U.S. Geological Survey Groundwater Resources Program and

prepared in cooperation with the Oregon Water Resources Department

Groundwater Status and Trends for the 

Columbia Plateau Regional Aquifer System, 

Washington, Oregon, and Idaho

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2012–5261
Cover: Lower left: Map showing select groups of wells with similar hydraulic 

response within portions of the Umatilla Basin, Oregon. Hydrographs showing 

the trend lines of spring median water levels for select groups of wells with 

similar hydraulic response in the Umatilla Basin, Oregon (East-West groups). 

(See figures 10 and 11B, respectively, in the report.)

Center foreground: Map showing distribution of groundwater-level trends in 

the Columbia Plateau Regional Aquifer System for the period 1968–2009. Red 

circles indicate groundwater-level declines, yellow circles indicate little or 

no change in groundwater levels, and blue circles indicate groundwater-level 

rises. Circle size is representative of magnitude of change.

(See figure 8 in the report.)

Lower right: Map showing select groups of wells with similar hydraulic 

response within portions of the Palouse Slope and the Columbia Basin Ground 

Water Management Area, Washington. Hydrographs showing the trend 

lines of spring median water levels for groups of wells with similar hydraulic 

response in the vicinity of the Palouse Slope and the Columbia Basin Ground 

Water Management Area, Washington (Eastern Flowpath groups).

(See figures 15 and 17, respectively, in the report)

Background: Photograph of Frenchman’s Coulee in the vicinity of Quincy, 

Washington. (Photograph taken by Peter Prehn, private citizen, 2008. Used 

with permission.)
Groundwater Status and Trends for the 

Columbia Plateau Regional Aquifer System, 

Washington, Oregon, and Idaho

By Erick R. Burns, Daniel T. Snyder, Jonathan V. Haynes, and Michael S. Waibel

U.S. Geological Survey Groundwater Resources Program and 

prepared in cooperation with the Oregon Water Resources Department

Scientific Investigations Report 2012–5261

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012

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

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Burns, E.R, Snyder, D.T, Haynes, J.V., and Waibel, M.S., 2012, Groundwater status and trends for the Columbia 

Plateau Regional Aquifer System, Washington, Oregon, and Idaho: U.S. Geological Survey Scientific Investigations 

Report 2012–5261, 52 p., http://pubs.er.usgs.gov/publication/sir20125261.
iii

Contents

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

Introduction.....................................................................................................................................................2

Purpose and Scope ..............................................................................................................................2

Description of Study Area ...................................................................................................................4

Hydrogeologic Setting ................................................................................................................4

Hydrogeologic Characteristics.........................................................................................5

Hydrologic Budget Components ......................................................................................7

Groundwater Occurrence and Movement .....................................................................7

Study Methods ......................................................................................................................................8

Sources of Data ...........................................................................................................................8

Groundwater-Level Information ......................................................................................8

Geologic Model.................................................................................................................10

Data Limitations..........................................................................................................................10

Measurement Accuracy and Data Compilation ..........................................................10

Representative Sampling ................................................................................................10

Localized Conditions and Stresses.......................................................................11

Influence of Well Construction..............................................................................11

Commingling .............................................................................................................12

Groundwater Status and Trends ...............................................................................................................15

Umatilla Area, Oregon........................................................................................................................18

Palouse Slope/Eastern Yakima Fold Belt, Washington.................................................................28

Summary and Conclusions.........................................................................................................................43

Acknowledgments.......................................................................................................................................45

Selected References...................................................................................................................................45

Appendix A. Interpretation Guide for Graphs of Stratigraphic Position Compared with 

Hydraulic Head for Wells in the Columbia River Basalt Group, Washington, Oregon, 

and Idaho.........................................................................................................................................49
iv

Figures

Figure 1. Map showing location of Columbia Plateau Regional Aquifer System of

Washington, Oregon, and Idaho ......................................................... 3

Figure 2. Map showing surficial geology and structures of the Columbia Plateau

Regional Aquifer System, Washington, Oregon, and Idaho ........................... 6

Figure 3. Map showing locations of wells with usable water-level data within or

immediately adjacent to the Columbia Plateau Regional Aquifer System,

Washington, Oregon, and Idaho, subsets of which were used for analysis in

this study .................................................................................... 9

Figure 4. Diagram showing effect of commingling on water levels in hypothetical wells

for aquifers of equal transmissivity ...................................................... 12

Figure 5. Diagram showing effect of commingling on water levels in hypothetical wells

for aquifers of unequal transmissivity ................................................... 13

Figure 6. Diagram showing effect of commingling on water levels in hypothetical wells

for aquifers with local variability ......................................................... 14

Figure 7. Map showing distribution of wells used for status and trends analysis for the

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

Figure 8. Map showing distribution of groundwater-level trends in the Columbia Plateau

Regional Aquifer System, Washington, Oregon, and Idaho, for 1968–2009 ......... 17

Figure 9. Map showing distribution of groundwater-level trends for 1968–2009 in Oregon

Water Resources Department administrative areas in the Umatilla Basin,

Oregon ....................................................................................... 19

Figure 10. Map showing groups of wells with similar hydraulic response near the Oregon

Water Resources Department administrative areas in the Umatilla Basin,

Oregon ....................................................................................... 20

Figure 11. Hydrographs showing groups of wells near the Oregon Water Resources

Department administrative areas in the Umatilla Basin, Oregon ..................... 21

Figure 12. Graphs showing estimated stratigraphic position (distance from the well

bottom to the simulated top of Grande Ronde Basalt) compared with winter

median water-level elevation in selected wells within and immediately

adjacent to the Oregon Water Resources Department administrative areas in

the Umatilla Basin, Oregon, 1968, 1984, and 2006 ....................................... 23

Figure 13. Graph showing stratigraphic position (distance from the well bottom to the

simulated top of Grande Ronde Basalt) compared with winter median water

level measured in wells for shallow–deep well pairs from groups 3 and 14 near

the Oregon Water Resources Department administrative areas in the Umatilla

Basin, Oregon .............................................................................. 27

Figure 14. Map showing distribution of groundwater-level trends for 1968–2009 relative to

the Washington State Department of Ecology administrative areas in parts of

the Palouse Slope/eastern Yakima Fold Belt and the Columbia Basin Ground

Water Management Area, Washington................................................... 29

Figure 15. Map showing generalized groundwater-flow paths under 2000–2010 conditions

through sets of well groups with similar hydraulic response near the

Washington State Department of Ecology administrative areas in parts of the

Palouse Slope/eastern Yakima Fold Belt and the Columbia Basin Ground Water 

Management Area, Washington ......................................................... 30
v

Figure 16. Hydrographs showing groups of wells corresponding to the western flow path

near the Washington State Department of Ecology administrative areas in

parts of the Palouse Slope/eastern Yakima Fold Belt and the Columbia Basin

Ground Water Management Area, Washington ....................................... 31

Figure 17. Hydrographs showing groups of wells corresponding to the eastern flow path

near the Washington State Department of Ecology administrative areas in

parts of the Palouse Slope/eastern Yakima Fold Belt and the Columbia Basin

Ground Water Management Area, Washington ....................................... 32

Figure 18. Hydrographs showing the middle groups of wells near the Washington State

Department of Ecology administrative areas in parts of the Palouse

Slope/eastern Yakima Fold Belt and the Columbia Basin Ground Water

Management Area, Washington ......................................................... 33

Figure 19. Hydrographs showing groups of wells corresponding to the southern flow path

near the Washington State Department of Ecology administrative areas in

parts of the Palouse Slope/eastern Yakima Fold Belt and the Columbia Basin

Ground Water Management Area, Washington ....................................... 34

Figure 20. Graphs showing distance of the well bottom from the simulated top of Grande

Ronde Basalt compared with water levels measured in wells near the

Washington State Department of Ecology administrative areas in parts of the

Palouse Slope/eastern Yakima Fold Belt and the Columbia Basin Ground Water

Management Area, Washington ......................................................... 36

Figure 21. Map showing generalized potentiometric surface for measured Columbia

River Basalt Group aquifer conditions during the pre-development period in

parts of the Palouse Slope/eastern Yakima Fold Belt and the Columbia Basin

Ground Water Management Area, Washington ....................................... 41

Figure 22. Map showing generalized potentiometric surface for measured Columbia

River Basalt Group aquifer conditions during 2000–2009 in parts of the Palouse

Slope/eastern Yakima Fold Belt and the Columbia Basin Ground Water

Management Area, Washington ......................................................... 42

Figures—Continued
vi

Conversion Factors, Datums, and Abbreviations and 

Acronyms

All conversion factors have been rounded to four significant digits. To reduce errors resulting 

from rounded conversion factors, reverse conversions should multiply by the inverse of the 

conversion factors originally used. For example, to convert feet into meters multiply by 0.3048. To 

convert meters into feet multiply by 1/0.3048.

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2) 259.0 hectare (ha)

square mile (mi2) 2.590 square kilometer (km2) 

Volume

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)

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

foot per year (ft/yr) 0.3048 meter per year (m/yr)

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)

SI to Inch/Pound

Multiply By To obtain

Length

meter (m) 3.281 foot (ft) 
vii

Datums

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

(NAVD 88). Vertical coordinate information for historical data collected and stored as National 

Geodetic Vertical Datum of 1929 (NGVD 29) has been converted to NAVD 88 for this publication. 

Conversion between NAVD 88 and the commonly used NGVD 29 varies spatially; however, over 

most of the study area the following conversion can be used:

NGVD 29 = NAVD 88 – 3.6 feet.

This conversion generally is accurate within about ± 0.5 feet for 95 percent of the study area. 

The reader is directed to either the National Geodetic Survey Web site for VERTCON at http://

www.ngs.noaa.gov/TOOLS/Vertcon/vertcon.html or the U.S. Army Corps of Engineers Web site 

for Corpscon at http://www.agc.army.mil/corpscon for more accurate conversions.

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

Horizontal coordinate information for historical data collected and stored as North American 

Datum of 1927 (NAD 27) has been converted to NAD 83 for this publication. Conversion between 

NAD 83 and the commonly used NAD 27 varies spatially, and the difference in horizontal 

positions can be greater than 300 feet within the study area. For assistance with conversions, 

the reader is directed to either the National Geodetic Survey Web site for NADCON at http://

www.ngs.noaa.gov/TOOLS/Nadcon/Nadcon.html or the U.S. Army Corps of Engineers Web site 

for Corpscon at http://www.agc.army.mil/corpscon.

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

Abbreviations and Acronyms

Abbreviation or 

Acronym Definition

CPRAS Columbia Plateau Regional Aquifer System

CRBG Columbia River Basalt Group

eYFB eastern Yakima Fold Belt

GWMA Columbia Basin Ground Water Management Area

MAF million acre-feet

NWIS National Water Information System

OWRD Oregon Water Resources Department

SOWAT SOil WATer balance model

USGS U.S. Geological Survey

WADOE Washington State Department of Ecology
viii

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Abstract

Well information and groundwater-level measurements 

for the Columbia Plateau Regional Aquifer System in 

Washington, Oregon, and Idaho, were compiled from data 

provided by the U.S. Geological Survey and seven other 

organizations. From the full set of about 60,000 wells 

and 450,000 water-level measurements a subset of 761 

wells within the aquifers of the Columbia River Basalt 

Group (CRBG) then was used to develop a simple linear 

groundwater-level trend map for 1968–2009. The mean of 

the trends was a decline of 1.9 feet per year (ft/yr), with 

72 percent of the water levels in wells declining. Rates of 

declines greater than 1.0 ft/yr were measured in 50 percent of 

wells, declines greater than 2.0 ft/yr in 38 percent of wells, 

declines greater than 4.0 ft/yr in 29 percent of wells, and 

declines greater than 8.0 ft/yr in 4 percent of wells.

Water-level data were used to identify groups of 

wells with similar hydraulic heads and temporal trends to 

delineate areas of overall similar groundwater conditions. 

