharge, surface-water use, and 

groundwater pumpage within the model domain are from 

Kahle and others (2011), and the groundwater components 

are from Ely and others (2014). The landscape-process 

components (precipitation, ET, and recharge) are shown in 

figure 24. The surface-water component is not described in this 

section because this component is accounted for and discussed 

as a part of the system described for the complete CPRAS. 

The primary emphasis of this section is on the groundwater 

budget, including the flow between hydrogeologic units.

Figure 24. Average annual values for selected water-budget components, Columbia Plateau Regional Aquifer System 

groundwater-model domain, Washington, Oregon, and Idaho, 1985–2007.

0

5

10

1988198719861985 1989 199819971996199519941993199219911990 1999 2006200520042003200220012000 2007

15

20

25

Values, in inches per year

Calendar year

Recharge

Evapotranspiration

Precipitation

EXPLANATION
50 Groundwater Availability of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

Precipitation

The average annual precipitation in the model domain 

was estimated as 14.5 in. or 35,000 ft3/s (25 MAF). The 

average was about 2.7 in. less than the average for the 

complete study area because much of the model domain 

is in the arid to semi-arid part of the CPRAS. Annually, 

precipitation ranged from 10.6 to 21.5 in., indicating the 

large temporal variation in the primary landscape hydrologic 

budget component. During the 23-year period, precipitation 

spatially ranged from a low of 3.2 in. in the central Columbia 

Plateau near Pasco, Washington, to a high of 113 in. in the 

Cascade Range. These large spatial and temporal variations 

affect ET, recharge, streamflow, and groundwater levels, and, 

ultimately, water availability. For example, in a dry year, the 

demand for irrigation water increases, which, in turn, can 

affect the ability of junior water-right holders to provide the 

necessary irrigation quantities to their crops—especially for 

surface-water users. Dry years also have a detrimental effect 

on aquatic ecosystems because of decreased groundwater 

discharge and increased water temperatures. In contrast, in wet 

years, water is available for all users and potential water-level 

declines can be moderated. Wet years also typically provide 

for a lower mortality rate than dry years for migrating juvenile 

salmonids because of increased spring-runoff “flushing” flows 

owing to increased unregulated runoff or additional reservoir 

releases (especially for the Columbia and Snake Rivers).

Evapotranspiration

The estimated average annual ET within the model 

domain was about 10.8 in. or 26,100 ft3/s (18.9 MAF) over 

the 23-year period. The average value was only about 1 in. 

less than the study area average because of the ET from the 

irrigation of croplands, most of which are within the model 

domain. Annual values ranged from 6.7 to 16 in. (representing 

a difference of 22,500 ft3/s or 16.3 MAF). These large 

annual variations are primarily influenced by the spatial and 

temporal variations in precipitation and air temperature, and 

ultimately affect the groundwater system through the recharge 

component. The spatial-temporal variations in ET provide new 

insight into the dynamics of the landscape processes that affect 

and influence the groundwater-flow system. The results also 

provide a perspective on the relative contribution of budget 

items to the total water budget. For example, the 16.3 MAF 

difference between the highest and lowest annual ET values 

within the model domain during 1985–2007 is 72 percent of 

the average annual flow of the Snake River at Anatone for the 

same period. 

Recharge

The interaction between the landscape hydrologic 

processes of precipitation and ET, and the additional effects 

from the delivery/pumpage and application of irrigation 

water controls both the groundwater recharge component of 

the water budget and the groundwater discharge component 

to streams. The SOWAT/regression-derived average annual 

recharge for the model domain was estimated at 3.8 in. 

or 9,100 ft3/s (6.6 MAF), which is within 8 percent of the 

difference between precipitation and ET. The average annual 

recharge during 1956–77 was estimated at 4.24 in. for the 

domain of the CP-RASA groundwater model, and differed by 

only 11 percent from the CPRAS estimate (which included 

7 dry years). Estimated annual recharge ranged from 3.3 in. 

(5.8 MAF) to 9.1 in. (15.9 MAF), and this large range (10.1 

MAF) potentially represents a large control on groundwater 

availability. The large range in annual precipitation and ET 

(12 and 16.3 MAF, respectively) cascades through the system 

as large temporal variations in recharge. During the 23-year 

period, annual recharge was estimated to spatially range 

from 0 to 92 in. The large spatial and temporal variations 

in recharge have a profound effect on the groundwater 

flow system.

Surface-Water Use and Groundwater Pumpage

There is no apparent long-term trend in either 

groundwater or surface water irrigation (fig. 25). Although 

information presented in figure 25 is for the entire study area, 

it also is representative of the model domain because more 

than 98 percent of the irrigated lands are within the model 

domain. The information suggests that there may be a small 

upward trend in the percentage of irrigation supplied by 

groundwater. Dry years when standby/reserve or supplemental 

groundwater rights were exercised are apparent (1987, 1988, 

1992, 1993, 1994, 2001, and 2005); however, there were other 

years (1997, 2000, and 2007) that were not considered dry, 

but groundwater was a larger percentage of total irrigation 

(fig. 25) relative to most other years. These 3 years were when 

areas predominately supplied by groundwater had abovenormal ET. However, even in dry years, surface-water use is 

much greater than groundwater pumpage. 

The growth of non-irrigation pumpage in the model 

domain (fig. 26) generally follows the same pattern as for 

the study area, with PWS as the largest category of use. 

The PWS use was more than 140,000 acre-ft in 2007, or 

about 26 percent of total surface water and groundwater 

irrigation use. In contrast to the study area as a whole, the 
Existing-Conditions Water Budget 51

industrial use through 2002 was larger than the domestic 

use. During 1985–2002, industrial use ranged from about 

41,000 to 54,000 acre-ft, and decreased to 39,000 acre-ft by 

2007. Through 2005, the “other water supply” uses were the 

smallest category, mirroring the study area results. The total 

non-irrigation pumpage in 2007 was about 193,000 acre-ft, of 

which PWS accounted for 76 percent. 

Groundwater

The groundwater system has undergone large changes 

since predevelopment conditions owing to the temporal 

and spatial variations in recharge (partly influenced by the 

delivery and application of irrigation water) and pumpage. 

For example, pumpage is negligible until the 1950s and 

increases significantly, especially during the 1970s and 1980s 

when it increased by about 0.75 MAF. Pumpage was only 

about 45 acre-ft in 1920, increased to about 100,000 acre-ft 

by 1950, and exceeded 1.2 MAF by 1985. Pumpage is only 

about 20 percent as large as recharge but, in major pumping 

centers (where recharge is minimal), it is the largest budget 

component. The connection between year-to-year variations 

in storage and stream discharges, and variations in recharge, 

is clearly shown in the model-simulated annual groundwater 

Figure 25. Estimated surface-water and groundwater irrigation, Columbia Plateau Regional Aquifer System, Washington, 

Oregon, and Idaho, 1985–2007 (from Kahle and others, 2011).

Figure 26. Estimated annual groundwater pumpage 

for public, domestic, industrial and other water supplies 

in the groundwater-model domain, Columbia Plateau 

Regional Aquifer System, Washington, Oregon, and 

Idaho, 1985–2007.

tac11-0648_fig25 Groundwater and surface-water irrigation, in million acre-feet 3

4

5

6

7

8

0

1

2

29

32

35

38

41

44

20

23

26

Percentage of irrigation from groundwater

1988198719861985 1989 199819971996199519941993199219911990 1999 2006200520042003200220012000 2007

Calendar year

Surface water irrigation

EXPLANATION

Groundwater irrigation

Percentage from groundwater

Groundwater pumpage, in acre-feet per year

Public water supply

EXPLANATION

Domestic water supply

Industrial water supply

Other water supply

0

40,000

80,000

120,000

160,000

19851986198719881989199019911992199319941995199619971998199920002001200220032004200520062007
52 Groundwater Availability of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

fluxes for the 88-year period of simulation (fig. 27; Ely and 

others, 2014). Although storage has increased significantly 

in the surface-water irrigated areas, there was a net decrease 

in storage owing to pumpage. Thus, natural recharge is a 

major control on the water fluxes of the groundwater system, 

but human activities, in place of irrigation recharge, as 

represented by pumpage, has affected the overall storage in 

the system. The control of recharge indicated in the annual 

water budgets represents typical hydrology with much of the 

recharge discharging along short-to-intermediate flow paths 

to surface-water features, with discharge to streams varying 

by 3.2 MAF over the model simulation period owing to the 

recharge variations.

Figure 27. Simulated annual water-budget fluxes for the groundwater-model domain, Columbia Plateau Regional Aquifer 

System, Washington, Oregon, and Idaho, 1920–2007 (from Ely and others, 2014).

Calendar year

1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

Outflows Inflows

-10

Predevelopment

12

-8

-6

-4

-2

0

2

4

6

8

10

Water-budget flux, in millions of acre-feet per year

Recharge

Storage

Pumpage

Streams

EXPLANATION

Hydrogeologic unit water budgets (fig. 28) show that 

all units are hydrologically active, with the importance and 

magnitude of the components varying by units. The budget for 

the hydrogeologic units indicates that, for most units, recharge 

is the dominant budget component, and that much of it 

discharges to streams (which is the second largest component 

for most units). In contrast to the other units, however, the 

location (spatial extent and outcrop extent) of the Saddle 

Mountains unit results in other budget components being 

more similar to its recharge quantity. Flow between units is 

relatively large and is representative of the extent of a unit 

relative to the other units. Overall, downward flows are much 

larger than upward flows. Annual storage changes seem to be 
Existing-Conditions Water Budget 53

relatively small compared to other components, but even small 

storage changes represent groundwater-level declines over 

large parts of the CPRAS. 

The water budget for the individual units follows the 

same general trend as the total water budget, with recharge 

and discharge to streams dominating. Interestingly, excluding 

recharge and discharge to streams, the water budget indicates 

Figure 28. Schematic diagram showing simulated water budget for existing conditions, groundwater-model domain, 

Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho, 1985–2007.

tac11-0648_fig28

-768 147

911

256

959

258

-245

-761

-2,835

-4,648

33 -344

114 -809

221 -480

636

134

904

57

271

3

60

51

2,416

921

2,737

3,851

Overburden

Saddle Mountains

Wanapum

Grande Ronde

-768

147

-245

911

2,461

Water budget, in cubic feet per second

Discharge to surface water features

Recharge

Pumpage

Storage

256

EXPLANATION

Grande

Ronde

Downward flux

Upward flux

Hydrogeologic unit

that pumpage is the next largest component (fig. 28), and 

highlights the importance of pumpage and its historical, 

current, and future effects on the system. Downward flux from 

overlying to underlying units is the next largest component 

and is more than four times larger than upward flux. Indeed, 

storage changes in the units are about as large as the upward 

flows between units. 
54 Groundwater Availability of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

Changes to the Water Budget between 

Predevelopment and Existing 

Conditions

Changes in the water budget from predevelopment to 

existing conditions show the effects of human activities on 

the hydrologic system. These changes directly influence 

groundwater availability. The changes are described in 

terms of the landscape hydrologic processes (surface 

water, ET, and recharge). As in previous discussions, 

precipitation and snowpack are assumed to be the same for 

both predevelopment and existing conditions. Additionally, 

surface-water use and groundwater pumpage are not described 

here because the information previously presented on these 

budget components for existing conditions represent the 

change from predevelopment conditions when there was no 

water use in the CPRAS study area. For the model domain, 

a detailed accounting of the changes to the flow system is 

presented based on the groundwater-flow model of Ely and 

others (2014). Changes are referenced to the predevelopment 

conditions simulated with the model and in most cases 

are calculated from the existing-conditions period values. 