Discontinuities in hydraulic head between well groups were 

used to help infer the presence of barriers to groundwater 

flow such as changes in lithology or the occurrence of folds 

and faults. In areas without flow barriers, dissimilarities in 

response of well groups over time resulted from the formation 

of groundwater mounds caused by recharge from irrigation 

or regions of decline caused by pumping. The areas of focus 

for this analysis included the Umatilla area, Oregon, and the 

Palouse Slope/eastern Yakima Fold Belt in the Columbia 

Basin Ground Water Management Area (GWMA) consisting 

of Adams, Franklin, Grant, and Lincoln Counties, Washington.

In the Umatilla area, water levels from 286 wells were 

used to identify multiple areas of high hydraulic gradient 

that indicate vertical and horizontal barriers to groundwater 

flow. These barriers divide the groundwater-flow system into 

several compartments with varying degrees of interconnection. 

Horizontal flow barriers commonly correspond to mapped 

geologic structure and result in horizontal hydraulic gradients 

that progressively become steeper from north to south 

corresponding to an increase in structural complexity that 

may be impeding recharge from the uplands into the heavily 

developed areas.

Most CRBG aquifers in the Umatilla area are declining 

and since 1970, cumulative declines range from about 100 

to 300 feet. Significant vertical hydraulic gradients are 

documented for relatively small areas near Umatilla, and since 

the 1970s, downward vertical gradients in these areas have 

been increasing as hydraulic heads in the deeper units have 

declined. The absence of vertical gradients over much of the 

area may be a consequence of flow through commingling 

wells that results in the equilibration of the heads between 

aquifers.

On the Palouse Slope in the central GWMA, large 

groundwater declines occurred during 1968–2009 along a 

north-south swath in the middle of the region. An analysis 

of 1,195 wells along major flow paths and through the 

area of persistent groundwater-level declines indicates that 

barriers to flow are not as evident in this area as in Umatilla. 

This is consistent with the geologic interpretation of the 

Palouse Slope as being a gently folded structure created 

by voluminous sheet flows of CRBG lavas. Groundwater 

discharge into the sediment-filled coulees, where the upper 

aquifers are intersected at land surface by incised canyons, 

is proposed as an alternative to explain local steepening of 

the hydraulic gradient along the Palouse Slope previously 

attributed to the presence of a groundwater dam. Comparison 

of generalized potentiometric surface maps developed for 

pre-development conditions and post-2000 conditions indicate 

that pre-development groundwater flow was from the uplands 

toward the Columbia and Snake River and that post-2000 

flow patterns in the area are controlled by irrigation practices 

that have resulted in broad regions of elevated or depressed 

hydraulic head. In some cases, irrigation-related changes in 

head have reversed groundwater flow directions. Evidence 

of significant vertical hydraulic gradients exists, although 

much of the aquifer thickness is affected by commingling of 

wells. The effect of commingling and its relative contribution 

to problems related to groundwater-level declines remains 

unclear.

Groundwater Status and Trends for the Columbia Plateau 

Regional Aquifer System, Washington, Oregon, and Idaho

By Erick R. Burns, Daniel T. Snyder, Jonathan V. Haynes, and Michael S. Waibel
2 Groundwater Status and Trends for the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

Introduction

The Columbia Plateau Regional Aquifer System 

(CPRAS) covers about 44,000 mi2 of southeastern 

Washington, northeastern Oregon, and western Idaho (fig. 1). 

The population of the region is more than 1.3 million people 

(U.S. Census Bureau, 2000), and an important agricultural 

industry valued in the billions of dollars annually has 

developed despite the arid to semi-arid climate and limited 

access to surface-water resources. Groundwater availability 

in the aquifers of the area is a critical water-resource 

management issue because of the high demand of water for 

agriculture, economic development, and ecological needs. 

Groundwater levels have declined throughout much of the 

Columbia Plateau (Whiteman and others, 1994, p. B61– B65; 

Porcello and others, 2009; Burns and others, 2012). A 

comparison of water-level measurements from 1984 and 

2009 from 470 wells in the CPRAS indicates that water 

levels declined in 83 percent of the wells and that declines 

were greater than 25 ft in 29 percent of the wells (Snyder and 

Haynes, 2010).

The primary aquifers of the CPRAS are basalts of 

the Columbia River Basalt Group (CRBG) and in places, 

overlying basin-fill sediments. Water-resources issues 

that have implications for current and future groundwater 

availability in the region include (1) widespread waterlevel declines associated with development of groundwater 

for irrigation and other uses; (2) reduction in base flow to 

rivers and associated effects on water temperature, water 

quality, fish, and other aquatic organisms; and (3) current and 

anticipated effects of global climate change on recharge, base 

flow, demand, and ultimately, groundwater availability.

The U.S. Geological Survey (USGS) Groundwater 

Resources Program began a study of the CPRAS in 2007 with 

the broad goals of (1) characterizing the hydrologic status 

of the system, (2) identifying trends in groundwater storage 

and use, and (3) quantifying groundwater availability. The 

study approach includes updating the regional geologic and 

hydrogeologic frameworks, documenting changes in the 

hydrologic status of the system, quantifying the hydrologic 

budget, and developing a groundwater-flow model for the 

system. The model, which will be outlined in a separate 

report, will be used to evaluate and test the conceptual model 

of the groundwater flow within the system and to evaluate 

groundwater availability. This report, which describes 

the relation between groundwater levels and trends and 

hydrogeologic controls, along with four recently published 

reports (Kahle and others, 2009; Snyder and Haynes, 2010; 

Burns and others, 2011; Kahle and others, 2011), provides 

comprehensive information about the physical hydrogeologic 

framework of the CPRAS based on historical and current 

investigations. This study, in part, relied on data collection and 

analysis conducted as part of a cooperative agreement between 

the USGS and the Oregon Water Resources Department to 

better define the hydrologic conditions in the Umatilla basin of 

Oregon.

Purpose and Scope

The purpose of this report is to describe the compilation 

of groundwater-level data for the CPRAS that will be used for 

comparison with a numerical groundwater-flow model and to 

evaluate the status and trends in the data and their relation to 

hydrogeologic controls that influence the hydraulic properties 

of the aquifer or hydraulic stresses from recharge or pumping. 

The scope of this report includes a regional assessment 

of the importance of these controls described through the 

presentation of maps of groundwater elevations, water-level 

changes, stresses, and hydrogeologic features in the CPRAS. 

This information will help to develop a broad understanding 

of how climatic, anthropogenic, and hydrogeologic factors 

combine to influence groundwater flow, and how climate 

change and groundwater development may influence the 

sustainability and availability of the water supply in the 

region. The analyses presented in this report generally are 

restricted to the primary CRBG aquifers within the CPRAS to 

demonstrate important features in key areas, and do not seek 

to be an exhaustive analysis of any area.
Introduction 3

tac12-0765_fig01

Pasco

Yakima

Pullman Moscow

Seattle Spokane

LewistonLewiston

La

Grande

WenatcheeWenatchee

The DallesThe Dalles

EllensburgEllensburg

Moses

Lake

Walla

Walla

Coeur

d'Alene

John Day River Crooked River

Palouse River

Snake River

Yakima River

Imnaha River

Clearwater River

Salmon River

Crab Creek

Hangman Creek

COLUMBIA

N Fk John Day R

Grande Ronde River

RIVER

Snake River

Lake Coeur d'Alene

Potholes

Res

Puget

Sound

COLUMBIA RIVER

Banks Lake

Naches R

McKay Cr

Lolo

Creek

CAS

CA

DE RA

N

GE CASCADE

RANGE

OKANOGAN HIGHLANDS

HORSE HEAVEN HILLS

FRENCHMAN HILLS

SADDLE MOUNTAINS

BLUE

MOUNTAINS

WALLOWA

MOUNTAINS

ROCKY

M

OUNTAINS

ROCKY

MOUNTAINS

MONTANA

Base map modified from USGS and other digital data, various scales. Coordinate 

system: State Plane-Washington South FIPS 4602, Projection: Lambert Conformal 

Conic; North American Datum of 1983.

GRANT

BAKER

UMATILLA

WASCO

UNION

WALLOWA

CROOK

MORROW

WHEELER

GILLIAM

JEFFERSON

LINN

LANE DESCHUTES MALHEUR

MARION

CLACKAMAS

SHERMAN HOOD

RIVER

IDAHO

VALLEY

SHOSHONE

BONNER

ADAMS

LATAH

CLEARWATER

KOOTENAI

WASHINGTON

BENEWAH

LEWIS

BOISE

BOUNDARY

NEZ

PERCE

GEM

YAKIMA

KING

GRANT

CHELAN

OKANOGAN

ADAMS

KITTITAS

LINCOLN

FERRY SKAGIT

WHITMAN

BENTON

STEVENS

LEWIS

KLICKITAT

DOUGLAS

SPOKANE

SNOHOMISH

PIERCE

SKAMANIA

FRANKLIN

WALLA

WALLA

ASOTIN

GARFIELD

COLUMBIA

PEND

OREILLE

0 20 40 60 80 100 Miles

0 20 40 60 80 100 Kilometers

122° 121° 120° 119° 118° 117° 116°

48°

47°

46°

45°

EXPLANATION

Geologic model extent of the Columbia River

Basalt Group (Burns and others, 2011, 

figure 8, p. 22) 14,410

5,000

10,000

Land-surface elevation, in feet (North 

American Vertical Datum of 1988)

Palouse

Slope

Yakima

Fold Belt

Blue

Mountains

Clearwater

Embayment

20

Structural regions of the Columbia Plateau 

Regional Aquifer System

Data extent

IDAHO

OREGON

WASHINGTON

B

C

P

Y

Palouse Slope

Clearwater Embayment

Blue Mountains

Yakima Fold Belt

B

C

P

Y

Structural regions

Figure 1. Location of Columbia Plateau Regional Aquifer System of Washington, Oregon, and Idaho.
4 Groundwater Status and Trends for the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

Description of Study Area

A complete description of the Columbia Plateau is 

available in Kahle and others (2011), parts of which are 

presented here. The Columbia Plateau (fig. 1) is a structural 

and topographic basin within the drainage of the Columbia 

River basin. It is bounded on the west by the Cascade 

Range, on the east by the Rocky Mountains, on the north 

by the Okanogan Highlands, and on the south by the Blue 

Mountains. The Columbia Plateau is underlain by massive 

basalt flows with thickness estimated to exceed 16,000 ft near 

Pasco, Washington (Reidel and others, 2002, p. 211, fig. 2.6). 

Sedimentary deposits locally exceeding 2,000 ft in thickness 

overlie the basalt over large areas of the plateau.

The Columbia Plateau was divided into four informal 

physiographic subprovinces that represent structural 

regions—the Yakima Fold Belt, Blue Mountains, Palouse 

Slope subprovinces, and the Clearwater Embayment (fig. 1). 

Groundwater characteristics in each of these regions can be 

different because of variations in stratigraphy, depositional 

environment, and post-deposition folding and faulting. The 

presence and importance of flow barriers in the CPRAS has 

been recognized and discussed in numerous studies (for 

example, Newcomb, 1959; Porcello and others, 2009). The 

Yakima Fold Belt includes most of the western half of the 

plateau and is characterized by a series of east-west trending 

anticlinal ridges and synclinal basins. The Palouse Slope 

occupies the northeast quarter of the plateau, is much less 

deformed, and has a gently southwestward dipping slope. 

The other structural regions within the CPRAS are the Blue 

Mountains, a composite anticlinal structure that forms the 

southeastern extent of the Columbia River basin, and the 

Clearwater Embayment, which marks the eastward extent of 

the CPRAS along the foothills of the Rocky Mountains and 

includes a series of folds extending into Idaho.

Much of the Columbia Plateau is semiarid, the mean 

annual precipitation for 1895–2007 (Kahle and others, 2011, 

p. 4) is about 17 in/yr (about 40 million acre-ft/yr) and ranges 

from about 7 in. in the center of the study area to more than 

60 in. in the north-westernmost extent of the study area. 