However, changes in surface-water fluxes and Wanapum 

unit groundwater levels are referenced to the model-base 

conditions of 2000–2007 to provide continuity to the same 

figures published in the model report of Ely and others (2014).

Surface Water

The surface-water system in the study area has been 

substantially altered over time with the construction of 

numerous reservoirs, diversion dams, canals, irrigation 

return-flow drains, and thousands of diversions. The timing 

and magnitude of streamflow has changed throughout the area 

to meet the power, irrigation, and to a much smaller extent, 

municipal demands that evolved over time. It was beyond the 

scope of this study to assess the changes in the streamflow 

throughout the study area. However, the groundwater model 

was used to estimate the changes in discharge to and from 

surface-water features from predevelopment through 2007. 

Changes in discharge have a direct effect on water availability 

because most of the streams in the study area are fully or 

nearly fully allocated for withdrawals; the amount available 

for allocation is on the order of one-thousandth of 1 percent 

of the total streamflow in the study area. The change in 

discharge from predevelopment to model-base conditions 

(fig. 29) indicates a wide range of patterns—from increased 

groundwater discharge in surface-water irrigated areas to 

decreases and increases in other areas. Some of these changes 

are owing to the differences in recharge in 1920 (4.6 MAF) 

compared to recharge in 2007 (6.7 MAF)—again showing the 

importance of recharge as a control on the hydrologic system. 

It should be noted that the sensitivity of the model to recharge 

and hydraulic conductivity variations, when combined with 

the water levels calculated over the model grid size, can 

lead to errors in the estimated surface-water fluxes. Small 

differences in the calculated shallow water levels may result 

in a losing reach being calculated as a gaining reach. Some of 

these differences are directly attributed to recharge differences 

between 1920 and 2007. This has been observed in the model 

results for Crab Creek in the northern part of the study area.

Evapotranspiration

The ET water-budget component increased by 2.1 in., 

from 8.7 in. (15.2 MAF or 21,000 ft3/s) to 10.8 in. (18.9 MAF 

or 26,100 ft3/s). The increased ET is owing primarily to the 

consumptive use of irrigation water applied to crops. For 

example, in the Yakima River Basin and CBP, increases in ET 

ranged from about 20 to 36 in. owing to the consumptive use 

by crops. Under predevelopment conditions, ET was about 

60 percent of precipitation and increased to 73 percent for 

existing conditions. The percentage increase indicates that 

irrigation effectively increased ET by 13 percent. 

Recharge

The change to the recharge component from 

predevelopment to existing conditions was about a 20 percent 

increase because of the infiltration of irrigation water owing to 

its delivery and application. The increase was from 5.5 MAF 

(7,600 ft3/s) to 6.6 MAF. (9,100 ft3/s). The percentage 

increase was about the same as for the CP-RASA model 

domain when comparing values for the CP-RASA modeling 

during 1983–85, showing consistency between the CP-RASA 

DPM-derived estimates and the SOWAT-derived estimates. 

A large percentage of the additional recharge in the surfacewater irrigated areas initially increased storage with rising 

water levels. With the stabilization of water-level rises under 

existing conditions, most of the additional recharge first fills 

up the seasonal storage as water-level rises (generally ranging 

from 1 to 10 ft, but as much as 30 ft in some locations) in the 

surface-water irrigated areas. Once the seasonal storage is 

filled, recharge then moves to streams and agricultural drains, 

with a smaller part discharging as shallow ET. 

Groundwater

The changes in water budgets for the hydrogeologic 

units are most pronounced for recharge, flow between units, 

discharge from the system, and pumpage (fig. 30). Changes 

in recharge are largest for the Overburden unit because of 

its association with surface-water irrigation in the numerous 

irrigation districts on parts of the Columbia Plateau. The 

additional recharge has increased water levels (storage) 
Changes to the Water Budget between Predevelopment and Existing Conditions 55

Figure 29. Change in simulated surface-water fluxes (base flow) from predevelopment (pre-1920) to model-base conditions

(2000–2007), Columbia Plateau Regional Aquifer System Washington, Oregon, and Idaho (from Ely and others, 2014).

tac110648_fig 29

Pasco

Yakima

Moscow

Pullman

Spokane

Lewiston

Wenatchee

The

Dalles

Ellensburg

Walla

Walla

Moses

Lake

Change in simulated base flow, in cubic feet per second

No

symbol

-144.95 to -100.00

-99.99 to -10.00

-9.99 to -1.00

-0.99 to -0.10

-0.09 to 0.10

0.10 to 1.00

1.01 to 10.00

10.01 to 100.00

100.01 to 615.04

Groundwater-flow model boundary

EXPLANATION

Walla Walla R

John Day

River Deschutes River

Palouse River

Yakima Rive

K

r

lickitat River

COLUMBIA

RIVER

Spokane River

Snake River

Lake

Coeur

d'Alene

COLUMBIA RIVER

Umatilla River

121° 120° 119° 118° 117°

48°

47°

46°

45°

0 20 40 60 80 MILES

0 20 40 60 80 KILOMETERS

Base modified from U.S. Geological Survey digital data, 1:2,000,000, 1972. Coordinate system: 

State Plane, Washington South, FIPS 4602: Projection: Lambert Conformal Conic. Horizontal 

datum: North American Datum 1983, Vertical datum: North American Vertical Datum of 1988.

Model

area

WASHINGTON

OREGON IDAHO

W A S H I N G T O N

O R E G O N

I D A H O
56 Groundwater Availability of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

and, consequently, also has increased discharges from these 

two units to surface-water features and flow between units. 

Concurrently, water-level declines and change in storage 

(in response to pumping) generally have been mitigated by 

the increased recharge (particularly in the Overburden unit); 

that is, surface-water irrigation has provided a new source of 

water to wells. Similar conclusions were described for the 

CP-RASA by Hansen and others (1994; p. 70): “The general 

effect of water development on the upper two units has been 

to raise the water-level altitudes over most of their extent...

Declines in these units were small and not widespread.” In 

contrast, groundwater-level declines owing to pumpage in the 

Wanapum (fig. 31) and Grande Ronde units have resulted in 

the loss of storage. Importantly, pumpage from basalt units is 

larger than the increase in recharge (fig. 30). For the Wanapum 

unit alone, model simulations suggest that pumpage has 

caused a total loss in storage of more than 10 MAF and that 

declines have encompassed more than 14,000 mi2. However, 

surface-water irrigation has resulted in rising water levels in 

the Wanapum unit where it underlies surface-water irrigation 

districts. These rises have generally balanced the declines as 

shown in figure 30, but model simulations and observed data 

indicate a net loss in storage after 1976. This loss in storage is 

continuing as of 2015 and likely into the future. 

Figure 30. Schematic diagram showing simulated water budget change from predevelopment (pre-1920) to existing 

conditions (1985–2007), groundwater-model domain, Columbia Plateau Regional Aquifer System, Washington, Oregon, 

and Idaho.

-337 -11,400

778

186

501

92

-245

-466

-216

8

-2,930 -344

-6,040 -809

4,540 -480

159

35

64

-9

16

0.2

-9

-6

1,611

308

375

81

Overburden

Saddle Mountains

Wanapum

Grande Ronde

-337

147

-245

778

1,611

Water budget, in cubic feet per second

Discharge to surface water features

Recharge

Pumpage

Storage

186

EXPLANATION

Grande

Ronde

Downward flux

Upward flux

Hydrogeologic unit
Changes to the Water Budget between Predevelopment and Existing Conditions 57

Figure 31. Simulated changes in groundwater levels in the Wanapum unit from predevelopment (pre-1920) to model-base 

conditions (2000–2007), groundwater-model domain, Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

(from Ely and others, 2014). tac110648_fig 31

Pasco

Yakima

Moscow

Pullman

Spokane

Lewiston

Wenatchee

The

Dalles

Ellensburg

Walla

Walla

Moses

Lake

EXPLANATION

10.1 to 50

50.1 to 100

100.1 to 150

150.1 to 200

200.1 to 250

250.1 to 300

300.1 to 350

350.1 to 400

400.1 to 450

-204.3 to -150

-149.9 to -100

-99.9 to -50

-49.9 to -10

No

symbol -9.9 to 10

Wanapum simulated drawdown, in feet

Active model domain

Base map modified from U.S. Geological Survey 

and other digital data, various scales. Coordinate 

Reference System: State Plane, Washington 

State, South. Horizontal datum is North 

American Datum of 1983. 

121° 120° 119° 118° 117°

48°

47°

46°

45°

0 20 40 60 MILES

0 20 40 60 KILOMETERS

John Day

River Deschutes River

Palouse River

Yakima River

Klickitat River

COLUMBIA

RIVER

Spokane River

Snake River

Lake

Coeur

d'Alene

COLUMBIA RIVER

Umatilla River

Walla Walla River

Model

area

WASHINGTON

OREGON IDAHO

W A S H I N G T O N

O R E G O N

I D A H O
58 Groundwater Availability of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

Irrigation practices, and to a smaller extent leakage 

from uncased wells, has changed the flow between units. The 

largest of these changes (fig. 30) are owing to the additional 

recharge from surface-water irrigation practices that have 

saturated more material in the system by raising water 

levels. The increase in storage has been well documented 

prior to this study. For example, Drost and others (1997) 

documented changes in storage exceeding 10,000 acre-ft/yr 

in an area covering 900 mi2 in Franklin County, Washington, 

that includes the southern part of the CBP and a small part 

of Benton County that includes a surface-water irrigation 

district in the Yakima River Basin. They estimated that 

storage has been increased by more than 5 MAF in this area 

since predevelopment time, and has largely altered the flow 

system. Overall, downward flow into the Saddle Mountains, 

Wanapum, and Grande Ronde units is a large component of 

the changes. Relative to upward flow, downward flow has 

increased in magnitude since the 1940s. This additional water 

to the units, in turn, has increased discharge to surface-water 

features in some areas. Indeed, agricultural drains (some 

more than 10 ft deep) have been constructed throughout 

the surface-water irrigated areas to prevent waterlogging of 

croplands. Although the change in storage in the units is not 

a large component of the changes, this change has resulted in 

declines exceeding 300 ft in some areas, which have affected 

water availability in the major pumping centers. Declines have 

exceeded 10 ft/yr in parts of the Odessa subarea in western 

Adams County, Washington.

Climate Variability and Climate Change

The water resources of the CPRAS and, thus, 

groundwater availability, are intricately connected to 

interannual to interdecadal climate variations, which, in turn, 

are tied to global and North Pacific sea-surface temperature 

and atmospheric pressure patterns (Namias, 1976; Wallace 

and Gutzler, 1981; Ropelewski and Halpert, 1986; Yarnal and 

Diaz, 1986; Cayan and Peterson, 1989; Redmond and Koch, 

1991; Cayan and Webb, 1992; Mantua and others, 1997; 

Vaccaro, 2002). The study and interpretation of natural climate 

variability is important for improving the understanding of 

the potential effects of climate change on precipitation and 

air temperature and, thus, on evapotranspiration, streamflow, 

and groundwater recharge. For example, the response of the 

hydrologic system during dry years in the historical record can 

be used as an indicator of possible changes in the landscape 

water fluxes and groundwater budgets during future droughts. 

Similarly, wet years in the historical record can be used as 

indicators of possible changes during future wet periods. The 

effects of interannual variations in climate on the snowpack 

and streamflow water-budget components are shown in 

figures 14, 16, and 20, especially the large differences between 

wet and dry years. For wet years, the changes in snowpack 

may not be as pronounced with warming temperatures.

Ultimately, climate variability is propagated through the 

groundwater system, first as groundwater recharge and then as 

storage changes (changes in groundwater levels). The changes, 

in turn, affect groundwater discharge to streams. Interannual 

variations in estimated predevelopment recharge in the lower 

Naches River Basin near Yakima, Washington (Vaccaro and 

Olsen, 2007), indicate the magnitude of variations owing to 

climate effects (fig. 32). Recharge variations are propagated 

through the system as interannual to interdecadal water-level 

changes, as shown by the water-level hydrograph in the upper 

120 ft of the Wanapum unit for a long-term monitoring well 

(USGS site identifier 473442118162201) located west of 

Spokane, Washington (fig. 33). These data are from the U.S. 

Geological Survey for the period of record (1989 is missing) 

and Washington State Department of Ecology (transducer data 

from October 1, 1997 through March 31, 2010; John Covert, 

Washington State Department of Ecology, written commun., 

2011). These water-level variations are correlated with 

precipitation variations as shown by the annual and cumulative 

departure from average values of precipitation for the same 

period (fig. 34) at a nearby weather station (Davenport 

weather station, National Weather Service cooperative station 

number 45-2007) located in Lincoln County, Washington. Wet 

years and dry years clearly stand out in figure 34, as does the 

persistence of wet and dry periods. This climatic variability 

and its subsequent effects have large implications for waterresource planning, management, and operations. 

In a seminal study of streamflow generation in western 

Washington, Gladwell (1970) showed that streamflow 

variability is “more climatically controlled than physically 

controlled.” Thus, changes in the climatic regime should be 

represented in streamflow characteristics for regions where 

streamflow variability is coherent (or in phase) with the 

climatic variability. The cumulative departure curve for the 

Columbia River at The Dalles (the longest hydrologic record 

in the study area) shows distinct shifts from wet periods to dry 

periods (fig. 35A). For about the first 25 years, streamflow was 

generally greater than average, followed by about 20 years 

of near-average streamflow, and then 25 years of less thanaverage. From 1977 to 2010, streamflow tended to be less than 

the long-term average but with a distinct, nearly decadal cycle, 

which is shown more clearly in the inset graph based on a 

shorter period of record (fig. 35B). 

Superimposed on the natural climate variability and 

its effects on the groundwater-flow system are changes 

in water use and water-budget components owing to this 

variability. For example, large variations in existing-conditions 

(referenced as current conditions by Vaccaro and Olsen, 2007) 

recharge in the lower Naches River Basin owing to variations 

in precipitation input and use of surface water for irrigation are 

shown in figure 32. In this case, irrigation allows for the soil 

moisture to be replenished and, as a result, recharge mimics 

precipitation variability. For some streams (for example, 

the Columbia River), the cycles described in the preceding 

paragraph are important in that junior “interruptible” 
Climate Variability and Climate Change 59

Figure 32. Calculated annual values of selected water-budget components for predevelopment and existing (current) 

conditions, lower Naches River Basin, Washington, water years 1950–2003 (from Vaccaro and Olsen, 2007).

Figure 33. Water levels for the Davenport monitoring well, Lincoln County, Washington, 1971–2009. Monitoring well location 

is shown in figure 4.

tac11-0648_fig32

Water year

5

10

15

20

25

30

0

Rate, in inches per year

1950 1960 1970 1980 1990 2000

EXPLANATION

Precipitation

Irrigation

Current actual evapotranspiration 

Predevelopment recharge

Current recharge

Predevelopment actual 

 evapotranspiration

55 1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010

25

50

45

40

35

30

Calendar year

Measured water levels

Measured water levels and

transducer - recorded water levels

Depth to water, in feet
60 Groundwater Availability of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

diversions can be limited or turned off to preserve water for 

more senior instream-flow rights. This is in contrast to junior 

surface-water users receiving less water in drought years to 

preserve water for more senior diversionary rights. Both cases, 

however, have the same effect of reducing irrigation recharge 

and, thus, return flows. In such drought years, some junior 

surface-water users switch to groundwater (supplemental or 

standby/reserve irrigation rights) to make up for their lack of 

water, which can locally aggravate groundwater-level declines 

(Vaccaro and others, 2009) and reduce streamflow.

Previous studies have identified “shifts” in the mean 

climate at interdecadal scales that are part of the natural 

variability (Vaccaro, 1995; Mantua and others, 1997; Vaccaro, 

2002). Although large shifts such as those associated with the 

Pleistocene-to-Holocene climate change are important for 

understanding large-scale natural variability, much smaller 

changes have a large effect on regional water resources, 

especially considering the typical time frame of many waterresource planning horizons and the near-total use of water 

resources in some parts of the study area. These smaller 

“shifts” to different regimes propagate through the system 

with respect to water use. Three major shifts in the Pacific 

Northwest occurred in about 1928–31, 1947, and 1977 

(Vaccaro, 1995), which were related to atmospheric circulation 

(Vaccaro, 2002) and to Pacific Ocean sea-surface temperatures 

as defined by the Pacific Decadal Oscillation (PDO; Mantua 

and others, 1997). The first period was drier than normal, the 

second wetter than normal, and the third drier than normal. 

Such shifts affect water management throughout the 

Columbia River Basin. For example, in the Yakima River 

Basin in the western part of the study area, there are five 

Reclamation reservoirs with a total capacity of 1.1 MAF that 

supply water for the irrigation of about 500,000 acres. Water 

demands are initially met by unregulated streamflow and 

then by reservoir releases, and the date of the first releases is 

termed the storage control date (SCD). During 1926–2010, 

the average SCD was June 24 (Julian day 175). The SCD 

not only has varied over time (fig. 36), but also has changed 

character with much more variability starting around the 1977 

climate shift. Indeed, of the 34 occurrences of releases before 

June 24, 17 occurred during the dry period after 1976 (50 

percent of the early releases occurred during 50 percent of 

the post-1976 record). Ten of the remaining 17 occurrences 

were before 1947 (the other dry period). For the post-1976 

period, SCDs were earlier by an average of 31 days compared 

to the complete pre-1977 period, indicating highly variable 

and more difficult reservoir operations. The cumulative 

departure of the SCD from the 1926–2010 average (fig. 37) 

Figure 34. Annual precipitation and cumulative departure from average annual precipitation for the Davenport weather 

station, National Weather Service cooperative station number 45-2007, Lincoln County, Washington, 1971–2009. Weather 

station location shown in figure 2.

tac11-0648_fig34

-5 1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010

Calendar year

25

0

5

10

15

20

Precipitation, in inches per year

Annual precipitation

EXPLANATION

Cumulative departure
Climate Variability and Climate Change 61

Figure 35. Cumulative departure of annual streamflow from average annual streamflow for the Columbia River at The 

Dalles, Washington, (A) 1879–2009, and (B) 1920–2009.

tac11-0648_fig35

Calendar year

188018851890189519001905191019151920192519301935194019451950195519601965197019751980198519901995200020052010

1920192519301935194019451950195519601965197019751980198519901995200020052010

0

100,000

200,000

300,000

400,000

500,000

600,000

700,000

800,000

900,000

1,000,000

Cummulative departure, in cubic feet per second

Cummulative departure, in 

cubic feet per second

-300,000

-400,000

-200,000

-100,000

0

100,000

200,000

300,000

B

A

indicates these three broad periods, especially the earlier 

SCDs of releases after 1976. The general relation to the PDO 

is recognizable in the SCD pattern, as is a general relation to 

the Southern Oscillation Index—a measure of the El NinoSouthern Oscillation phenomena (fig. 37). Most of the early 

(pre-May 31) SCDs after 1976 correspond to years in which 

junior surface-water users obtained less appropriated water 

and groundwater pumpage increased because of the use of 

standby/reserve water rights. Thus, these interdecadal climate 

variations have a large effect on water use, water management, 

and water-budget components.
62 Groundwater Availability of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

Figure 36. Storage control dates for reservoirs in the Yakima River Basin, Washington, 1926–2010. Data from C. Lynch, 

Bureau of Reclamation, written commun. (2011).

Figure 37. Cumulative departure from 1926–2010 average of standardized values of the storage control date; streamflow at 

the Yakima River at Parker, Washington; the Pacific Decadal Oscillation; and the Southern Oscillation Index, 1926–2010. Data 

from C. Lynch, Bureau of Reclamation, written commun. (2011), (http://www.jisao.washington.edu/data/pdo/), and University of 

East Anglia (2011).

tac11-0648_fig36

90

110

130

150

170

190

210

230

250

Calendar year

192619301935194019451950195519601965197019751980198519901995200020052010

Julian day

Apr May June July Aug Sept

Calendar year

1926

1930

19351940194519501955

1960

19651970197519801985

1990

1995200020052010

-15

-10

-5

0

5

10

15

Cumulative departure, in standardized units

EXPLANATION

Storage control date, Julian day

Yakima River at Parker

Southern Oscillation Index, 1926–2010

Pacific Decadal Oscillation
Climate Variability and Climate Change 63

Future Climate Projections

Global Climate Models (GCMs) are ideally suited 

for analyzing projected climate change because the 

hydrometeorological regime in the Pacific Northwest is forced 

by and clearly linked to atmospheric-oceanic circulation. 

Information suggests that the climate of the Pacific Northwest 

is already changing (Bureau of Reclamation, 2011) and that 

the trends are partially owing to human effects (Barnett and 

others, 2008; Levi, 2008). Results from GCMs indicate that 

the hydrologic conditions of the study area will continue to 

shift from historical conditions. Although the exact extent 

and timing of the long-term changes remain unknown, 

the projections include increased air temperatures, some 

changes in precipitation, reductions to the snowpack (more 

precipitation in the form of rain), earlier snowmelt, possibly 

larger floods, and other phenomena (Bureau of Reclamation, 

2011). 

Using GCMs, the Intergovernmental Panel on Climate 

Change (IPCC) (2001, 2007) analyzed four scenarios, 

including three future emission scenarios that represented 

different levels of carbon emission during the 21st century. 

The future emission scenarios represent different patterns 

in greenhouse gas emissions and concentrations, from 

relatively low (B1), to medium (A1B), to high concentrations 

(A2) during the 21st century (Intergovernmental Panel on 

Climate Change, 2007). Results from 20 of the 2007 IPCC 

GCMs for the A1B scenario were used for a climate-change 

impact assessment for Washington State, and indicate an 

average increase in air temperature of about 2.2 °F by the 

2020s, 3.5 °F by the 2040s, and 5.9 °F by the 2080s (Climate 

Impacts Group, 2009). According to a 2011 Washington State 

legislative report (Washington State Department of Ecology, 

2011), the 2030 water-demand forecasts using model input 

forced by the IPCC GCMs indicate that “Water availability 

will shift away from the irrigation season when demands are 

highest” and water supply will increase between November 

and May and decrease by up to 21 percent from June through 

October. With increased temperatures, ET rates are expected 

to increase, as would the length of the growing season. As 

with the Central Valley in California, “other indirect effects 

of a temperature increase include earlier budding of orchard 

crops, premature ripening of crops (particularly grapes), the 

increased ability to grow more than one crop in a season, and 

reduced milk production from dairy herds” (Faunt, 2009). 

Together, all these projected changes “undermine a basic 

assumption that historically has facilitated management of 

water supplies, demands, and risks” (Milly and others, 2008).