The types and amounts of natural vegetation growing on the 

Columbia Plateau are largely dependent on precipitation and 

land-surface elevation. The vegetation ranges from sagebrush 

and grasslands at lower elevations to grasslands and forest at 

mid elevations to barren rock and conifer forests at the upper 

elevations. Dry land agriculture mainly includes winter and 

spring wheat and lentils. Irrigated agriculture includes apples, 

hops, and other crops.

Overviews of the geology and hydrology of the CPRAS 

presented summarize detailed descriptions in reports by 

(1) Kahle and others (2009), who discuss the geologic 

framework used in this report; (2) Burns and others (2011), 

who describe the three-dimensional characteristics of the 

geology of the CPRAS; and (3) Kahle and others (2011), who 

discuss the hydrogeologic framework and the hydrologic 

budget components of the CPRAS.

Hydrogeologic Setting

The Columbia Plateau is an intermontane basin between 

the Rocky Mountains and the Cascade Range that is filled with 

mostly Cenozoic basalt and sediment. Most rocks exposed in 

the region are the CRBG, intercalated sedimentary rocks, and 

overlying younger sedimentary rocks and deposits that include 

Pleistocene cataclysmic flood deposits, eolian deposits, 

and terrace gravels of modern rivers. The CRBG consists 

of a series of more than 300 flows that erupted in various 

stages during the Miocene, 17 million to 6 million years ago. 

Individual flows range in thickness from 10 to more than 

300 ft (Tolan and others, 1989; Drost and others, 1990). Soil 

and sediments that were formed or deposited on top of older 

lava flows and then covered and preserved by a subsequent 

lava flow are called sedimentary interbeds.
Introduction 5

Generalized geologic stratigraphy discussed in this 

investigation is broadly based on recognized formations within 

the Columbia River Basalt Group. The primary geologic 

formations of interest, listed in order of increasing age, include 

the Saddle Mountains Basalt, the Wanapum Basalt, and the 

Grande Ronde Basalt (fig. 2; Swanson and others, 1979, 

p. G4–G8; Whiteman and others, 1994, p. B32–B33). Younger 

sedimentary deposits cover parts of the CRBG across the 

study area and are referred to informally as the Overburden. 

The Overburden consists of undivided, unconsolidated to

semi-consolidated sedimentary deposits ranging from 

Miocene to Holocene in age (Drost and others, 1990). These 

include many types of deposits of local and (or) regional 

extent including flood gravels and slack water sediments, 

terrace gravels of modern rivers, and eolian deposits that can 

range in thickness from 0 to 1,300 ft. The Saddle Mountains 

Basalt formation consists mostly of basalts and associated 

sedimentary interbeds and is the least extensive and youngest 

formation of the CRBG. Most of the formation is in the 

west-central part of the study area, with less continuous 

occurrences in the Blue Mountains and eastward into Idaho. 

Thickness of the Saddle Mountains Basalt ranges from 0 to 

about 1,000 ft. The Wanapum Basalt formation, composed 

mostly of basalt and sedimentary interbeds, is in the central 

part of the study area. Much of the formation lies beneath the 

Overburden and Saddle Mountains Basalt. Thickness of the 

Wanapum Basalt ranges from 0 to about 1,200 ft. The Grande 

Ronde Basalt formation is the oldest and most extensive of the 

basalt formations discussed and comprises the vast majority 

of the CRBG. This formation underlies most of the study area, 

except for an area along the southern boundary of the CPRAS 

in Oregon and along the eastern edge of the aquifer system in 

Idaho. The Grande Ronde Basalt formation contains basalt and 

sedimentary interbeds. Thickness of the formation is largely 

unknown, but may be greater than 15,000 ft near the central 

part of the basin (Burns and others, 2011, p. 30, fig. 10D, and 

digital data).

Folding and faulting of the basalts occurred during the 

period of deposition and more recently. Younger basalt flows 

commonly were less-voluminous intra-canyon flows, so 

the distribution and thickness of these lavas are controlled 

by the shape of the valleys through which the lava flowed. 

Pleistocene outburst floods shaped the area by causing incision 

in the basalt, and erosion and deposition of the 

overlying sediment.

These geologic formations correspond to the 

hydrogeologic units defined for use in the USGS Regional 

Aquifer-System Analysis (RASA) Program and Groundwater 

Availability studies of the CPRAS (Whiteman and others, 

1994, p. B32–B33; Kahle and others, 2009), and consist 

of the Overburden unit, the Saddle Mountains unit, the 

Wanapum unit, and the Grande Ronde unit. The informal 

term “unit” is used to differentiate from the formal geologic 

formations simplified for hydrologic discussions. For example, 

the geologic formations were simplified by grouping the 

Mabton and Vantage Interbeds into the Saddle Mountains 

and Wanapum units, respectively. The older rocks bordering 

and underling the CPRAS form the basement confining unit, 

referred to as the Older Bedrock hydrogeologic unit, and 

is composed of various rock types older than the CRBG, 

generally with much lower permeabilities than the basalts and 

considered the base of the regional flow system.

Hydrogeologic Characteristics

The hydraulic characteristics of the geologic materials 

determine how a groundwater-flow system functions and 

how it will respond to stresses such as pumpage. These 

characteristics include horizontal and vertical hydraulic 

conductivity and the storage coefficient. Because of the 

heterogeneity of the geologic materials that comprise the 

CPRAS, the hydraulic characteristics can vary considerably. 

The Overburden deposits are diverse in lithology and the 

large variation in grain size, depositional regimes, and age 

of the deposits account for the large range of their hydraulic 

characteristics (Kahle and others, 2011, p. 20). Each of 

the CRBG geologic units consist of tens to hundreds of 

individual layered basalt flows. The layers are highly variable 

in thickness and extent, but over much of the Columbia 

Plateau, the lava flows are comprised of flood basalts that 

form laterally extensive deposits. Hydraulic characteristics 

vary greatly within and between the individual basalt flows. 

Horizontal hydraulic conductivities generally are greatest in 

the interflow zones formed from the combination of basalt 

flow tops with the base of an overlying basalt flow, and an 

intervening sedimentary interbed, if present. Flow tops and 

bases commonly are brecciated, although they can exhibit a 

wide range of depositional textures. Because flow tops and 

bottoms commonly have open and highly-connected pore 

structures, the basalt interflow zones frequently exhibit high 

horizontal hydraulic conductivity (Lindolm and Vaccaro, 

1988). The interflow zones are separated by the low hydraulic 

conductivity flow interior in which most fractures are cooling 

joints that often are vertically oriented (columnar jointing). 

Despite the fact that joints exist, flow interiors commonly are 

effective confining units. Porosity and permeability generally 

are lower in the older bedrock than in the Overburden and 

CRBG units.
6 Groundwater Status and Trends for the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

tac12-0765_fig02

0 20 40 60 80 100 Miles

0 20 40 60 80 100 Kilometers

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 Oreille River

Coeur

d'Alene

Lake

F. D. Roosevelt

COLUMBIA RIVER

SNOHOMISH

SKAGIT

PIERCE

LEWIS

CHELAN

OKANOGAN

KITTITAS

YAKIMA

SKAMANIA

KLICKITAT

HOOD

RIVER

CLACKAMAS

MARION

LINN

FERRY STEVENS

LINCOLN

SPOKANE

ADAMS

FRANKLIN

WALLA WALLA

WHITMAN

COLUMBIA

GARFIELD

ASOTIN

SHERMAN

JEFFERSON

DESCHUTES

CROOK

GILLIAM

MORROW

WHEELER

MALHEUR

BAKER

WALLOWA

BONNER

KOOTENAI

BENEWAH

LATAH

CLEARWATER

NEZ PERCE

WASHINGTON

VALLEY

ADAMS

IDAHO

DOUGLAS

WASCO

GRANT

UNION

SHOSHONE

LEWIS

UMATILLA

PEND

ORIELLE

BENTON

GRANT

KING

OREGON

IDAHO

WASHINGTON MONT.

Blue Mountains

Wallowa Mountains

The Palouse

Horse Heaven Hills

C

A

S

C

A

D

E

R

A

N

G

E

Ellensburg

Walla

Walla

Yakima

Pasco

Lewiston

Moscow

Spokane

The

Dalles

La Grande

Pullman

Wenatchee

Moses

Lake

Coeur

d'Alene

Pendleton

Hermiston

122° 121° 120° 119° 118° 117° 116°

122° 121° 120° 119° 118° 117° 116°

48°

47°

46°

45°

48°

47°

46°

45°

Digital data: U.S. Geological Survey, various scales. Projection: State Plane, Washington, South, FIPS 4602: 

Projection: Lambert Conformal Conic; False easting: 1640416.666667; False northing: 0.000000; Central meridian: 

-120.500000; Standard parallel 1: 45.833333; Standard parallel 2: 47.333333; Latitude of origin: 45.333333; 

Horizontal datum: North American Datum 1983

Fault

Fold

Older Bedrock

Grande Ronde Basalt

Wanapum Basalt

Saddle Mountains Basalt

Overburden (sedimentary)

Geologic units

EXPLANATION

Geologic data not used for modeling 

(post-CRBG volcanics)

Structural regions of the Columbia Plateau 

Regional Aquifer System

Data extent

IDAHO

OREGON

WASHINGTON

B

C

P

Y

Palouse Slope

Clearwater Embayment

Blue Mountains

Yakima Fold Belt

B

C

P

Y

Structural regions

Figure 2. Surficial geology and structures of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho.
Introduction 7

Kahle and others (2011, p. 21) estimated the median 

horizontal hydraulic conductivity values for the Overburden, 

basalt units, and older bedrock as 161, 70, and 6 ft/d, 

respectively, based on specific capacity data reported in 

previous studies. Vertical hydraulic conductivity of the 

geologic units in the CPRAS generally is known to be low 

but is poorly quantified. Estimates of vertical hydraulic 

conductivity range from about 0.009 to 2 ft/d for the 

Overburden unit, although values for some parts of this unit 

may be as low as 10-10 to 10-7 ft/d; values for the CRBG units 

range from 4×10-7 to 4 ft/d (Kahle and others, 2011, p. 57).

The storage characteristics of an aquifer are described 

by the storage coefficient, a dimensionless property defined 

as the volume of water that an aquifer takes into or releases 

from storage per unit of surface area per unit change in head. 

Previous estimates of the storage coefficient in the CPRAS 

typically range from about 0.1 to 0.2 for the unconfined parts 

of the Overburden unit and from about 0.01 to 10-6 for the 

CRBG basalt units (Kahle and others, 2011, p. 26–27).

Hydrologic Budget Components

The following discussion and estimates of the 

regional-scale hydrologic budget components for the 

CPRAS is from the recent work by Kahle and others (2011). 

Mean annual recharge from infiltration of precipitation was 

estimated based on annual precipitation data and previous 

model simulation results of recharge. The estimates of the 

other budget components were developed using a monthly 

SOil WATer (SOWAT) balance model to determine irrigation 

water demand, groundwater flux to the underlying modeled 

soil zone (recharge or discharge), direct runoff, and soil 

moisture in irrigated areas (Kahle and others, 2011, p. 36). 

The SOWAT model was developed to make use of estimates 

of actual evapotranspiration available from a new Simplified 

Surface Energy Balance method that uses remotely sensed 

land-surface temperature data (Senay and others, 2007).

Mean annual recharge from the infiltration of 

precipitation of 4.6 in/yr (10.8 million acre-ft/yr) for 

1985–2007 was estimated for the CPRAS. The spatial 

distribution in recharge mirrors that of annual precipitation, 

with the highest recharge (more than 20 in/yr) occurring along 

the Cascade Range and the Blue Mountains. Mean annual 

recharge from infiltration of precipitation is less than 1 in/yr 

for a large part of the study area adjacent to the Columbia and 

Yakima Rivers where precipitation is limited.