Decreases in snowpack would affect reservoir storage 

and would be complicated not only by earlier runoff, but by 

any increased variability in precipitation that would place 

more stress on the reliability of existing flood management 

and water-storage systems. Increased ET would result in 

increased water demands, which would occur with concurrent 

reductions in late spring and early summer runoff. Less 

groundwater recharge in the mountain uplands for part of the 

study area may occur if there is no increase in precipitation 

(Mastin, 2008; Climate Impacts Group, 2009). Reductions in 

the amount of water available for recharge and storage during 

the dry season are likely to occur, and such reductions would 

affect the amount and timing of groundwater pumpage. 

To improve our understanding of the potential effects 

of climate change on the groundwater system, a simplified 

analysis was completed for this report. In this analysis, the 

projected monthly maximum and minimum air temperature 

changes and percent change in precipitation calculated from 

six GCMs were down-scaled to the central part of the study 

area (L. Hay, written commun., U.S. Geological Survey, 

2011). The median of the six GCM results (mostly changes 

in the maximum/minimum air temperature and precipitation) 

from the A1B medium emission scenario of the IPCC 

assessment were used in this simplified analysis. It should be 

noted that as of 2012, the current global emissions are larger 

than the “high” emission scenario (A2) of the IPCC and, thus, 

the results likely are conservative in terms of warming. The 

median projected A1B changes were used to modify, on a 

monthly basis, historical daily climate at five locations (all 

of which were existing weather sites). The modified climate 

series were then used to drive the DPM under various crop 

types and land covers for each site. Potential changes in 

recharge and projected increases in irrigation pumpage owing 

to increased crop potential evapotranspiration were estimated 

from DPM. 

Results from the six GCMs used in this study and the 

20 GCMs used in the Washington State impact assessment all 

show reasonably consistent trends (“consensus”) in warming, 

but large differences in precipitation changes—from decreases 

to increases. Results from the operation of DPM under 

different projected GCM changes in precipitation indicated 

that any change in precipitation can have a corresponding 

large effect on recharge and ET. The CPRAS is a transition 

area between the 20-GCM “consensus” on increasing or 

decreasing precipitation and, thus, it is unknown as of 2012 

what changes may occur in precipitation under the emissions 

scenarios. This aspect is very important because any changes 

in precipitation (increases, decreases, and seasonality) will 
64 Groundwater Availability of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

have a large effect on the groundwater system, including water 

demand. Therefore, the groundwater model was operated 

with the “current” conditions (2000–2007; Ely and others, 

2014) recharge, but with pumpage increasing by 13 percent 

from 2007 to 2050. The pumpage increase was the average 

of the increase in crop-water demand for the specific crops 

types, which included wine grapes, hops, corn, mint, wheat, 

apples, and row crops, at the locations with irrigation that 

were modeled with DPM. The largest increase in crop water 

demand was for wheat, apples, and mint, and the smallest 

increases were for grapes and hops (the lower increase for 

those two crops is partly related to them being modeled with 

DPM using below-canopy/drip irrigation in contrast to above 

canopy irrigation). The primary result of this analysis is that 

crop water demand can be expected to increase in the future 

and add more competitive demands on the available water 

supply. Some of the results from this analysis are presented in 

the next section.

Groundwater Availability and 

Sustainability

Groundwater availability is a function of the quantity 

and quality of water, and also the laws, rules, regulations, 

and socioeconomic factors that control its demand and uses 

(Reilly and others, 2008). Groundwater sustainability can 

be defined as the achievement of an acceptable tradeoff 

between groundwater use and the long-term effects of that 

use (Alley, 2006). Sustainability requires an iterative process 

of monitoring, analysis, and application of management 

practices (invariably through State regulations). Groundwater 

availability and, ultimately, its sustainability in the study area 

is an issue that is interrelated to groundwater management and 

the availability and use of surface water. Water management, 

in turn, generally is intricately related to environmental 

protection and public health, and availability, thus, is related 

to conflicts between consumptive use and instream uses (both 

diversionary and instream flows).

The concept of sustainability is inherently subjective 

and ambiguous. This is because what is or is not considered 

sustainable is based partly on social and philosophical issues 

that change with time (Alley and Leake, 2004). Factors that 

limit sustainability include physical, chemical, economic, 

environmental, legal, philosophical, or institutional factors 

(Faunt, 2009). These issues were discussed as far back as 

1958 by Leopold (1958), who also described the importance 

of the interrelationship between economics and sustainability. 

Leopold (1990) also stated that the “integrity of the hydrologic 

continuum is implied in the explicit term ‘sustainability’ and 

the dynamic flexibility to adjust constantly through changing 

circumstances.” Like the Central Valley Aquifer study (Faunt, 

2009), this study focused on the physical constraints that may 

affect groundwater sustainability. 

An overarching control is the laws, rules, and regulations 

codified in State law (see appendix 1) that ultimately affect 

water availability and, thus, sustainability. Limited surfacewater and groundwater availability during the “dry” season 

(when water demand is greatest) in large parts in or near 

the study area is shown in figure 38; these constraints on 

availability are defined by existing laws and rules, or are 

based on recommendations of State, Federal, and Tribal 

fishery biologists. For the recommendations, there are Water 

Resource Inventory Areas (WRIAs) not shown on figure 38

that are administratively limited, and water-rights decisions 

are based on these recommendations. For example, as of 2012 

for the Columbia River upstream of Bonneville Dam, there 

are 379 water-right holders in Washington who are subject to 

instream flow requirements for salmonids; these interruptible 

rights total some 309,000 acre-ft of water (Washington State 

Department of Ecology, 2014). There are numerous added 

complexities related to legal constraints and water availability. 

In Washington State, for the Columbia River Basin upstream 

of The Dalles and downstream of the confluence of the 

Spokane River with the Columbia River, annual water-right 

allocations for surface water and groundwater are 9.5 and 

3.4 MAF, respectively, and at The Dalles, annual instream 

flows are on the order of 72.5 MAF (Washington State 

Department of Ecology, 2011). Even with this large disparity 

in quantities, the diversions for interruptible rights have been 

reduced or shut off in dry years, and, at times, instream flows 

were not met. As a further complicating factor, in Washington, 

there are an order of magnitude more surface-water and 

groundwater claims with a listed quantity (many do not list a 

quantity) than permitted and certificated rights. Many of these 

claims may not be valid because of non-additive rights—

claims that were filed for the same uses authorized through 

permits and certificates, groundwater claims that may be 

covered under the exempt statutes (a permit already exists), or 

spurious claims. For example, Vaccaro and Sumioka (2006) 

estimated that of 16,605 groundwater claims in the Yakima 

River Basin, less than about 2,000 may be valid. Until the 

claims are analyzed through a judicial process (adjudication), 

they are legal ‘paper rights” that must be brought into the 

mix of managing for water availability. In the overallocated 

Walla Walla River Basin, there are 45 times more claims 

than existing rights. If only 3 percent of these claims were 

valid, there would still be more claims than existing rights, 

indicating the potential extent of this complicating factor. 

Sorting out the claims “would probably require a team to 

look at each water right permit, certificate and claim (both 

ground and surface water), and compare uses to prevent 

double-counting, as well as conduct some simplified extent 

and validity analysis. That effort would be considerable.” 

(M. Shuppe, written commun., Washington State Department 

of Ecology, 2011.) Entering into this dynamic are a mosaic 

of rulings from court cases and Federal intervention in State 

water policy in response to a State request or when State law 

encroaches on the U.S. Constitution.
Groundwater Availability and Sustainability 65

Figure 38. Existing “dry”-season water-availability constraints for parts of Washington, Oregon, and Idaho (see appendix 2).

tac11-0648_fig38

Ellensburg

Umatilla

Walla

Walla

Yakima

Heppner

MiltonFreewater

Pasco Lewiston

Moscow

Spokane

The

Dalles

La Grande

Deschutes

John Day

Umatilla

Powder

Hood Sandy

37

39

49

60 61

62

48

45

38

59

55

53 54

43

34

35

Grande Ronde

32

SNAKE

NAKE

Klickitat RiverKlickitat River COLUMBIA

Moses

Banks

Lake

Moses

Lake

Coulee

RIVER

Toppenish Creek

Yakima RiverYakima River Yakima Rive r

Yakima Rive r

Naches

Satus CreekSatus Creek

River

River

John Day RiverJohn Day River

Deschutes Deschutes

Lower Crab Cr

er Crab Cr

Crab

SpokaneSpokane

Creek

River

Hangman CreekHangman Creek

RIVER

Sa ml on Sa ml on

SNAKE

Coeur

d'Alene

Lake

Lake

Pend

Oreille

F. D. Roosevelt LakeF. D. Roosevelt Lake

Umatilla Umatilla

River River

GrandeGrande

Ronde

John Day RiverJohn Day River

John

Day River

Fork

North

RIVER

Palouse

River

Palouse

River

Clearwater Clearwater River

Lake

Chelan

Walla Walla River alla Walla River

Clearwater R iverClearwater R iver

Potholes

Reservoir

Rocky

Ford Cr 

River

Imnaha River

WASH INGTON

OREGON

IDAHO

BONNEVILLE

DAM

122° 120° 118° 116°

48°

47°

46°

45°

Snake River—Water may be limited during April–August

Groundwater-management area

EXPLANATION

IDAHO

WASHINGTON OREGON

Boundary of study area

Quincy Groundwater Management area—Limited groundwater 

availability

Odessa subarea—New groundwater not available for irrigation 

or M&I

32 Surface-water basin—WRIA: over-appropriated, fullyappropriated, or limited appropriable; groundwater availability

limited, typically with mitigation

Columbia River—No new irrigation water right during 

April–August, M&I water is available with mitigation

Snake River—No new irrigation water right during April–August; 

M&I water is available with mitigation

Critical groundwater and restrictively classified management areas— Groundwater 

availability limited for most uses 

Hood Basins with limited available surface water—During low-flow period; groundwater 

available for selected uses 

Columbia and Snake River—No new water rights during April 15–

September 30 (limited exceptions)

Water available through appropriation process—However, surface water or 

groundwater may be limited locally (permit denied or conditioned)

Note: Domestic (exempt) groundwater available in most of the study area; Columbia River is for 

the river upstream of Bonneville Dam; M&I, municipal and industrial; WRIA, Water Resource 

Inventory Area

0 20 40 60 80 Miles

0 20 40 60 80 Kilometers

Base modified from U.S. Geological Survey digital data, various scales. Coordinate system: 

State Plane, Washington South, FIPS 4602: Projection: Lambert Conformal Conic. Horizontal 

datum: North American Datum 1983, Vertical datum: North American Vertical Datum of 1988.
66 Groundwater Availability of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

Limited surface water indicates that groundwater would 

necessarily need to be part of the future water-availability 

dynamic. However, in overallocated basins such as the Yakima 

and Walla Walla River Basins, most new, larger (and in some 

cases even small exempt) groundwater withdrawals need 

to be mitigated for senior surface-water diversionary rights 

and instream flows. In Oregon basins, during times of water 

shortage the junior users are regulated, especially during 

the summer low-flow season, and by the end of summer for 

some streams, water is available only for users with water 

rights established in the late 1800s (Oregon Water Resources 

Department, 2014). As with the Yakima River Basin in 

Washington, many of these junior surface-water users switch 

to groundwater when their surface-water right is unavailable. 

Most of the junior surface-water rights are senior to 

groundwater rights in Oregon; thus, “dry” season groundwater 

availability may be limited in the future. Overall, groundwater 

availability in the study area is controlled by (1) the problem 

of variation and timing of precipitation (resulting in the “dry” 

season legal constraints shown in figure 38), and (2) the 

relation between pumpage, recharge, and supply of water 

to wells—that is, the actual and potential for decreases in 

aquifer storage. For the problem of variation and timing of 

precipitation, Wolman (1986), in discussing water and human 

health, identified this aspect in simple terms: “Water is not 

and will not always be available at the right place, at the right 

time, and at a minimum cost.” 