Mean monthly irrigation throughout the study area peaks 

in July at 1.6 million acre-ft (MAF) (1985–2007 average), of 

which 0.45 and 1.15 MAF are from groundwater and 

surface-water sources, respectively. Annual use of irrigation 

water in the study area averaged 5.3 MAF during 1985–2007, 

with 1.4 MAF (or 26 percent) supplied from groundwater 

and 3.9 MAF supplied from surface water. Mean annual 

groundwater recharge from deep percolation of applied 

irrigation water in the study area was 4.2 MAF (1985–2007); 

2.1 MAF (50 percent) occurred within the predominately 

surface-water irrigated regions of the Yakima Basin, Umatilla 

Basin, and Columbia Basin Project. The Columbia Basin 

Project, located in east central Washington, consists of more 

than 1,000 mi2 of land irrigated with Columbia River water 

through a series of dams, pumping plants, and canals. Annual 

recharge rates range from less than 5 in/yr in predominately 

sprinkler-irrigated areas where groundwater is the source 

to more than 20 in/yr in surface-water supplied areas where 

conveyance losses and less-efficient application methods are 

more common.

Annual groundwater use (1984–2009) for purposes 

other than irrigation was estimated for the study area using 

information from several agencies. Public-supply groundwater 

use increased from about 201,000 acre-ft/ yr in 1984 to 

about 269,000 acre-ft/yr in 2009. Domestic self-supplied 

groundwater use increased from about 54,600 acre-ft/ yr 

in 1984 to about 71,200 acre-ft/yr in 2009. Industrial 

groundwater use decreased from about 53,400 acre-ft/yr in 

1984 to about 43,900 acre-ft/yr in 2009. Other groundwater 

use, including for mining, thermoelectric needs, livestock, and 

aquaculture combined, increased from 16,900 acre-ft/yr in 

1984 to about 43,600 acre-ft/yr in 2009.

Groundwater Occurrence and Movement

Groundwater moves through the regional aquifer system 

from the uplands to surface drainage features in the lowlands, 

mainly to the Columbia River and its major tributaries. 

Groundwater movement is affected by the geometry of the 

land surface and stream system, the extent, thickness, and 

hydraulic properties of the aquifers, the presence and nature of 

geologic structures, and the rate and location of groundwater 

recharge and discharge. Groundwater flow within the basalt 

units moves horizontally and vertically in the basalt interflow 

zones, flow centers, and sedimentary interbeds (Kahle and 

others, 2011, p. 27). Horizontal hydraulic conductivities 

generally are greatest in the interflow zones and consequently 

the interflow zones support most of the horizontal groundwater 

movement, whereas movement in the typically thick and 

lower horizontal hydraulic conductivity flow centers mainly is 

vertical. Therefore, the interflow zones in the basalt sequence 

form numerous, thin, semiconfined aquifers whose physical 

and hydraulic characteristics vary horizontally and vertically. 

Geologic complexity consisting of changes in lithology and 

folds and faults affect the geometry of flow paths by forming 

flow barriers or preferential pathways for groundwater flow 

(Snyder and Haynes, 2010, p. 7; Kahle and others, 2011, p. 27).
8 Groundwater Status and Trends for the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

Study Methods

Recent and historical water-level data were compiled 

from a large number of wells located throughout the CPRAS. 

These data were used to identify “well groups” each consisting 

of individual wells with similar hydraulic heads (groundwater 

levels) and temporal trends. Comparisons were made between 

adjacent well groups to delineate sets of well groups, which 

define areas of overall similar groundwater-flow conditions 

with regard to groundwater-flow directions and horizontal 

and vertical hydraulic gradients (the change in hydraulic head 

per unit of distance in a given direction). Discontinuities 

in groundwater-flow conditions between the sets of well 

groups were used to help infer the presence of features 

that contribute to control the regional flow of groundwater. 

These discontinuities can result from: (1) geologic features 

that influence the hydraulic properties of the aquifer such 

as changes in lithology or the occurrence of structural folds 

and faults, or (2) the presence of hydrologic features such as 

groundwater mounds or troughs caused by stresses such as 

recharge from irrigation or discharge to pumping.

Sources of Data

The evaluation of groundwater status and trends for the 

CPRAS relied on information that was routinely collected 

by many agencies or that was developed in the current or 

previous studies of groundwater availability in the CPRAS. In 

particular, this report relies heavily on the development of the 

geologic framework and three-dimensional geologic model 

by Kahle and others (2009) and Burns and others (2011), 

respectively, and the hydrologic budget components developed 

by Kahle and others (2011).

Groundwater-Level Information 

The primary source of well information, including 

location, construction, and water-level measurements, was 

the USGS National Water Information System (NWIS). 

Additional well and water-level data were provided by the 

U.S. Department of Energy, Washington State Department 

of Ecology, Oregon Water Resources Department, Columbia 

Basin Ground Water Management Area, GSI Water Solutions, 

Inc., Confederated Tribes of the Umatilla Indian Reservation, 

and Walla Walla Basin Watershed Council. Due to constraints 

on project resources, not all wells or water-level measurements 

from the CPRAS were included in this study. Emphasis was 

placed on incorporating datasets with the greatest number of 

wells, which included information on well construction and 

many water-level measurements made over relatively long 

periods. Wells are primarily within the extent of the CPRAS, 

although some wells as much as 20 mi outside of the CPRAS 

boundary also were included to help identify groundwaterlevel conditions across the boundary of the CPRAS (fig. 3). 

Information was acquired for 60,115 wells, although usable 

water-level data were available for only 39,610 wells. A total 

of 447,992 water-level measurements were collected from the 

39,610 wells. The period of record for the water-level data 

extended from 1891 to 2010. The longest period of record for 

water-level measurements at an individual well is 96 years. 

Measurements at the longest annually monitored well began 

in 1940 and continue to the present (2011). From these data 

a subset of 7,772 wells and 147,563 water levels were used 

for the analyses presented in this report and are on file at the 

USGS office in Tacoma, Washington (http://wa.water.usgs.

gov/projects/cpgw), and available upon request. Much of 

the data processing for analyzing groundwater levels was 

accomplished using Excel® spreadsheet tools developed by 

Tillman (2009).

Information on land-surface elevation was needed 

to calculate groundwater elevation from depth-to-water 

measurements. Land-surface elevations for many wells were 

reported by the agencies that provided the well information. 

Land-surface elevations were independently determined using 

a 10-m lateral resolution Digital Elevation Model (DEM; 

U.S. Geological Survey, 1999) as verification for these wells 

and to obtain land-surface elevations for the remaining wells 

with no reported values. Values of the DEM derived landsurface elevations were used if the difference between the 

reported and DEM derived elevations differed by greater than 

100 ft.
Introduction 9

Figure 3. Locations of wells with usable water-level data within or immediately adjacent to the Columbia Plateau Regional Aquifer 

System, Washington, Oregon, and Idaho, subsets of which were used for analysis in this study.

tac12-0765_fig03

14,410

20

5,000

10,000

Land-surface elevation, in feet (North 

American Vertical Datum of 1988)

MONTANA

Pasco

Yakima

Moscow

Pullman

Spokane

Lewiston

La Grande

Wenatchee

The

Dalles

Ellensburg

Moses

Lake

Walla

Walla

Coeur

d'Alene

GRANT BAKER

UMATILLAUMATILLA

WASCO

UNION

WALLOWA

MORROW

WHEELER

GILLIAM

JEFFERSONJEFFERSON

CROOK

MALHEUR

SHERMAN

HOOD

RIVER

IDAHO

VALLEY

SHOSHONESHOSHONE

ADAMS

LATAH

BONNER

KOOTENAIKOOTENAI

CLEARWATER

WASHINGTON

BENEWAH

LEWIS

NEZ

PERCE YAKIMA

GRANT

CHELAN

KITTITASKITTITAS ADAMS

LINCOLN KING

WHITMAN

BENTON

KLICKITATKLICKITAT

DOUGLAS

SPOKANE

OKANOGANOKANOGAN FERRY

STEVENS

FRANKLINFRANKLIN

SNOHOMISH

WALLA

WALLA

LEWIS

ASOTIN

COLUMBIACOLUMBIA

GARFIELDGARFIELD

PIERCE

SKAGIT

SKAMANIASKAMANIA

PEND

OREILLE

John Day River Crooked River

Palouse River

Snake River

Yakima River

Imnaha River

Clearwater River

Sa ml on River

Crab Creek

Hangman Creek

COLUMBIA

N Fk John Day R

Grande Ronde River

RIVER

Snake River

Lake Coeur d'Alene

Potholes

Res

COLUMBIA RIVER

Banks Lake

Naches R

McKay Cr

Lolo

Creek

EXPLANATION

Wells

Geologic model extent of the Columbia River

Basalt Group (Burns and others, 2011, 

figure 8, p. 22)

0 20 40 60 80 100 Miles

0 20 40 60 80 100 Kilometers

Base map modified from USGS and other digital data, various 

scales. Coordinate system: State Plane-Washington South FIPS 

4602, Projection: Lambert Conformal Conic; North American 

Datum of 1983.

121° 120° 119° 118° 117° 116°

48°

47°

46°

45°

IDAHO

OREGON

WASHINGTON

B

C

P

Y

Palouse Slope

Clearwater Embayment

Blue Mountains

Yakima Fold Belt

B

C

P

Y

Structural regions
10 Groundwater Status and Trends for the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

Geologic Model

A geologic model (Burns and others, 2011) provided 

improved estimates of CRBG and Overburden unit volumes 

and refined location of large structural features. This model 

was used to interpret the presence and significance of 

hydrogeologic controls on the groundwater system presented 

in this report. An on-line interactive tool was developed to 

serve point information and cross sections developed from the 

geologic model to the public (U.S. Geological Survey, 2012).

Data Limitations

The accuracy and representativeness of the 

groundwater-level measurements are dependent on various 

factors pertaining to measurement accuracy, quality assurance 

procedures, local conditions in the aquifer at the time of 

measurement, and well construction.

Measurement Accuracy and Data Compilation

Typical methods used to measure the depth to water in a 

well have precisions ranging from 0.01 ft to several feet. The 

precision of most measurements is expected to be in the range 

of about 0.1 ft. However, the accuracy of the determination 

of the groundwater elevation (how it compares to the actual 

value) depends on how the depth to water measurement 

is transformed to a groundwater elevation at a particular 

location, how it is associated with a particular aquifer, and 

the associated quality assurance procedures. A major factor 

affecting measurement accuracy is related to how the elevation 

for the well was determined, and the errors associated with 

assigning an elevation to the well. Errors also can relate to 

the location of the well. Errors in the latitude and longitude 

coordinates for the site can create errors of hundreds of feet in 

the assigned elevation of the water level, especially in areas 

with steep relief. For this report, well locations and elevations 

were mostly based on information reported from the respective 

agencies. A comparison was made between reported well 

elevations and those determined from a 10-m DEM on the 

basis of the reported latitude and longitude locations and 

lateral datums from the respective agencies. The median of 

the absolute values of the elevation differences is 4.3 ft and 

90 percent of the differences are less than 44 ft.

The well and water-level data were compiled from many 

different agencies. Each agency has its own protocols for 

the description and documentation of measured wells, for 

the collection and recording of water-level data, and quality 

assurance procedures. Errors can occur when compiling 

large datasets due to either data entry, arithmetic errors, or 

treatment of exceptions when using rule-based algorithms 

to write all data into a common database. Common errors 

include incorrect well identification or location, incorrect or 

incomplete well construction information, errors in depth 

to water measurements, and incorrect determination of 

land-surface elevations. Additionally, some well information 

and water-level measurements for the same well may be 

reported by multiple agencies resulting in duplication of 

information. An effort was made to identify and correct errors 

in the data used in this study. However, it was not possible 

to ascertain the quality of the well information or water-level 

measurement for each well and undoubtedly, errors are present 

in the data. The large number of wells and measurements 

used in the analyses in this study should help to minimize the 

influence of these errors and provide a robust estimate of the 

groundwater elevation.

Water-level measurements in reports filed by well 

drillers at the time of new well installations were included in 

the compilation of water levels. However, measurements of 

water levels in newly constructed wells may not have been 

at equilibrium at the time of measurement and therefore may 

not represent static water-level conditions (Snyder, 2008, 

p. 11–14). Water-levels where the status indicated the well 

was dry, obstructed, or influenced by pumping, were excluded 

from the analyses as these water levels may not represent 

static conditions.