The “dry” season constraints are a result of the need 

to (1) manage the effects of historical water-development 

practices (described for the groundwater system in the section, 

“Changes to the Water Budget between Predevelopment 

and Existing Conditions”) and legal constraints; and (2) 

manage existing conditions for environmental protection, 

especially for salmon recovery, including its interrelationship 

to ESA listings. That is, salmon recovery efforts in the 

Pacific Northwest greatly influence water availability by the 

establishment of target or instream flows by the Washington 

State Department of Fish and Wildlife, Indian Tribes, the 

National Oceanographic and Atmospheric Administration, 

or the U.S. Fish and Wildlife Service. Water availability 

and sustainability can be addressed by estimating the 

water in storage in the system, fluxes within and through 

the system, and uses of water, and by describing the legal 

and ecological framework. This was partly achieved by 

overlaying the groundwater changes simulated by the CPRAS 

model for long-term (2050) equilibrium conditions with 

existing water-management strategies shown in figure 38. 

The potential future (2050) system under a set of conditions 

yielding increased pumpage demand estimated to occur with 

a changing climate is additive to the existing conditions in the 

future with large water-level declines. 

The term “groundwater reserves” is used to emphasize 

the fact that groundwater, like other limited natural resources, 

can be depleted (Alley, 2006). Depletion of aquifer-system 

storage by pumpage has been substantial for many areas in the 

study area, especially for the Odessa subarea in Washington 

and the critical groundwater management areas in Oregon. 

Water-level records and previous studies (Whiteman and 

others, 1994; Vaccaro, 1999) confirm that large amounts of 

water were removed from storage in the basalt units prior 

to 1985. The groundwater-model simulation results for this 

study also indicate that, since predevelopment times, the 

basalt storage in areas of groundwater-level declines has been 

depleted by more than 10 MAF in the Wanapum unit alone. 

Measured water levels from this study also verified continued 

declines in the basalt units over large areas and that declines 

have expanded to other areas (Snyder and Haynes, 2010; 

Burns and others, 2011). It is unknown if the loss in storage 

has caused any subsidence or a decrease in pore space, but if 

so, it may affect the ability to artificially recharge the system 

in areas of widespread and large water-level declines. The 

long-term decrease in aquifer-system storage represents a 

substantial wedge of the water stored in the upper 500 ft of 

the basalt part of the CPRAS. In comparison, the decrease 

in storage relative to the storage in the complete system is 

very small. However, as a practical matter, it is impossible 

to remove all water from storage by pumpage because many 

factors limit the amount of water that can be recovered. 

Important institutional factors (such as use restrictions, basin 

adjudication, and surface-water rights) limit the availability 

of water because even small storage changes can have large 

effects or repercussions because of these institutional factors. 

Similarly, aquifer-system hydraulic characteristics, well yields, 

the cost of drilling wells, the cost of energy for lifting water, 

and the design of the well and pump can limit the availability 

of water. These hydrologic limitations are amplified by the 

institutional factors.

Given long-term equilibrium conditions (using 2007 

pumpage and average recharge for 2000–2007 simulated 

through 2050), results of CPRAS model simulations indicated 

large areas of water-level declines (fig. 39), and these results 

were consistent with those estimated by the CP-RASA (Bauer 
Groundwater Availability and Sustainability 67

and Hansen, 2000). In these areas of declines, groundwater 

availability may be limited hydrologically or by regulations, 

or it already is limited (such as in the Odessa subarea, the 

Walla Walla River Basin, and the critical groundwater and 

restrictively classified areas in Oregon [fig. 38]). In some 

areas (especially those with downward vertical gradients), 

these declines have been aggravated by the draining of upper 

units owing to numerous large-capacity uncased wells (Ely 

and others, 2014). Declines may result in changes in surfacewater quantity and quality, and (or) reduction of the economic 

viability of groundwater irrigation because of increased 

energy costs with the increased pumping lifts. Declines can 

factor into environmental issues, in turn, by changing and (or) 

degrading habitats if groundwater discharge to surface-water 

bodies decreases, and these related effects may constitute the 

primary constraint to groundwater development. For example, 

in the Yakima River Basin, Ely and others (2011) determined 

that groundwater pumping with junior rights is affecting 

streamflow and, thus, senior surface-water rights, including 

time-immemorial Tribal rights for sustaining anadromous 

salmonid populations. In particular, Washington State defines 

impairment of surface-water rights by groundwater pumping 

if one molecule of surface water is captured. Under long-term 

2043–2050 conditions with 2007 pumpage, groundwater 

discharge to streams in the model area was estimated to 

decrease by an additional 623 ft3

/s. Thus, during periods of 

drought (and perhaps even the normal summer low-flow 

period), some streams may not have enough groundwater 

inflow to sustain environmental flows. Surface-water 

diversions and groundwater pumpage aggravate this problem. 

Indeed, with the average potential increase of 13 percent in 

pumpage owing to increased water demand under a changing 

climate, the model simulated a large decrease in groundwater 

discharge by 2050. This decrease was owing to groundwater 

declines resulting in an additional loss in storage in 2050 

of more than 713 ft3/s (this is compared to 2007 and is not 

cumulative loss, which is much larger); the decrease in 

basalt storage by 2050 exceeds 5 MAF and declines extend 

over more than 20,000 mi2 in the Wanapum unit alone. The 

13 percent increase represents an average crop-water demand 

change for various crop types. However, some crops that 

are prevalent on the Columbia Plateau, such as orchards and 

wheat, were estimated to have a more than 20-percent increase 

in water demand. Thus, the effects of increased pumping 

owing to warming likely will be larger than estimated. This 

aspect is especially important because of the total dollar 

value of perennial crops such as apples, cherries, grapes, and 

mint. If senior surface-water users have an increased need 

for irrigation to meet crop-water demand, then both junior 

surface-water uses and groundwater uses may be limited. This 

aspect was not addressed in this study but would need to be 

analyzed for future water-management strategies.

The population in the study area is projected to 

increase by 150,000 people by 2030, but growth in most 

of the counties will be less than 1 percent. Although small, 

this projected growth will add some 27,000 acre-ft to the 

competing demands for limited water resources (Washington 

State Department of Ecology, 2011), and the economics 

of municipal water supply may result in the purchase of 

agricultural water rights. Even with population growth, 

however, agriculture will continue to consume substantially 

more water than public water-supply systems and domestic 

uses. Although agricultural acreage has stayed relatively 

constant since about 1992, water deliveries for irrigation have 

been reduced through greater efficiencies in recognition of 

environmental needs. Although these trends in efficiencies 

are projected to continue, there is a forecasted increase in 

irrigation demand of about 10 percent by 2030 (Washington 

State Department of Ecology, 2011), as shown by the potential 

increase in crop-water demand due to climate change. 

Ongoing negotiations between the United States and Canada 

on a new Columbia River Treaty also may affect future 

demand, especially for users of Columbia River water.

Without increased storage, the projected changing 

surface-water regime with earlier runoff because of less 

snowpack will lead to larger water demands during the 

summer low-flow season. If the amount of surface water 

available during the dry season decreases, the amount and 

timing of groundwater pumpage ultimately may be affected. 

These effects are in addition to increased crop-water demand, 

and also to a potential decrease in summer base flow caused 

by the earlier snowmelt and recharge period. The projected 

effects also may decrease the number of years in which water 

demands for agricultural uses are met, which in some years 

already are not met. Assuming these climate projections are 

correct, decreases in groundwater storage likely will continue 

in some areas, perhaps at an accelerated rate. Additionally, 

new environmental uses may lead to even larger instream 

water demands. 

The potential effects of global climate change on 

agricultural water demand are not understood thoroughly. 

Agricultural water demand is influenced by climate, crop 

selection, input costs, and farming practices. Crop selection 
68 Groundwater Availability of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

Figure 39. Simulated long-term (2007–2050) water-level declines in the Wanapum unit given average 2000–2007 recharge, and 

their relation to existing water-availability constraints, Columbia Plateau Regional Aquifer System, Washington, Oregon, and 

Idaho.

tac11-0648_fig39

Ellensburg

Umatilla

Walla

Walla

Yakima

Heppner

MiltonFreewater

Pasco Lewiston

Moscow

Spokane

The

Dalles

La Grande

SNAKE

NAKE

Klickitat RiverKlickitat River COLUMBIA

Moses

Banks

Lake

Moses

Lake

Coulee

RIVER

Toppenish Creek

Yakima RiverYakima River Yakima Rive r

Yakima Rive r

Naches

Satus CreekSatus Creek

River

River

John Day RiverJohn Day River

Deschutes Deschutes

Lower Crab Cr

er Crab Cr

Crab

SpokaneSpokane

Creek

River

Hangman CreekHangman Creek

RIVER

Sa ml on Sa ml on

SNAKE

Coeur

d'Alene

Lake

Lake

Pend

Oreille

F. D. Roosevelt LakeF. D. Roosevelt Lake

Umatilla Umatilla

River River

GrandeGrande

Ronde

John Day RiverJohn Day River

John

Day River

Fork

North

RIVER

Palouse

River

Palouse

River

Clearwater Clearwater River

Lake

Chelan

Walla Walla River alla Walla River

Clearwater R iverClearwater R iver

Potholes

Reservoir

Rocky

Ford Cr 

River

Imnaha River

WASH INGTON

OREGON

IDAHO

BONNEVILLE

DAM

122° 120° 118° 116°

48°

47°

46°

45°

Base modified from U.S. Geological Survey digital data, various scales. Coordinate system: 

State Plane, Washington South, FIPS 4602: Projection: Lambert Conformal Conic. Horizontal 

datum: North American Datum 1983, Vertical datum: North American Vertical Datum of 1988.

Study

area

WASHINGTON

OREGON IDAHO

No decline

1 to 50

50.1 to 100

100.1 to 150

150.1 to 200

200.1 to 283.9

Groundwater-level decline, in feet 

EXPLANATION

Boundary of study area

Management area—Areas shown on figure 38.

Active model domain

Snake River—Water may be limited during April–August

WASHINGTON

Columbia River—No new irrigation water rights during 

July–August; M&I water is available with mitigation

Snake River—No new irrigation water rights during 

April–August; M&I water is available with mitigation

Columbia and Snake River—No new water rights during April 15–

September 30 (limited exceptions)

OREGON

Note: M&I, municipal and industrial

0 20 40 60 80 MILES

0 20 40 60 80 KILOMETERS
Groundwater Availability and Sustainability 69

typically is influenced by market prices of agricultural 

commodities (both domestic demand and foreign markets, 

for which Washington State agricultural exports exceeded 

$10 billion in 2012), but water availability enters into 

the mix. The large quantity of irrigated lands in the study 

area dedicated to perennial crops such as apples, cherries, 

asparagus, grapes, hops, hay, and alfalfa (most of which have 

a large water demand) makes changing crop types difficult, 

but water limitations may result in a changing crop mix or a 

decrease in total acreage planted. As an example, with the dry 

conditions in 2014 for the Central Valley in California, some 

almond farmers have pulled at least 20 percent of the trees 

from their orchards because of the lack of irrigation water 

(NBC News, 2014). Almonds are one of the most high-value 

crops in the Central Valley; however, energy costs and poor 

water quality generally limit additional groundwater pumping. 

Indeed, in parts of the CPRAS, the deepening of irrigation 

wells because of declining water levels has led to the need to 

mix water because of the poor quality of warm, sodium-rich 

water from the deeper part of the system. Effects of historical 

usage indicate that in many areas, the groundwater system 

cannot meet existing demands indefinitely, and any increased 

groundwater use under a changing climate will only increase 

the competitive demand for the limited resource. For example, 

the cities in the Odessa subarea cannot sustainably “chase” 

declining levels without encountering poor-quality water with 

higher energy costs and attendant costs of treatment.

Thus far, farmers have adjusted their practices to 

grow more crops per acre-foot of applied water, especially 

in drought years such as 2001 and 2005. The increased 

efficiencies result from changes in crop type, increased 

efficiency in the delivery and use of irrigation water, improved 

productivity, and other changes in farming practices. In some 

areas of groundwater irrigation, less land is being cropped, 

for example, by irrigating three 160-acre circles instead of 

four, because of declining water levels and or additional water 

needs during dry, warm years. Even with improvements or 

less cropping, groundwater storage loss has continued to 

occur from 1985 through 2014. Based on the simulated flow 

system under 2050 conditions, groundwater will continue 

to be removed from storage (fig. 39) and the availability of 

groundwater will decrease.

Implications for Groundwater Management

Since the late 1970s, State and Federal water projects 

have not expanded with growing urbanization and the 

increased agricultural and environmental uses of water 

on the Columbia Plateau. Although irrigated agriculture 

continues to use most of the groundwater, other categories 

of groundwater use have increased (fig. 23B). The study area 

faces ongoing and projected increases in competing demands 

for water resources, none of which are likely to be met by 

improvements in agricultural efficiencies. These demands 

include providing water for a growing population, agriculture, 

and environmental uses. The demand for water resources 

by people directly competes with environmental uses such 

as maintaining minimum streamflows and preserving fish 

habitat, and this competition also is expected to increase. The 

interrelationship between these competing demands and legal 

constraints is clearly indicated by the spatial distribution of 

more than 45,000 groundwater (exempt wells not shown) 

and surface-water rights for Washington and Oregon, and 

30,000 claims for Washington (fig. 40). Importantly, figure 40

does not show many of the instream or target flows, nor does 

is show the thousands of outstanding applications for new 

water rights.

An integrated water-management approach could help 

meet competing demands. An analysis of the future conditions 

for water demand in the Columbia River Basin stated that 

groundwater dynamics need to be incorporated in the analysis 

of water supply and agricultural irrigation demand modeling 

because of its relation to surface water (Washington State 

Department of Ecology, 2011). Ongoing and increased use of 

management actions includes enhancements in conjunctive 

use of surface water and groundwater, artificial recharge, and 

the use of recycled or reclaimed water. The development of 

new, off-stream surface-water storage also can be a viable 

option (Washington State Department of Ecology, 2012b), but 

finding sites and mitigating environmental consequences is 

proving to be elusive. Artificial recharge may be approached in 

two ways. The first way is the use of injected water by major 

users, which currently is limited to several large municipal 

systems. The second way includes the spreading of water to 

recharge larger areas of the groundwater system and taking 

advantage of irrigation canal losses by running water through 

the delivery/drain network prior to the irrigation season. 

The second way would produce the most benefit for longterm sustainability of the resource under existing and future 

conditions (replacing lost snowpack) within the areas where 

surface-water networks exist. This method could provide 

water in the shallow system for both groundwater and surfacewater uses, and perhaps delay the release of reservoir water 

to meet demands. Karlinger and Hansen (1983), however, 

indicate that an artificial-recharge-irrigation operation may 

not be an economic alternative to a surface-water irrigation 

system in groundwater areas. Indeed, the Washington State 

Department of Ecology and Reclamation have analyzed the 

expansion of the CBP into the groundwater-irrigated areas 

in place of artificial recharge (Bureau of Reclamation and 

Washington State Department of Ecology, 2010; see also 

Washington State Department of Ecology, 2015).
70 Groundwater Availability of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

Figure 40. Distribution of points of withdrawal for groundwater and surface-water rights and claims in Washington and Oregon in or 

near the Columbia Plateau Regional Aquifer System study area, Washington, Oregon, and Idaho. 

watac11-0648_fig40

Grand Coulee Dam

B L U E M O U N T A I N S

CASCADE RANGE

OKANOGAN HIGHLANDS

CASCADE RANGE

WALLOWA

MOUNTAINS

R O C K Y M O U N T A I N S 

R O C K Y M O U N T A I N S

HORSE HEAVEN HILLS

BEEZLEY HILLS

Snake

River

Crab Creek

Columbia

River

COL

UMBIA

Naches River

Yakima River

Umatilla River

Columbia

River

RIVER

N orth Fork John Day River

Snake River

Imnaha River

Banks

Lake Coeur

D'Alene

Lake

Grande Ronde River

Salmon River

John D ay River

Moses Coulee

Walla Walla River

Deschutes River

Klickitat River

White Salmon R 

Clearwater

River

Hangman Cr

 F.D Ro osevelt La ke

Hood R

Wenatch e e R.

Palouse River

Satus

T

Cr

oppenish Creek

Spokane

River

Potholes 

Reservoir

Moses

Lake

Rocky Ford Cr.

Ellensburg

Umatilla

Walla Walla

WASHINGTON

OREGON

IDAHO

Yakima

Heppner

MiltonFreewater

Pasco

Lewiston

Moscow

Spokane

The

Dalles

La Grande

122° 120° 118° 116°

46°

48°

47°

45°

Base modified from U.S. Geological Survey digital data, various scales. Coordinate system: 

State Plane, Washington South, FIPS 4602: Projection: Lambert Conformal Conic. Horizontal 

datum: North American Datum 1983, Vertical datum: North American Vertical Datum of 1988.

John Day RiverJohn Day River

0 20 40 60 80 MILES

0 20 40 60 80 KILOMETERS

Study

area

WASHINGTON

OREGON IDAHO

EXPLANATION

Study area boundary

Right

Claim

Point of withdrawal for groundwater 

and surface water
Monitoring the Hydrologic System 71

Monitoring the Hydrologic System

The assessment of water availability, both groundwater 

and surface water, is dependent on monitored data. Surfacewater data are intricately tied to groundwater availability 

because most senior water rights are for surface water, and 

these rights may be impaired by groundwater pumping—there 

is a limit on available water. Monitored data include streamflow, 

water-level, and water-use. Each of these data components 

was integral in the assessment of groundwater availability for 

the CPRAS. The first two categories are discussed in the two 

subsections that follow. Water use was discussed in depth by the 

National Research Council (2002); within the study area, the 

USGS does not directly monitor or estimate water use except as 

part of individual water-resource investigations such as in the 

CPRAS study. The USGS compiles water-use information by 

county and State every 5 years (for example, see Lane, 2009) 

as part of the National Water-Use Information Program (http://

water.usgs.gov/watuse/wunwup.html). Water quality can affect 

water availability, and water quality in the CPRAS is monitored 

by many entities. Water quality has been monitored and 

described for large parts of the study area as part of the USGS 

National Water-Quality Assessment Program, and a summary 

of the studies in the study area and the program is available at 

http://wa.water.usgs.gov/projects/ccyk/. The potential waterquality effects on availability were beyond the scope of this 

study, as was a description of the monitoring of quality. 

Surface Water

Monitoring of surface water on the Columbia Plateau 

has a much longer history than groundwater monitoring. Early 

settlers used the surface waters for irrigation and livestock 

needs, but the apportioning of water among users (water 

law) quickly became an issue. Only with measurements of 

streamflow could water be properly apportioned.

 The monitoring of surface water by the USGS in the study 

area began in 1878 for the Columbia River at The Dalles. After 

that time, USGS streamgage sites were established on streams 

in or near the study area starting about 1893, especially in the 

Yakima River Basin. Many of these early sites were used to 

monitor streamflow to provide information on the potential for 

power production and (or) irrigation supply (for example, see 

Parker and Storey, 1916). Later, some sites were established to:

• Estimate the amount of surface water that was available 

in a basin for allocation,

• Develop information on peak flows (floods) for floodcontrol studies,

• Monitor streamflow used for power production/power 

revenue forecasts (such as along the Columbia and 

Snake Rivers),

• Conduct hydroregulation studies,

• Analyze environmental effects,

• Plan for operations, or

• Define the contribution of smaller tributary basins to a 

larger river system.

As of water year 2012, there were 77 active and 131 

discontinued streamflow measurement sites in the study area 

and 28 active sites measuring streamflow entering the study 

area (fig. 41). Various users rely on water that enters the 

study area, which is under legal constraints that affect water 

availability. Thus, measurement of streamflow entering the 

study area is important for understanding the hydrologic system 

and its uses. The discontinued streamgages include historical 

sites that may have operated for numerous years, such as from 

1906 to 1948, and sites that were established as part of studies 

that may have operated for several months to several years. 

More than 1,800 miscellaneous measurements were made at 

sites in or near the study area. Data were collected at these 

sites for various reasons—for example, seepage investigations 

to estimate gains or losses along a stream, flood studies, 

low-flow analyses, fish-flow analyses for salmonids, and 

water-quality studies. 

The historical, current, or real-time data for monitoring 

sites, therefore, are used for many purposes:

• For estimating recurrence intervals for floods,

• For flood warnings, 

• For monitoring instream and target-flow quantities,

• For management of irrigation supply, and

• For management of diversions.

The sites with longer periods of record provide a base for 

calculating descriptive statistics (such as the 1- to 7-day lowflow characteristics) and for assessing trends over time, such as 

an early runoff owing to changing climate.

Groundwater

Knowledge of current and historical groundwater levels 

are a key component for the assessment of groundwater 

availability, including the status and trends evaluation (Burns 

and others, 2011), groundwater flow modeling (Ely and 

others, 2014), and availability/sustainability analysis of the 

CPRAS. Water-level data have been collected in the study 

area since about 1896, but most measurements in the study 

area occurred after the 1940s (fig. 42A), with about 5,600 

water levels measured through 1939, 12,610 measured in the 

1940s, and a more than doubling to about 31,100 measured 

in the 1950s (fig. 42B). The largest increase in the number 
72 Groundwater Availability of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

Figure 41. Location of U.S. Geological Survey streamflow measurement sites, Columbia Plateau Regional Aquifer System, 

Washington, Oregon, and Idaho.

Ellensburg

Umatilla

Walla

Walla

Yakima

Heppner

MiltonFreewater

Pasco

Lewiston

Moscow

Spokane

The

Dalles

La Grande

B L U E M O U N T A I N S

CASCADE RANGE

OKANOGAN HIGHLANDS

CASCADE RANGE

WALLOWA

MOUNTAINS

R O C K Y M O U N T A I N S 

R O C K Y M O U N T A I N S

HORSE HEAVEN HILLS

BEEZLEY HILLS

W A S H I N G T O N

O R E G O N

I D A H O

122° 120° 118° 116°

48°

47°

46°

45°

0 20 40 60 80 MILES

0 20 40 60 80 KILOMETERS

Base modified from U.S. Geological Survey digital data, various scales. Coordinate system: 

State Plane, Washington South, FIPS 4602: Projection: Lambert Conformal Conic. Horizontal 

datum: North American Datum 1983, Vertical datum: North American Vertical Datum of 1988.