Representative Sampling

The groundwater-level measurements used in this study 

are assumed to be representative of the groundwater positions 

within the aquifers of the CPRAS. However, some of the 

data may suffer from a number of possible biases. These 

biases may arise from a variety of sources such as influences 

due to localized conditions and stresses, well construction, 

and commingling of groundwater. In addition, the spatial 

distribution and selection of wells used for the collection 

of water-level measurements may be biased as a result of 

increased scrutiny and monitoring in areas with groundwater 

declines. This can result in the clustering of wells and an 

over-representation of wells showing water-level declines.
Introduction 11

Localized Conditions and Stresses

Pumping and recharge stresses can cause geographically 

localized or transient perturbations of water levels. Water-level 

measurements affected by such localized perturbations may 

not be representative of true conditions in the aquifer over a 

broader area.

Recharge in the CPRAS is dominated by precipitation 

(Kahle and others, 2011), which generally has gradual 

geographic variation. However, other recharge sources 

such as irrigation water, artificial recharge, or leakage from 

streams, canals, or lakes can influence groundwater levels 

over short distances. Stresses due to discharge by pumping, 

or to gaining streams, springs, seepage faces, wetlands, or 

evapotranspiration similarly can have large variations over 

short distances and influence groundwater levels.

The aquifer properties that influence the movement and 

storage of water in the aquifer and, therefore the groundwater 

level in the aquifer, include the extent, thickness, shape, 

hydraulic conductivity, storage coefficient, and degree of 

confinement. These properties can be highly heterogeneous 

over short distances due to variations in the original lithology 

or process of emplacement, subsequent modification by 

physical process such as erosion or fracturing due to structural 

deformation, or chemical changes such as dissolution, 

alteration, or mineralization. Variations in these properties 

over short distances may affect the groundwater levels and (or) 

the timing and magnitude of groundwater changes in response 

to changes in stresses relative to groundwater levels elsewhere 

in the aquifer.

Influence of Well Construction

Well construction may substantially influence the water 

levels in wells. Ideally, a well used for groundwater-level 

monitoring should be constructed to ensure good hydraulic 

connection between the well and the intended aquifer and that 

the water level and water-level fluctuations in the well broadly 

represent conditions in the aquifer. The well design must take 

into consideration the placement of open intervals and include 

appropriately-sized well screens to permit a good hydraulic 

connection between water in the aquifer and in the well. The 

use of sanitary and flow seals and well casings, where needed, 

help to isolate the well and the contributing aquifer from other 

units to prevent commingling, the collapse of rock into the 

well, and movement of water between the rock and the outside 

of the well casing. Incorrect well design, construction defects 

at the time of installation, insufficient well development 

(repeated purging and filling of a well to remove fine materials 

that may clog the well screen), or degradation due to age 

including silting, corrosion, or bacterial growth may affect 

how a well responds to changes in the aquifer (Taylor and 

Alley, 2002, p. 9).

Complete and accurate documentation of the well 

construction is important to ensure that the water levels 

measured in the well are properly interpreted. Documentation 

of well construction for some wells often is incomplete and 

does not include sufficient information, such as lithology 

or position of open intervals, to identify the contributing 

aquifer(s). This creates uncertainty on how to associate a 

water-level measurement from a well with the appropriate 

aquifer. Uncertainty with regard to the aquifer represented 

in a water-level measurement can contribute to misleading 

interpretations.

Over time, water-level declines in the CPRAS have 

resulted in dry wells, some of which have been deepened 

and reconstructed. Because water-levels from a deepened 

or reconstructed well represent conditions at a different 

location in the aquifer system, measurements always should 

be associated with well depth and construction at the time of 

measurement. However, in most instances the information on 

the deepening of wells was not associated with the original 

well for the data compilation sources used for analysis in this 

study. As a result, water-level measurements made subsequent 

to the deepening of some wells may remain associated with 

the depth of the original well. Hydrographs for these wells 

may show an initial step change in the water-levels measured 

in the well following deepening that could be mistakenly 

attributed to other causes. The water-level response of the well 

subsequent to deepening also may reflect the conditions in the 

aquifer at the new well depth. To address these limitations, 

robust methods of analysis were used to ensure that errors 

at individual wells did not strongly affect overall study 

conclusions.
12 Groundwater Status and Trends for the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

Commingling

Commingling is the term used to describe the condition 

that occurs when a well is constructed so water can move from 

one aquifer to another through the well bore. This can occur 

in wells that are open to multiple aquifers through screens or 

uncased intervals. If the aquifers have different heads, then 

water will move through the well bore from the aquifer(s) 

with higher head to the aquifer(s) with lower head. When 

such commingling occurs, the static water level in the well is 

a composite water level, averaging conditions between all the 

aquifers open to the well.

In figure 4, well 3 is a hypothetical multiple completion 

well with open intervals in aquifers 1 and 2, which results 

in commingling of waters between aquifers. Because 

the hydraulic head in aquifer 1 (as represented by the 

potentiometric surface of the aquifer at the well) is greater 

than the hydraulic head in aquifer 2 a downward hydraulic 

gradient enables flow to enter the well 3 bore from aquifer 1, 

traversing down the well 3 bore and exiting the well 3 bore 

into aquifer 2. For this example, the transmissivities of aquifer 

1 and 2 are equal; therefore, the resulting hydraulic head in 

well 3, from a transmissivity-weighted average of the heads 

 

Aquifer 2

Aquifer 1

Potentiometric surface

Aquifer 2

Potentiometric surface

Aquifer 1

Confining

unit

Confining

unit

Confining

unit

Well 1:

Single 

completion 

well,

Aquifer 1 

Well 2:

Single 

completion 

well,

Aquifer 1 

(affected by 

commingling)

Well 3:

Multiple 

completion 

well,

Aquifers 1 

and 2

Well 4:

Single 

completion

well,

Aquifer 2

(affected by 

commingling)

Well 5:

Single 

completion 

well,

Aquifer 2

TransmissivityAquifer 1 TransmissivityAquifer 2 

Figure 4. Effect of commingling on water levels in hypothetical wells for aquifers of equal transmissivity. Near the commingled 

well (well 3), the potentiometric surface is strongly affected by aquifer cross-connection. Far from the commingled well, the 

potentiometric surface reflects regional conditions.
Introduction 13

in aquifers 1 and 2, is exactly one-half the vertical distance 

between the head in the two aquifers. Well 2 and well 4 are 

single completion wells with open intervals within aquifer 

1 and aquifer 2, respectively, that are in close proximity 

to commingling well 3. These wells are within the cone of 

depression and cone of impression (the inverse of a cone of 

depression) resulting from the commingling in well 3. The 

hydraulic head in these wells is intermediate to the head at 

well 3 and the initial unaffected heads in aquifers 1 and 2 as 

represented by the heads in wells 1 and 5, respectively.

In figure 5, the transmissivity of aquifer 1 is much greater 

than the transmissivity of aquifer 2; therefore, the head in well 

3 is dominated by the hydraulic head of aquifer 1. Because 

the cone of depression for aquifer 1 is greatly subdued, water 

levels in proximal wells open to aquifer 1, such as well 2, are 

minimally affected. However, water levels in proximal wells 

open to aquifer 2, such as well 4, are strongly affected due to 

the exaggerated cone of impression for aquifer 2.

 

Aquifer 2

Aquifer 1

Potentiometric surface

Aquifer 2

Potentiometric surface

Aquifer 1

Confining

unit

Confining

unit

Confining

unit

Well 1:

Single 

completion 

well,

Aquifer 1 

Well 2:

Single 

completion well,

Aquifer 1 

(slightly affected 

by commingling)

Well 3:

Multiple 

completion 

well,

Aquifers 1 

and 2

Well 4:

Single 

completion well,

Aquifer 2

(strongly affected 

by commingling)

Well 5:

Single 

completion 

well,

Aquifer 2

TransmissivityAquifer 1 TransmissivityAquifer 2 

Figure 5. Effect of commingling on water levels in hypothetical wells for aquifers of unequal transmissivity. Water level in a well 

most closely resembles the aquifer with the higher transmissivity because the well has better hydraulic connection to this aquifer. 

Near the commingled well (well 3), the potentiometric surface is strongly affected by aquifer cross-connection. Far from the 

commingled well, the potentiometric surface reflects regional conditions.
14 Groundwater Status and Trends for the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

The basalt aquifers of the CPRAS consist of a series of 

permeable interflow zones separated by less permeable flow 

interiors (Kahle and others, 2011, p. 20). The transmissivities 

of the aquifers can vary over several orders of magnitude 

(Kahle and others, 2011). The resulting water levels in 

commingled wells open to multiple aquifers depend on the 

relative transmissivity of the aquifers, which is a function of 

the thickness and the permeability of the aquifers. Figure 6

illustrates some possible examples for aquifers of varying 

transmissivity (thickness and (or) permeability).

Well 1: Well 2: Well 3: Well 4:

Areas of low permeability

EXPLANATION

T Transmissivity

Confining

unit

Confining

unit

Confining

unit

Aquifer 2

Aquifer 1

Potentiometric

surface, Aquifer 2

Potentiometric

surface, Aquifer 1

TAquifer 1 TAquifer 2 TAquifer 1 TAquifer 2 TAquifer 1 TAquifer 2 TAquifer 1 TAquifer 2 

Figure 6. Effect of commingling on water levels in hypothetical wells for aquifers with local variability. Water level 

in a well most closely resembles the aquifer with which it is in best connection. The flow of water can be restricted 

by changes in aquifer thickness, aquifer lithology, or well construction, resulting in reduced hydraulic connection. 

Near a commingled well, the potentiometric surface is strongly affected by aquifer cross-connection. Far from the 

commingled well, the potentiometric surface reflects regional conditions.

The ratio of water-level fluctuations in a well to the 

groundwater-level fluctuations in an aquifer penetrated by 

that well is equal to the ratio of transmissivity of the aquifer 

in which the fluctuation occurs to the total transmissivities of 

all aquifers perforated by the well. As a result, the effect of the 

change in groundwater level of any aquifer where the well is 

open imposes a smaller change on the water level in the well 

(Sokol, 1963, p. 1,080).
Groundwater Status and Trends 15

Groundwater Status and Trends

Trend analyses were done on the compiled groundwaterlevel data for the CRBG aquifers to characterize and document 

changes in the hydrologic status of the system. Water levels in 

individual wells vary in response to natural and anthropogenic 

stresses on daily, seasonal, decadal, and longer time scales. 

Because the purpose of the USGS Groundwater Resources 

Program study is to evaluate the long-term availability and 

sustainability of groundwater resources within the CPRAS, the 

focus of the data analysis is to examine persistent decadal or 

longer trends that have occurred in many wells since the onset 

of widespread irrigation and groundwater pumping.

For the trend analysis, groundwater measurements 

representing conditions within the CRBG aquifers were used. 

The geologic model of Burns and others (2011) was used to 

identify wells with bottom elevations between the simulated 

top of the CRBG geologic units and the older pre-Miocene 

basement rocks. Water-level measurements from these 

identified wells then were used in the analyses. To remove 

daily and seasonal variation in groundwater levels induced by 

irrigation pumping, the median groundwater level measured 

in winter between January and March of each year was used 

in the analysis. The resulting data reflect the influence of 

multi-year precipitation patterns and the cumulative effects of 

pumping and irrigation recharge.

Water-level measurements made prior to 1951 are 

assumed to represent pre-development conditions because 

large scale irrigation started after the beginning of 1951. 

Because summer water levels in wells were not yet affected by 

large scale groundwater pumping, annual median water levels 

were used when winter water-level data were not available 

prior to 1951.

Water-level measurements in reports filed by well drillers 

at the time of new well installation were included to obtain 

the broadest spatial and temporal distribution possible in areas 

or periods of sparse data. Groundwater-level measurements 

reported as non-static measurements were excluded from 

analysis. The subsampled dataset includes 7,735 CRBG wells, 

with data representing pre-development conditions in 1,265 

wells (fig. 7).

Although pumping and irrigation effects began to 

have appreciable effects on groundwater levels during the 

1950s in the CPRAS, most persistent regional declines from 

pumping started after 1970. A map of the linear trends in 

groundwater levels for 1968–2009 was constructed to show 

areas of widespread declines in CRBG aquifers (fig. 8). 

Groundwater-level trends were computed as the slope of the 

best-fit line to the winter median water-level data for each 

well, provided that at least four data points were available 

that spanned at least 50 percent of the period 1968–2009. Of 

the 761 wells in the CRBG aquifers with sufficient data to 

compute water-level trends, overall declines were measured in 

72 percent of the wells. The mean of the slopes of the waterlevel trends for all wells was a decline of 1.9 ft/yr. Rates of 

declines greater than 1.0 ft/yr were measured in 50 percent of 

wells, declines greater than 2.0 ft/yr in 38 percent of wells, 

declines greater than 4.0 ft/yr in 29 percent of wells, declines 

greater than 6.0 ft/yr in 9 percent of wells, and rates of decline 

greater than 8.0 ft/yr in 4 percent of wells. These results are 

similar to the values obtained by Snyder and Haynes (2010, 

table 1, p. 8) for 273 CRBG aquifer wells in the CPRAS 

during 1984–2009 where water levels declined in 81 percent 

of wells and water levels changed at an average rate of a 

1.5 ft/yr decline (calculated as a weighted-mean average 

for all CRBG aquifer wells). Because complex anomalous 

behavior can occur for any single well hydrograph (for 

example, step changes in water level associated with nearby 

well construction activities), the linear water-level trend for 

any single well may not represent conditions over an area 

of interest, but the general pattern of water-level declines in 

multiple wells illustrates the persistent patterns across 

the CPRAS.

The clusters of wells with linear declines generally 

correspond to areas that are the subject of previous 

hydrogeologic studies and to continued data collection efforts 

by local, State, Federal, or non-governmental agencies in 

Oregon in the Mosier Watershed and Umatilla Basin, and 

in Washington in the Yakima Basin and the Palouse Slope/

eastern Yakima Fold Belt in the Columbia Basin Ground 

Water Management Area (GWMA). The Columbia Basin 

GWMA, which encompasses Adams, Franklin, Grant, and 

Lincoln Counties (fig. 8), was designated by the Washington 

State Department of Ecology (WADOE), for the protection 

of groundwater in the area. Detailed analyses of well 

hydrographs for the Mosier Watershed, Oregon, and Yakima 

Basin, Washington, are available in Burns and others (2012) 

and Keys and others (2008), respectively. Well hydrographs 

are examined for parts of the Umatilla Basin, Oregon, and the 

Palouse Slope/eastern Yakima Fold Belt (within the GWMA), 

Washington, in this report. Groundwater-level hydrographs 

for the Umatilla Basin show that barriers to groundwater 

flow are readily identifiable. Conversely, groundwater-level 

hydrographs from the Columbia Basin GWMA exhibit a 

significantly different behavior, with fewer well-defined 

barriers to flow and with groundwater-level changes being 

dominated by the large-scale irrigation projects in the lowland 

near the Columbia River. An analysis describing the relation 

between well hydrographs and geologic features in the 

GWMA is presented by Porcello and others (2009); therefore, 

the discussion here is restricted to a complementary discussion 

of regional-scale flow patterns and hydraulic changes resulting 

from development of water resources in the GWMA since 1950.
16 Groundwater Status and Trends for the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

tac12-0765_fig07

Pasco

Yakima

Moscow

Pullman

Spokane

Lewiston

La Grande

Wenatchee

The

Dalles

Ellensburg

Moses

Lake

Walla

Walla

Coeur

d'Alene

GRANT BAKER

UMATILLA

WASCO

UNION

WALLOWA

MORROW

WHEELER

GILLIAM

JEFFERSON

CROOK

MALHEUR

SHERMAN

HOOD

RIVER

IDAHO

VALLEY

SHOSHONE

ADAMS

LATAH

BONNER

KOOTENAI

CLEARWATER

WASHINGTON

BENEWAH

LEWIS

NEZ

YAKIMA PERCE

GRANT

CHELAN

KITTITAS ADAMS

LINCOLN KING

WHITMAN

BENTON

KLICKITAT

DOUGLAS

SPOKANE

OKANOGAN FERRY

STEVENS

FRANKLIN

SNOHOMISH

WALLA

WALLA

LEWIS

ASOTIN

COLUMBIA

GARFIELD

PIERCE

SKAGIT

SKAMANIA

PEND

OREILLE

John Day River Crooked River

Palouse River

Snake River

Yakima River

Imnaha River

Clearwater River

Sa ml on River

Crab Creek

Hangman Creek

COLUMBIA

N Fk John Day R

Grande Ronde River

RIVER

Snake River

Lake Coeur d'Alene

Potholes

Res

COLUMBIA RIVER

Banks Lake

Naches R

McKay Cr

Lolo

Creek

IDAHO

OREGON

WASHINGTON

B

C

P

Y

Palouse Slope

Clearwater Embayment

Blue Mountains

Yakima Fold Belt

B

C

P

Y

Structural regions

EXPLANATION

Geologic model extent of Columbia

River Basalt Group

Columbia Basin Ground Water

Management Area

Mosier Watershed

Umatilla Basin

Yakima Basin

Wells open to the Columbia River Basalt 

Group with water-level measurements

Earliest measurement: 1895-1950 (pre-development)

Earliest measurement: 1951-2009

0 20 40 60 80 100 Miles

0 20 40 60 80 100 Kilometers

Base map modified from USGS and other digital data, various 

scales. Coordinate system: State Plane-Washington South FIPS 

4602, Projection: Lambert Conformal Conic; North American 

Datum of 1983.

121° 120° 119° 118° 117° 116°

48°

47°

46°

45°

Figure 7. Distribution of wells used for status and trends analysis for the Columbia Plateau Regional Aquifer System, Washington, 

Oregon, and Idaho. The water levels represent the period each year when seasonal pumping effects are minimal.
Introduction 17

tac12-0765_fig08

GRANT BAKER

UMATILLA

WASCO

UNION

WALLOWA

MORROW

WHEELER

GILLIAM

JEFFERSON

CROOK

MALHEUR

SHERMAN

HOOD

RIVER

IDAHO

VALLEY

SHOSHONE

ADAMS

LATAH

BONNER

KOOTENAI

CLEARWATER

WASHINGTON

BENEWAH

LEWIS

NEZ

YAKIMA PERCE

GRANT

CHELAN

KITTITAS ADAMS

LINCOLN KING

WHITMAN

BENTON

KLICKITAT

DOUGLAS

SPOKANE

OKANOGAN FERRY

STEVENS

FRANKLIN

SNOHOMISH

WALLA

WALLA

LEWIS

ASOTIN

COLUMBIA

GARFIELD

PIERCE

SKAGIT

SKAMANIA

PEND

OREILLE

Palouse Slope

Clearwater Embayment

Blue Mountains

Yakima Fold Belt

B

C

P

Y

Structural regions

John Day River Crooked River

Palouse River

Snake River

Yakima River

Imnaha River

Clearwater River

Sa ml on River

Crab Creek

Hangman Creek

COLUMBIA

N Fk John Day R

Grande Ronde River

RIVER

Snake River

Lake Coeur d'Alene

Potholes

Res

COLUMBIA RIVER

Banks Lake

Naches R

McKay Cr

Lolo

Creek

OR ID

WA

B

C

P

Y

EXPLANATION

Modeled extent of Columbia River Basalt Group

Columbia Basin Ground Water Management Area

Mosier Watershed

Umatilla Basin

Yakima Basin

7.5 to 10.8

5.0 to 7.5

3.5 to 5.0

1.0 to 3.5

0.1 to 1.0

-0.1 to -1.0

-1.0 to -3.5

-3.5 to -5.0

-5.0 to -7.5

-7.5 to -25.1

Slope increases, in feet per year Slope declines, in feet per year

Groundwater level trends—1968–2009

Slope, flat -0.1 to 0.1

Structural regions of the Columbia Plateau 

Regional Aquifer System (see inset map)

0 20 40 60 80 100 Miles

0 20 40 60 80 100 Kilometers

Base map modified from USGS and other digital data, various 

scales. Coordinate system: State Plane-Washington South FIPS 

4602, Projection: Lambert Conformal Conic; North American 

Datum of 1983.

121° 120° 119° 118° 117° 116°

48°

47°

46°

45°

Pasco

Pendleton

Yakima

Moscow

Pullman

Spokane

Lewiston

La Grande

Wenatchee

The

Dalles

Ellensburg

Moses

Lake

Walla

Walla

Coeur

d'Alene

Figure 8. Distribution of groundwater-level trends in the Columbia Plateau Regional Aquifer System, Washington, Oregon, and 

Idaho, for 1968–2009.
18 Groundwater Status and Trends for the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

Umatilla Area, Oregon

The Oregon Water Resources Department (OWRD) 

established several administrative areas (Critical Ground 

Water Areas and Ground Water Limited Areas) in the area 

of the Umatilla Basin that cover most locations with large 

long-term groundwater declines. Administrative areas 

include Butter Creek, Ella Butte, Ordnance Basalt, and Stage 

Gulch (fig. 9; Oregon Water Resources Department, 2012). 

A total of 286 wells within and immediately adjacent to the 

OWRD administrative areas in the CRBG aquifers were 

divided into clustered groups of wells with similar water 

levels and trends (fig. 10). Figure 11 presents hydrographs 

showing the winter median water levels for individual wells 

within each well group shown in figure 10 and a trend line 

representing the overall water-level trend of all wells in the 

group constructed using the LOESS algorithm in the statistical 

software program TIBCO Spotfire S+ (TIBCO Software, Inc., 

1988–2008). Zones of low permeability may separate the 

groups of wells with similar groundwater levels and trends. 

These zones represent leaky barriers to groundwater flow and 

compartmentalize the CRBG aquifer system. The degree of 

compartmentalization is variable, but it occurs in both the 

vertical and horizontal directions in the Umatilla area.

Horizontal flow barriers (barriers prohibiting or reducing 

horizontal groundwater flow) were identified between adjacent 

well groups when water levels and trends were different for 

wells open to the same aquifer. Because aquifers have not been 

mapped extensively within the study area, drilling of each well 

was presumed stopped when the target aquifer was found. The 

elevation where drilling stopped was used as an estimate of the 

elevation of an aquifer at that location. However, the originally 

flat-lying CRBG lava flows have been deformed over geologic 

time, so the CRBG interflow zones hosting most productive 

aquifers in the CPRAS are not horizontal. To correct for the 

departure from horizontality, the geologic model of Burns 

and others (2011) was used to compute estimates of the 

stratigraphic positions for water producing zones in each well. 

Correction was accomplished by subtracting the estimated 

top elevation of the Grande Ronde Basalt geologic model 

unit from each well bottom elevation. Positive values indicate 

aquifers are in the lavas above the top of the Grande Ronde 

Basalt, and negative values indicate aquifers are in the Grande 

Ronde Basalt. Plotting the groundwater level against this 

stratigraphic position for each well allows a rapid assessment 

of whether or not hydraulic head values are representative 

of the same aquifers (fig. 12). However, the flow margin of 

several younger (post Grande Ronde Basalt) lava flows are 

within the area of interest with thicker deposits to the north. 

The resulting wedge shape of the overlying lavas complicates 

the interpretation of positive elevations (fig. 12) corresponding 

to aquifer horizons, especially for north-south transects. 

Additional details regarding this method are provided in 

appendix A. 

An example of strong horizontal compartmentalization is 

provided by comparing groups 2 (dark blue triangles) and 12 

(gray triangles) (fig. 10). Measurements for these two groups 

likely are from the same aquifer within the Grande Ronde 

Basalt although water levels differ on average by about 500 ft 

(figs. 11B and 12D–F). Within each group, water levels for 

nearby wells commonly are within a few tens of feet of each 

other. Group 2 wells have a wider range of values than group 

12 wells, which corresponds to a relatively smooth hydraulic 

gradient from Pendleton, Oregon, to the center of the Stage 

Gulch administrative area.