Study

area

WASHINGTON

OREGON IDAHO

watac11-0648_fig41

EXPLANATION

Streamflow measurement site

Active

Discontinued

Miscellaneous

Study area boundary
Monitoring the Hydrologic System 73

tac11-0648_fig42

Calendar year

B

A

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

1930 1940 1950 1960 1970 1980 1990 2000 2010

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

80,000

90,000

100,000

110,000

1900–1929 1930–1939 1940–1949 1950–1959 1960–1969 1970–1979 1980–1989 1990–1999 2000–2009

Number of water level measurements Number of water level measurements, summed by decade 

Decade

Figure 42. Number of water levels in the National Water Inventory System (A) by year, and (B) by decade, Columbia 

Plateau Regional Aquifer System, Washington, Oregon, and Idaho.

of measurements (27,000) occurred from the 1980s to the 

1990s, with the total number of about 108,000 measurements 

in the 1990s accounting for about 24 percent of the more than 

444,000 measurements in the USGS National Water Inventory 

System (NWIS; fig. 42B). The number of measurements 

ranged widely from year to year and indicated the need for 

more measurements as a result of population growth, irrigation 

increases, and environmental needs such as instream flows. The 

annual number of measurements is strongly related to increased 

measurements during droughts and the timing of groundwater 

studies. The distribution of more than 39,000 wells with water 

levels (fig. 43) indicates that much of the study area has some 

measurements; the distribution does not include all wells with 

water levels, but is a subset used by Burns and others (2011) 

to assess the status and trends of the groundwater system. The 

high density of data in selected areas corresponds to areas 

where groundwater sustainability is of particular concern 

(see fig. 38), and studies were done in these areas many years 
74 Groundwater Availability of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

Figure 43. Distribution of wells with water levels in the National Water Inventory System, Columbia Plateau Regional Aquifer 

System, Washington, Oregon, and Idaho.

watac11-0648_fig43

Snake

River

Crab Creek

Columbia

River

COL

UMBIA

Naches River

Yakima River

Umatilla River

Columbia

River

RIVER

N orth Fork John Day River

Snake River

Imnaha River

Banks

Lake Coeur

D'Alene

Lake

Grande Ronde River

Salmon River

John D ay River

Moses Coulee

Walla Walla River

Deschutes River

Klickitat River

White Salmon R 

Clearwater

River

Hangman Cr

 F.D Ro osevelt La ke

Hood R

Wenatch e e R.

Palouse River

Satus

T

Cr

oppenish Creek

Spokane

River

Potholes 

Reservoir

Moses

Lake

Rocky Ford Cr. Grand Coulee Dam

B L U E M O U N T A I N S

CASCADE RANGE

OKANOGAN HIGHLANDS

CASCADE RANGE

WALLOWA

MOUNTAINS

R O C K Y M O U N T A I N S 

R O C K Y M O U N T A I N S

HORSE HEAVEN HILLS

BEEZLEY HILLS

Ellensburg

Umatilla

Walla Walla

WASHINGTON

OREGON

IDAHO

Yakima

Heppner

MiltonFreewater

Pasco

Lewiston

Moscow

Spokane

The

Dalles

La Grande

122° 120° 118° 116°

46°

48°

47°

45°

0 20 40 60 80 MILES

0 20 40 60 80 KILOMETERS

Base modified from U.S. Geological Survey digital data, various scales. Coordinate system: 

State Plane, Washington South, FIPS 4602: Projection: Lambert Conformal Conic. Horizontal 

datum: North American Datum 1983, Vertical datum: North American Vertical Datum of 1988.

Study

area

WASHINGTON

OREGON IDAHO

EXPLANATION

Study area boundary

Wells
Monitoring the Hydrologic System 75

ago (for example, Newcomb, 1965). Such areas of concern 

have expanded, as shown by the high density of data in the 

northeastern part of the study area. In the areas with a high 

density of data, the groundwater resources have been studied 

intensively over time, with concomitant measurements of 

water levels.

There are two categories of water-level monitoring. 

The first category includes wells that are monitored from 

over the course of a season to as many as several years for 

a specific purpose (these represent most of the water levels 

in NWIS), typically in support of area-specific groundwater 

investigations. The second category consists of networks 

of wells for systematically collecting long-term water-level 

data. Such networks were established in about the 1980s in 

Washington, the 1960s in Oregon, and the 1950s in Idaho 

by the respective State water agencies (for Idaho, USGS 

initiated measuring water levels in the 1950s). These longterm networks include about 905 wells that withdraw water 

from various hydrogeologic units; these wells are included in 

the distribution shown in figure 43. Many of these networkmonitoring wells are in areas of declining water levels. 

The Washington network also includes more than 10 deep 

wells that contain multiple piezometers. The Yakama Nation 

monitors about 113 wells on Tribal lands, and hundreds of 

wells are monitored on the Hanford Site as part of historical 

and ongoing nuclear waste restoration activities. As of 2014, 

the USGS monitors about 110 wells in the study area. 

Use of Water-Level Data

The two categories of water-level monitoring are 

complementary. For example, the CP-RASA monitoring 

of water levels over a 3-year period (1983–85) provided 

information to: 

• Map groundwater levels by hydrogeologic units;

• Assess water-level changes over this period and 

compare these changes to those mapped for 1967–81 

(Cline, 1981);

• Calibrate the CP-RASA regional-flow model; and

• Describe the flow system in a detailed manner, 

including lateral and vertical hydraulic gradients, 

and possible effects of geologic structure on the flow 

system.

Afterward, State monitoring networks (which included many 

of the CP-RASA monitored wells) provided information on 

the status and trends of the mapped groundwater levels. The 

CPRAS monitoring (Snyder and Haynes, 2010) incorporated 

the previous data to estimate the spatial changes in water 

levels from 1984 to 2009, and described local changes based 

on the information from State networks. 

Hydrographs from several wells (fig. 44A–D) show the 

complementary nature of the monitoring. Early measurements 

were made to improve the understanding of the flow system 

as part of groundwater-resource investigations. Wells were 

then added to a monitoring network, resulting in longterm records that indicate the trends or lack of trends in 

water levels and potential effects on sustainability. For 

example, the 1,510-ft-deep well (fig. 44A) in the Toppenish 

Creek Basin in the Yakima River Basin, was measured 

during three groundwater investigations (the CP-RASA 

measurements were made in the mid-1980s), and was then 

added to the Yakama Nation network in 1992; additional 

1960 19651970197519801985199019952000

A. 10N/17E-27Q01, Well depth 1,510 feet

Saddle Mountains-Wanapum-Grande Ronde unit composite

Water level below land surface, in feet

150

200

250

300

350

400

1960 19651970197519801985199019952000

B. 13N/24E-27M01, Well depth 723 feet

Saddle Mountains-Wanapum unit composite

Water level below land surface, in feet

150

200

250

300

350

400

Figure 44. Water levels for selected wells in the (A) 

Toppenish Creek Basin, (B) Cold Creek drainage, (C) 

Umatilla Basin, and (D) Toppenish Basin, Columbia Plateau 

Regional Aquifer System, Washington, Oregon, and Idaho.
76 Groundwater Availability of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

tac11-0648_fig44c

C

EXPLANATION

Water-level elevations and regression line 

fits to elevations for each group of wells

200

100

1940 2010

300

400

500

600

700

800

900

1,000

1,100

1,200

1,300

1,400

1,500

1,600

1,700

1,800

1,900

2,000

Water-level elevation, in feet (North American Vertical Datum of 1988)

1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

D. 11N/19E-10A02, Well depth 765 feet

Overburden unit (Ellensburg Formation)

0

5

10

15

20

25

30

35

40

1960

Water level below land surface, in feet

19651970197519801985199019952000

Figure 44.—Continued.

measurements were made in the earlier 2000s as part of the 

YRBAS study. The hydrograph of the well clearly shows 

a trend of decreasing water levels that are consistent with 

measurements from nearby deep basalt wells. Similarly, 

the 723-ft-deep basalt well (fig. 44B) north of the Hanford 

Site near the Cold Creek drainage was measured as part 

of past investigations, with Washington State adding it to 

their monitoring network after the CP-RASA ended. The 

hydrographs for wells in the Umatilla Basin (fig. 44C, from 

Burns and others, 2011) are primarily based on measurements 

from the Oregon Water Resources Department network, but 

also include measurements made during USGS groundwater 

investigations. These wells are in critical groundwater and 

restrictively classified management areas of Oregon (fig. 38), 

and the water levels provide information for managing the 
References Cited 77

resources and understanding the complex flow system in this 

area (Burns and others, 2012). The complexity of the response 

in the aquifer system to pumping is clearly shown by the 

variations in water levels between these groups of wells. Even 

without a monitoring network, water levels measured as part 

of investigations at different time periods provide valuable 

information. For example, the 765-ft-deep well (fig. 44D) in 

the Toppenish Basin was measured during the 1970s and later 

during the YRBAS, and the water levels over this 30-year 

period indicate that there are only seasonal but no long-term 

trends—groundwater use seems sustainable at this location.

Future Uses of Model and Challenges 

to Assessing Groundwater Availability

The CPRAS hydrologic model uses a set of mathematical 

equations to represent an extremely complex natural system 

that has been perturbed by human activities. The model was 

constructed to simulate regional-scale groundwater flow; 

therefore, it can be used to address questions regarding issues 

at that scale that are relevant to water management. For 

example, on a regional scale, interactions can be considered 

between hydraulic heads, groundwater discharge, pumping, 

and flow direction and magnitude. The annual time step 

used in the model was selected to provide adequate temporal 

resolution for analyses of variations in pumping and recharge 

rates corresponding to numerous years of changes in recharge, 

irrigation practices, and pumpage. Effects of management 

decisions on time scales of less than a year are unlikely to be 

adequately simulated, but the model can appropriately be used 

to simulate annual changes to water-use practices, crop types, 

or potential future climate.

As shown by this study, groundwater availability can be 

evaluated with the model by examining the effects of human 

and climatic temporal changes on the groundwater-budget 

components provided by the model. An additional benefit is 

the ability to forecast system response to these same effects 

(including potential management scenarios), thus providing 

insights into the longer-term sustainability of the groundwaterflow system. Alternative conceptualizations of the flow system 

that are likely to have a regional effect also can be evaluated. 

These conceptualizations might include the effects of changes 

in recharge caused by climate change, different interpretations 

of the extent of geologic structure, or other conceptual models 

(Faunt and others, 2004). Flow direction and magnitude also 

may be appropriately represented using particle tracking 

methods, as long as the particle paths are interpreted to 

represent advective-transport flow paths that are at least 

several times longer than the length of a model cell (Tiedeman 

and others, 2003).

Future uses of the flow model could include optimization 

techniques to assist in the management of local or sub-regional 

groundwater problems such as substantial water-level declines 

at concentrated pumping centers. The potential effects of new 

wells on streamflow and existing groundwater users could be 

simulated and evaluated by using the local grid refinement 

package available for MODFLOW-2000.

The model cannot represent completely, or “capture,” 

all the physical processes within the hydrologic system. 

Determining if a model weakness is attributable to input 

data error or model shortcomings is almost impossible, but 

the simplifying assumptions and generalizations that are 

incorporated into a model undoubtedly affect the results of 

the simulation. The model in this study was not designed to 

represent every detail of the hydrologic system (a task beyond 

the scope of the investigation). Model results will vary based 

on which details were and were not included.

Acknowledgments

A project of this magnitude could not be completed 

without the help of many individuals and organizations. 

The authors acknowledge the U.S. Geological Survey 

Groundwater Resources Program for its support of largescale multidisciplinary studies of groundwater availability 

across the United States, including this study. The authors also 

wish to thank the government agencies, Tribal governments, 

groups, and citizens that provided valuable support, access to 

their records, and data during this study. Special thanks are 

extended to John Covert (Washington State Department of 

Ecology); Kenneth Lite, Jon LaMarche, and Dwight French 

(Oregon Water Resources Department); and Shelley Keen, 

Pamela Skaggs, and Sean Vincent (Idaho Department of Water 

Resources) for providing information and their review of 

various sections of this report.