An example of a vertical flow barrier is provided by 

groups 3 (dark green squares) and 14 (light blue squares) 

(fig. 10). In map view, these groups apparently occupy much 

of the same area; therefore, horizontal compartmentalization 

does not explain the hydraulic head contrast between the 

groups that began in the 1960s (fig. 11A). However, based on 

the shallow and deep well pairs shown in figure 10, group 3 

well bottoms are consistently above group 14 well bottoms 

(fig. 13), indicating that the aquifers are separated vertically, 

with the shallow group 3 wells exhibiting no persistent 

water-level declines, and the deeper group 14 wells exhibiting 

substantial declines since the 1960s. Whereas small groups 

of wells show ideal vertical separation (fig. 13), the pattern 

is obscured when considering all wells in groups 3 and 14 at 

the same time (fig. 12A–F). Well bottoms for group 3 wells 

trend to lower elevations of stratigraphic position to the south 

(not shown), indicating that the group 3 aquifer(s) are sloping 

relative to the estimated top of Grande Ronde Basalt consistent 

with the thickening of the younger lavas to the north. If 

aquifers were mapped based on similarity in hydraulic head, 

the apparent overlap of groups 3 and 14 (fig. 12) would 

disappear and the ideal vertical separation (compare fig. 13

with appendix A, fig. A1A) would be more apparent.
Groundwater Status and Trends 19

Figure 9. Distribution of groundwater-level trends for 1968–2009 in Oregon Water Resources Department administrative areas in 

the Umatilla Basin, Oregon.

tac12-0765_fig09

BAKER

UMATILLA

UNION

MORROW

WHEELER

GILLIAM

YAKIMA

BENTON

KLICKITAT

WALLA

WALLA

Condon

Umatilla

Pendleton

Hermiston

MiltonFreewater

Yakima River

COLUMBIA RIVER

Umatilla River

Palouse Slope

Clearwater Embayment

Blue Mountains

Yakima Fold Belt

B

C

P

Y

Structural regions

OR ID

WA

B

C

P Y

EXPLANATION

Geologic model extent of the Columbia River

Basalt Group

Umatilla Basin

Oregon Water Resources Department 

Administrative Areas

Butter Creek

Ella Butte

Ordnance Basalt

Stage Gulch

7.5 to 9.8

5.0 to 7.5

3.5 to 5.0

1.0 to 3.5

0.1 to 1.0

-0.1 to -1.0

-1.0 to -3.5

-3.5 to -5.0

-5.0 to -7.5

-7.5 to -12.9

Slope increases, in feet per year Slope declines, in feet per year

Groundwater level trends—1968–2009

Slope, flat -0.1 to 0.1

0 10 20 Miles

0 10 20 Kilometers

Base map modified from USGS and other digital data, various 

scales. Coordinate system: State Plane-Washington South FIPS 

4602, Projection: Lambert Conformal Conic; North American 

Datum of 1983.

46°

45°

120° 119°
20 Groundwater Status and Trends for the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

tac12-0765_fig10

COLUMBIA RIVER

Umatilla River

Ione

Echo

Heppner

Umatilla

Boardman

Stanfield

Pendleton

Lexington

Hermiston

Pilot

Rock

Irrigon

UMATILLA

MORROW

YAKIMA

BENTON

KLICKITAT

WALLA

WALLA

WILLOW CREEK MONOCLINE

 

REITH

ANTICLINE

SERVICE ANTICLINE 

AGENCY SYNCLINE

DALLES - UMATILLA SYNCLINE

Palouse Slope

Clearwater Embayment

Blue Mountains

Yakima Fold Belt

B

C

P

Y

Structural regions

OR ID

WA

B

C

P Y

EXPLANATION

Wells by group

Group 2

Group 6

Group 9

Group 1

Group 3

Group 4

Group 5

Group 7

Group 8

Group 12

Group 13

Group 11

Group 10

Group 14

Group 15

Group 16

Group 17

Group 18

Shallow-deep pairs

Fault

Fold

Yellow pair

Blue pair

Purple pair

Orange pair

Umatilla Basin

Oregon Water Resources Department

Administrative Areas

Butter Creek

Ella Butte

Ordnance Basalt

Stage Gulch

0 10 20 Miles

0 10 20 Kilometers

Base map modified from USGS and other digital data, various 

scales. Coordinate system: State Plane-Washington South FIPS 

4602, Projection: Lambert Conformal Conic; North American 

Datum of 1983.

119°

46°

Figure 10. Groups of wells with similar hydraulic response near the Oregon Water Resources Department administrative areas in 

the Umatilla Basin, Oregon. Wells are grouped to show a general North-South transect (circles and squares) and a general EastWest transect (triangles and squares) (fig. 11).
Groundwater Status and Trends 21

tac12-0765_fig11a

Group 1

EXPLANATION

Group 3

Group 4

Group 5

Group 7

Group 8

Group 10

Group 14

Group 15

Group 16

Group 17

Group 18

Well

200

100

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

Winter median water-level elevation, in feet (NAVD88)

A. Hydrograph corresponding to North-South well groups shown in figure 10

1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010

Year

Figure 11. Groups of wells near the Oregon Water Resources Department administrative areas in the Umatilla Basin, Oregon. 

(A) North–South group (circles and squares); and (B) East–West group (triangles and squares). Hydrographs depict the winter 

median water level for individual wells within each well group and LOESS curve representing the overall water-level trend of all 

wells in the group. Locations of wells are shown in figure 10.
22 Groundwater Status and Trends for the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

tac12-0765_fig11b

Group 1

EXPLANATION

Group 2

Group 3

Group 6

Group 7

Group 8

Group 9

Group 11

Group 12

Group 13

Group 14

Well

200

100

300

400

500

600

700

800

900

1,000

B. Hydrograph corresponding to East-West well groups shown in figure 10

1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010

Winter median water-level elevation, in feet (NAVD88)

Year

Figure 11.—Continued
Groundwater Status and Trends 23

Figure 12. Estimated stratigraphic position (distance from the well bottom 

to the simulated top of Grande Ronde Basalt) compared with winter median 

water-level elevation in selected wells within and immediately adjacent to 

the Oregon Water Resources Department administrative areas in the Umatilla 

Basin, Oregon, 1968, 1984, and 2006. (A–C) North–South group; and (D–F) 

East–West group. Locations of groups are shown in figure 10.

tac12-0765_fig12a Winter median water-level elevation, in feet (NAVD88)

A. Umatilla North-South well groups for 1968

300

400

500

600

700

800

900

1,000

1,100

1,200

1,300

1,400

1,500

1,600

-1,200 -1,000 -800 -600 -400 -200 0 200 400 600 800 1,000 1,200

Stratigraphic position relative to top of the Grande Ronde Basalt, in feet

Well bottom in the Grande Ronde Basalt Well bottom above top of Grande Ronde Basalt

EXPLANATION

Group 3

Group 4

Group 8

Group 14

Group 15

Well

Estimated top of Grande Ronde Basalt
24 Groundwater Status and Trends for the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

tac12-0765_fig12b Winter median water-level elevation, in feet (NAVD88)

B. Umatilla North-South well groups for 1984

100

200

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

-1,200 -1,000 -800 -600 -400 -200 0 200 400 600 800 1,000 1,200

Stratigraphic position relative to top of the Grande Ronde Basalt, in feet

Well bottom in the Grande Ronde Basalt Well bottom above top of Grande Ronde Basalt Estimated top of Grande Ronde Basalt

EXPLANATION

Group 1

Well

Group 3

Group 4

Group 5

Group 7

Group 8

Group 10

Group 14

Group 15

Group 16

Group 17

Figure 12.—Continued
Groundwater Status and Trends 25

tac12-0765_fig12c Winter median water-level elevation, in feet (NAVD88)

C. Umatilla North-South well groups for 2006

200

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

-1,200 -1,000 -800 -600 -400 -200 0 200 400 600 800 1,000 1,200

Stratigraphic position relative to top of the Grande Ronde Basalt, in feet

Well bottom in the Grande Ronde Basalt Well bottom above top of Grande Ronde Basalt Estimated top of Grande Ronde Basalt

EXPLANATION

Group 1

Well

Group 3

Group 4

Group 5

Group 7

Group 8

Group 10

Group 14

Group 15

Group 16

Group 17

Group 18

Winter median water-level elevation, in feet (NAVD88)

D. Umatilla East-West well groups for 1968

-200 200

300

350

400

-400 400

450

500

550

600

-600 600

650

700

750

800

-800 800

850

900

950

1,000

-1,600 -1,200 -1,400 -1,000 0 1,000 1,200

Stratigraphic position relative to top of the Grande Ronde Basalt, in feet Estimated top of Grande Ronde Basalt

Well bottom in the Grande Ronde Basalt Well bottom above top of Grande Ronde Basalt

Well 

EXPLANATION

Group 2

Group 6

Group 3

Group 8

Group 12

Group 13

Group 11

Group 14

-1,800

Figure 12.—Continued
26 Groundwater Status and Trends for the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

Figure 12.—Continued

tac12-0765_fig12e Winter median water-level elevation, in feet (NAVD88)

E. Umatilla East-West well groups for 1984

150

-200 200

200

250

300

350

400

-400 400

450

500

550

600

-600 600

650

700

750

800

-800 800

850

900

950

-1,600 -1,200 -1,400 -1,000 0 1,000 1,200

Stratigraphic position relative to top of the Grande Ronde Basalt, in feet Estimated top of Grande Ronde Basalt

Well bottom in the Grande Ronde Basalt Well bottom above top of Grande Ronde Basalt

Group 12

Group 13

Group 2

Group 6

Group 9

Group 11

Group 1

Well 

EXPLANATION

Group 3

Group 7

Group 8

Group 14

-1,800

Winter median water-level elevation, in feet (NAVD88)

F. Umatilla East-West well groups for 2006

-200 200

200

250

300

350

400

-400 400

450

500

550

600

-600 600

650

700

750

800

-800 800

850

900

-1,600 -1,200 -1,400 -1,000 0 1,000 1,200

Stratigraphic position relative to top of the Grande Ronde Basalt, in feet Estimated top of Grande Ronde Basalt

Well bottom in the Grande Ronde Basalt Well bottom above top of Grande Ronde Basalt

-1,800

Group 12

Group 13

Group 2

Group 6

Group 9

Group 11

Group 1

Well

EXPLANATION

Group 3

Group 7

Group 8

Group 14
Groundwater Status and Trends 27

Figure 13. Stratigraphic position (distance from the 

well bottom to the simulated top of Grande Ronde Basalt) 

compared with winter median water level measured 

in wells for shallow–deep well pairs from groups 3 

and 14 near the Oregon Water Resources Department 

administrative areas in the Umatilla Basin, Oregon. Colors 

of points correspond to colors of shallow-deep pair circles 

in figure 10.

tac12-0765fig13

Group 3

Group 14

300

350

400

450

500

550

-300 -200 -100 0 100 200 300 400 500 600 700 800

Umatilla area well groups 3 and 14 shallow-deep

well pairs for 1983-86 

Winter median water-level elevation, in feet (NAVD88)

Stratigraphic position relative to top of the Grande Ronde Basalt, in feet

EXPLANATION

Yellow Pair: Group 3

Yellow Pair: Group 14

Blue Pair: Group 3

Blue Pair: Group 14

Purple Pair: Group 3

Purple Pair: Group 14

Orange Pair: Group 3

Orange Pair: Group 14

Shallow-Deep Pairs

Barriers to groundwater flow result from primary 

characteristics of the basalt, the depositional environment, 

and post-depositional folding and faulting. Vertical flow 

barriers typically result when aquifers are separated by dense 

impermeable CRBG lava flow interiors that are laterally 

extensive. Near flow margins, however, the dense interiors 

are discontinuous, and aquifers may be well-connected 

hydraulically. In the OWRD administrative areas, vertical 

separation is apparent only in the groundwater-level data 

near groups 6 (light blue triangles), 11 (pink triangles), and 

14 (light blue squares), which have higher hydraulic heads 

than nearby wells completed in lower stratigraphic units. For 

example, the head in group 3 wells stayed nearly constant at 

about 500 ft, whereas the deeper group 14 wells have shown 

declines in groundwater elevation from about 450 to about 

300 ft since the 1960s (fig. 11B).