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84 Groundwater Availability of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

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Appendix 1 85

The development of water resources with the concomitant 

economic growth in Washington, Oregon, and Idaho has 

been, is, and will be controlled by State water law, which, as 

in most of the Western United States, is the doctrine of “prior 

appropriation” for both surface water and groundwater. As 

early as 1873, the Washington territorial legislature approved 

the principle of appropriative water rights—the first in time 

is the first in right. The priority date, common to all the State 

water codes in the study area, establishes the temporal relation 

between water rights, with the oldest or most senior rights 

receiving water first, before later, more junior rights. This 

concept is important in overallocated basins or in drought 

years when there may not be enough water available to meet 

all appropriated rights and, thus, there may be no water 

remaining for the junior water-right holders after the more 

senior water-right holders receive their water. With respect 

to groundwater availability, the potential impairment of more 

senior surface-water rights by junior groundwater rights is an 

issue in all three states. An important aspect of the priority 

date is the “Winters Doctrine.” This 1908 U.S. Supreme Court 

ruling established the principle of reserved water rights for 

Indian Tribes with reservations in sufficient quantities under 

the terms of the establishment of the reservations; the priority 

date of each reserved right is the date of the treaty. These 

rights, many of which have not been quantified, generally 

have much earlier priority dates than non-Indian rights. 

Additionally, some tribal rights associated with treaty rights in 

the study area are considered to have a priority date of “time 

immemorial.” Laws of each of the three States also include a 

beneficial use and a “use-it-or-lose-it” provision. Beneficial 

uses include irrigation, municipal, livestock, domestic, and 

industrial uses. Use-it-or-lose-it means that if a water right is 

not used for beneficial use for 5 consecutive years, it may be 

forfeited; however, the Indian reserved rights are not bound by 

this provision. The creation and implementation of the water 

laws for the three States in the study area has directly affected 

the development of water resources and, thus, the economic 

development of the region. A brief overview of the creation 

of the water codes for each of the three States is provided in 

this appendix; a more detailed summary is available at https://

archive.org/details/westernstateswat4002heco for the three 

States, and at http://www.idwr.idaho.gov/WaterManagement/

default.htm for Idaho, http://egov.oregon.gov/OWRD/LAW/

index.shtml for Oregon, and http://www.ecy.wa.gov/programs/

wr/rights/water-right-home.html for Washington. The 

constitutions and statutes of all three States define all waters 

within the State as public waters administered by the State, 

and these definitions underlie the water codes of these States. 

Note that the brief description of State water law captures the 

essence of the law, but it does not capture the true complexity 

and nuances inherent in State water law.

Although prior appropriation was established as the 

general principle of water law in the study area, many of 

the settlers came from States with riparian water rights 

and assumed that similar rights prevailed in the territories 

and, later, in the three States. The riparian doctrine allows 

landowners with property bordering a stream to use the water 

as long as that use does not unreasonably affect downstream 

water uses or earlier rights. For example, as late as 1912, 

Parker and Storey (1916) state that in the Yakima River Basin 

“the scarcity of water resulting from overappropriation has 

led to much litigation and adjudication of water rights, and 

the rights of the valley are in a condition of chaos, owing to 

the absence of a water code in the State of Washington, to 

conflicting claims of riparian owners and prior appropriators, 

and to the exorbitant filings made for many canals.” They 

further state that the water-appropriation filings in the basin 

were enough to irrigate several States as large as Washington. 

It was not until 1917 that Washington passed a water code for 

surface water (8 years after Oregon) that in effect established 

the prior appropriation principle, but also included remnants 

of the riparian doctrine. Note that Idaho does not recognize 

riparian water rights but Oregon does for rights with priority 

dates prior to 1909.

Some laws were passed by the Washington legislature 

for groundwater, such as capping artesian wells when not in 

use, but a State groundwater code (modeled after the surfacewater code) was not implemented until 1945. This code has an 

exemption in the permitting process for single wells producing 

up to 5,000 gallons per day (gal/d) (referred to as “exempt 

wells”), and an unlimited exemption for livestock wells. As 

in Idaho and Oregon, there is an exemption for irrigating as 

much as one-half acre. The Washington groundwater code also 

explicitly states that surface-water rights cannot be impaired 

by groundwater rights. Surface-water and groundwater 

rights applied for after 1917 and 1945, respectively, are 

based on first in time. Beneficial use of surface water put to 

use prior to 1917 is acceptable, but the use occurs without 

a permit or certificate; the actual water right is only clearly 

defined by adjudication. To date (2014), 82 drainage basins 

(mostly small streams) have been adjudicated. More than 

165,000 Statements of Water Right Claims were filed in the 

Washington State during the four claims registry periods. 

Only a small number (about 4,500) of these claims has been 

adjudicated. There is no current timeframe for adjudicating the 

remaining claims. As of 2014, only one adjudication is active 

in the State. Adjudication of the Yakima River Basin is nearly 

complete. This adjudication has been ongoing for more than 

30 years at a cost of tens of millions of dollars. 

Similarly, users of groundwater prior to 1945 would need 

to file a claim for the water, and the claims and post-1945 

water rights would need to be adjudicated to clearly define 

the purpose, quantity, and location of use of the claims. As 

in Oregon and Idaho, groundwater claims provide a means 

for identifying water rights with no previous records, but 

importantly, claims are not confirmations of water rights. As 

of 2011, only 400 groundwater claims have been adjudicated 

in Washington.

Appendix 1. An Overview of Water Law in Washington, Oregon, and Idaho
86 Groundwater Availability of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

In 1909, Oregon created its surface-water code based 

on the prior appropriation doctrine. However, prior to the 

code, Oregon recognized riparian water rights; thus, Oregon 

has a dual system. The groundwater code initially was 

established for the area east of the Cascade Range in 1927 

and was modeled after the surface-water code. Applications 

for groundwater use are assessed for their potential effect on 

other wells and surface-water rights/claims. Oregon also has 

an exempt well provision for wells that are not required to go 

through the permitting process. These provisions allow for 

single or group domestic uses not exceeding 15,000 gal/d, 

lawn watering of less than one-half acre, stock watering (as 

in Washington and Idaho), and single industrial/commercial 

uses not exceeding 5,000 gal/d. For surface-water uses prior to 

1909, adjudications are used to determine water rights. As in 

Washington, a claim needs to be filed. The State reviews these 

claims and makes a determination of their validity. If there 

is an appeal of the State determination, a circuit court would 

then agree with the State or modify the State determination, 

generally resulting in a water right with a certificate. Most of 

the pre-1909 claims in the study area in Oregon have been 

adjudicated.

Idaho established a groundwater code in 1963 and a 

surface-water code in 1971. As in Washington and Oregon, 

the Idaho codes allow water rights to be granted only under 

an application and permitting procedure and the priority date 

for the rights is the date of filing of the application. Prior 

to the establishment of codes for groundwater (1963) and 

surface water (1971), there were two methods to obtain water 

rights. The first method was simply to put water to beneficial 

use. These rights, referred to as “historic” or “beneficial” use 

rights, have a priority date of when the water was first put to 

use. However, until adjudication is completed, these rights 

are not associated with a permit/certificate/decree. The second 

method made use of the posted-notice statute that was in 

effect in Idaho prior to 1903. Under this statute, people would 

post a notice at the point of diversion and record the notice 

with the county where the diversion occurred. These rights 

would have a priority date of when the notice was posted, and 

are essentially the same as beneficial rights. Idaho also has 

an exempt well exclusion where domestic uses do not have 

to go through the permitting process. Idaho allows one-half 

acre of land to be irrigated under this exemption (the same as 

Washington and Oregon) and to use as much as 13,000 gal/d 

or 0.04 ft3

/s. Instream livestock water use also is a defined 

exempt use in Idaho. The water-rights adjudication process in 

Idaho is similar to the process in Washington. To date (2011), 

the only major adjudication has been completed for the Snake 

River Basin. 

Various other statutes, laws, and rules have been 

implemented by the three States that also affect water 

availability in the study area. For example, each State now 

(2014) considers instream flows a beneficial use and has 

passed legislation to protect instream flows that can reduce the 

quantity of surface water available for future appropriation. As 

part of a 1955 groundwater law, Oregon can regulate water by 

designating “Critical Groundwater Areas.” As of 2011, there 

were four such areas on the Columbia Plateau in Oregon and 

two other areas are designated “restrictively classified.” In 

one area, the Moiser area, new appropriation of groundwater 

mostly has been stopped. Oregon also manages groundwater 

and surface water conjunctively by basin; within a basin, 

further appropriation of water for parts or the entire basin 

can be restricted. Idaho also manages water by basin and has 

about 50 administrative units or basins under this management 

strategy. In several areas, parts or all of these units have 

a moratorium on some uses. For groundwater, Idaho has 

legislation that allows for the creation of water-measurement 

districts and groundwater districts, and has authority for 

establishing Ground Water Management and Critical Ground 

Water Areas; there are two Ground Water Management 

Areas in the study area. Washington legislation allows for 

the development of groundwater-management areas, and 

for declaring critical areas where groundwater levels are 

declining and no new appropriations are allowed. Other laws 

in Washington established a watershed-management program 

in which the water resources in a basin, including water rights, 

are analyzed. In some cases, such analyses have resulted in 

limiting future appropriations, including limiting exempt 

wells. For basins considered fully or over-appropriated, the 

three States generally require the retiring (purchase) of a 

surface-water right (based on its estimated consumptive use) 

in exchange for allowing new groundwater withdrawals; this 

process typically is referred to as “mitigation.” Finally, a major 

issue in the study area and the West generally is the status of 

exempt-well statutes (http://www.lclark.edu/live/files/4541-

401bracken.html) because it is recognized that exempt wells 

affect water-management strategies and may affect senior 

surface-water and groundwater rights.
Appendix 2 87

The information presented in figure 38 is from John 

Covert (Washington State Department of Ecology, written 

commun., 2011); Kenneth Lite, Jon La Marche, and Dwight 

French (Oregon Water Resources Department, written 

commun., 2011); and Shelley Keen, Pamela Skaggs, and 

Sean Vincent (Idaho Water Resources Department, written 

commun., 2011). Water-availability information for Water 

Resource Inventory Areas in Washington is available at http://

www.ecy.wa.gov/programs/wr/rights/wrpenapp_avail.html, 

Washington State digital archives contain a map of overappropriated basins in Washington presented in the Statewide 

Strategy to Recover Salmon (http://www.digitalarchives.

wa.gov/GovernorLocke/gsro/strategy/summary/background.

htm), various maps for Oregon are available at http://egov.

oregon.gov/OWRD/MAPS/index.shtml, and Groundwater 

Management Areas in Idaho are available at http://www.

idwr.idaho.gov/WaterInformation/GroundWaterManagement/

designated_areas.htm.

Appendix 2. Sources of Information for “Dry” Season Water-Availability Map

Publishing support provided by the U.S. Geological Survey

Science Publishing Network, Tacoma Publishing Service Center

For more information concerning the research in this report, contact the

 Director, Washington Water Science Center

U.S. Geological Survey

934 Broadway, Suite 300

Tacoma, Washington 98402 

http://wa.water.usgs.gov
Groundwater Availability of the 

Columbia Plateau Regional Aquifer System, 

Washington, Oregon, and Idaho

Vaccaro and others

—Groundwater Availability of the Columbia Plateau Regional Aquifer System

—Professional Paper 1817

ISSN 1044-9612 (print)

ISSN 2330-7102 (online)

http://dx.doi.org/10.3133/pp1817 Printed on recycled paper