For all other well groups, there is a conspicuous lack 

of vertical hydraulic gradients across thick sequences 

of lava that should contain many individual lava flows. 

Within most groups a uniform hydraulic head exists across 

thousands of feet of basalt thickness despite the fact that 

many (approximately 100-ft thick) lavas are intersected 

(fig. 12A–F; ideal behavior is shown in fig. A1D). These 

hydraulic heads are uniform even though the older lava flows 

were more extensive than the younger flows, which implies 

that dense flow interiors should separate the aquifers, creating 

the conditions necessary for vertical hydraulic gradients. 

The uniformity of hydraulic heads in these aquifers may be 

the result of hydraulic equilibration through commingling 

wells. For confined or other low storage aquifers, time for 

equilibration can be short. In the Mosier Watershed, Oregon, 

commingled wells equilibrated within 2 years (Burns and 

others, 2012). Because groundwater-level monitoring typically 

begins after multi-year declines have been documented, data 

representing pre-commingling conditions are limited.
28 Groundwater Status and Trends for the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

Examination of the mapped geologic structure shows 

horizontal compartmentalization is frequently correlated to 

the structure, although not always. The correlation along the 

Willow Creek monocline in groups 4 (orange circles), 5 (dark 

red circles), 10 (purple circles), and 15 (dark blue circles) 

show consistent head patterns along the sinuous fold structure 

and high head differences between the groups across the fold 

indicating that the fold functions as a longitudinal conduit 

and a perpendicular barrier to groundwater flow (figs. 10 and 

11A). The Service anticline also is an apparent horizontal 

flow barrier, although the deeper aquifers have complex, yet 

similar patterns particularly later in time, which indicates that 

some aquifers may be better hydraulically connected across 

the structure than others, indicating that commingling may be 

rendering this barrier less effective over time. To the contrary, 

the Reith anticline apparently is not a horizontal barrier to 

flow between the Pendleton area and the eastern side of the 

Stage Gulch administrative area as evidenced by group 2 

wells (dark blue triangles) in figure 11B. From north to south, 

the hydraulic gradients increase proportionally to the amount 

of geologic structure (compare water levels from figure 11A

to the well locations in figure 10), indicating that geologic 

structure may be impeding lateral recharge from the uplands. 

Curiously, the exceptionally high hydraulic head contrast 

between groups 2 and 12 (gray triangles) (figs. 10 and 12D–E) 

does not correspond to a mapped geologic structure, indicating 

the presence of a previously unmapped geologic feature.

Near the OWRD administrative areas, a few shallow 

CRBG aquifers (groups 3 [dark green squares], 6 [light blue 

triangles], 7 [yellow squares], and 11 [pink triangles]) are 

receiving recharge from irrigation projects and have stable 

or slightly rising hydraulic heads (fig. 11B). Water levels in 

many of the deeper CRBG aquifers, in contrast, have declined 

100–300 ft since 1970 as shown in groups 4 (orange circles), 

8 (red squares), 10 (purple circles), and 14 (light blue squares) 

(fig. 11A). Hydraulic heads in groups 15 [dark blue circles], 

16 [green circles], 17 [white circles], and 18 [magenta circles] 

(figs. 10 and 11A) to the south have lower total decline 

because they are protected from the high pumping rates in the 

north by horizontal flow barriers immediately north of 

these groups.

Palouse Slope/Eastern Yakima Fold Belt, 

Washington

The Palouse Slope and the eastern Yakima Fold Belt 

(fig. 1), which forms a transition area between the two 

physiographic provinces, encompass the entire area of the 

Columbia Basin GWMA (hereafter referred to as the Palouse 

Slope/eYFB). Within the GWMA, WADOE has established 

several administrative areas (Ground Water Management 

Subareas) that cover most locations with large long-term 

groundwater-level declines (Washington State Department 

of Ecology, 1988a, 1988b, and 1988c). These include 

the Odessa, Quincy, and 508-14 subareas (fig. 14). The 

largest groundwater-level declines measured in the central 

GWMA (1968–2009) are along a north-south swath near 

the center of the area (fig. 14). Hydrographs for 1,195 wells 

blanketing the area of largest declines in the CRBG aquifers 

were examined and divided into groups of wells exhibiting 

similar changes in hydraulic head over time and a subset of 

these groups was selected to illustrate these trends (fig. 15). 

Horizontal barriers to flow are not as evident in this area as 

in the Umatilla area, but groups are still identifiable based 

on similar response to hydraulic stresses. This is consistent 

with the geologic interpretation of the Palouse Slope as being 

a gently folded structure created during subsidence where 

CRBG lavas were deposited in voluminous sheet flows. For 

this area, hydrographs of well groups are most easily viewed 

along general flow paths that have developed as the result of 

irrigation stresses on the aquifer system (fig. 15). These flow 

paths generally trend toward the area with large declines (fig. 14).

Figures 16–19 present hydrographs showing the 

winter median water levels for individual wells within each 

well group shown in figure 15 and the trend line for each 

group constructed using the statistical software program, 

TIBCO Spotfire S+ (TIBCO Software, Inc., 1988–2008). 

Water levels in wells in groups 1 (pink circles), 2 (light 

blue circles), 3 (dark green circles), and 4 (black circles 

and white circles for shallow and deep wells, respectively) 

associated with the western flow path (shown as circles in 

figure 15) start to rise during the 1950s (fig. 16). Prior to 

1950, the limited data suggest lower groundwater levels and 

flatter hydraulic gradients. After 1950, groundwater levels 

rise and the hydraulic gradient steepens from west to east 

indicating increased groundwater flow toward Moses Lake and 

farther east where groundwater declines are associated with 

widespread irrigation from groundwater.
Groundwater Status and Trends 29

Figure 14. Distribution of groundwater-level trends for 1968–2009 relative to the Washington State Department of Ecology 

administrative areas in parts of the Palouse Slope/eastern Yakima Fold Belt and the Columbia Basin Ground Water Management 

Area, Washington.

tac12-0765_fig14

Pasco

FERRY STEVENS

OKANOGAN

CHELAN

DOUGLAS

SPOKANE GARFIELD

LINCOLN

GRANT

KITTITAS

WHITMAN

ADAMS

YAKIMA

FRANKLIN

BENTON

COLUMBIA

FRENCHMAN HILLS

SADDLE MOUNTAINS

Moses

Lake

WALLA

WALLA

COLUMBIA

Banks Lake

RIVER

East Low Canal

McCarteney

Creek

West Canal

Lower Crab Creek

COLUMBIA

Palouse River

Snake River

Yakima River

Potholes

Reservoir

Crab Creek

48°

47°

30'

47°

46°

30'

0 5 10 15 20 25 Miles

0 5 10 15 20 25 Kilometers

Base map modified from USGS and other digital data, various 

scales. Coordinate system: State Plane-Washington South FIPS 

4602, Projection: Lambert Conformal Conic; North American 

Datum of 1983.

120° 119°30' 119° 118°30' 118°

EXPLANATION

Slope, flat -0.1 to 0.1

7.5 to 10.8

5.0 to 7.5

3.5 to 5.0

1.0 to 3.5

0.1 to 1.0

-0.1 to -1.0

-1.0 to -3.5

-3.5 to -5.0

-5.0 to -7.5

-7.5 to -25.1

Slope increases, in feet per year Slope declines, in feet per year

Groundwater level trends—1968–2009

EXPLANATION

508-14

Odessa

Quincy

Geologic model extent of Columbia River

Basalt Group

Columbia Basin Ground Water Management Area

Washington Department of Ecology Ground

Water Management Subareas 

Palouse Slope

Clearwater Embayment

Blue Mountains

Yakima Fold Belt

B

C

P

Y

Structural regions

OR ID

WA

B

C

P

Y
30 Groundwater Status and Trends for the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

tac12-0765_fig15

Pasco

FERRY STEVENS

OKANOGAN

CHELAN

DOUGLAS

SPOKANE GARFIELD

LINCOLN

GRANT

KITTITAS

WHITMAN

ADAMS

YAKIMA

FRANKLIN

BENTON

COLUMBIA

FRENCHMAN HILLS

SADDLE MOUNTAINS

Moses

Lake

WALLA

WALLA

COLUMBIA

Banks Lake

RIVER

East Low Canal

McCarteney

Creek

West Canal

Lower Crab Creek

COLUMBIA

Palouse River

Snake River

Yakima River

Potholes

Reservoir

Crab Creek

EASTERN

SOUTHERN

WESTERN

120° 119°30' 119° 118°30' 118°

48°

47°

30'

47°

46°

30'

0 5 10 15 20 25 Miles

0 5 10 15 20 25 Kilometers

Base map modified from USGS and other digital data, various 

scales. Coordinate system: State Plane-Washington South FIPS 

4602, Projection: Lambert Conformal Conic; North American 

Datum of 1983.

EXPLANATION EXPLANATION

508-14

Odessa

Quincy

Geologic model extent of Columbia River

Basalt Group

Columbia Basin Ground Water Management Area

Washington Department of Ecology Ground 

Water Management Subareas 

Fault

Fold

Generalized groundwater 

flow path under current 

conditions

Well

Group 6

Group 7

Group 5

Middle groups

Group 13

Group 14 -1

Group 14 - 2

Group 15

Southern groups

Group 11 - Deep

Group 11 - Shallow

Group 8

Group 10

Group 9

Group 12

Eastern groups

Group 2

Group 1

Group 3

Group 4 - Deep

Group 4 - Shallow

Western groups

Palouse Slope

Clearwater Embayment

Blue Mountains

Yakima Fold Belt

B

C

P

Y

Structural regions

OR ID

WA

B

C

P

Y

Figure 15. Generalized groundwater-flow paths under 2000–2010 conditions through sets of well groups with similar hydraulic 

response near the Washington State Department of Ecology administrative areas in parts of the Palouse Slope/eastern Yakima Fold 

Belt and the Columbia Basin Ground Water Management Area, Washington.
Groundwater Status and Trends 31

Group 1

EXPLANATION

Group 2

Group 3

Group 4 - shallow

Group 4 - deep

Winter median water-level elevation, in feet (NAVD88)

1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010

750

800

850

900

950

1,000

1,050

1,100

1,150

1,200

1,250

1,300

1,350

Year

Figure 16. Groups of wells corresponding to the western flow path near the Washington State Department of Ecology 

administrative areas in parts of the Palouse Slope/eastern Yakima Fold Belt and the Columbia Basin Ground Water Management 

Area, Washington. Hydrographs depict the winter median water level for individual wells within each well group and the trend 

line representing the overall water-level trend of all wells in the group. Well locations and flow paths are shown in figure 15.

The wells in groups 8 (brown squares), 9 (orange 

squares), 10 (dark green squares), 11 (black squares and 

white squares, shallow and deep), and 12 (gray squares) 

associated with the eastern flow path (shown as squares in 

figure 15) show a hydraulic gradient sloping from northeast 

to southwest, down the Palouse Slope (fig. 17). Water-level 

declines for most of these wells begin after 1970. Group 11 

shallow wells (black squares) are less affected, indicating 

hydraulic separation between most of these wells and the 

wells downslope (to the west). The wells showing significant 

declines exhibit various decline patterns, although total 

drawdown is similar in most wells.

Between the eastern and western flow paths and well 

groups, there is a middle set of well groups, groups 5 (dark 

blue triangles), 6 (orange triangles), and 7 (pink triangles), 

(shown as triangles in figure 15), with transitional behavior of 

groundwater levels. Groundwater levels in most of these wells 

begin to increase about 1950, as observed with the western 

well groups. Some of these wells also show groundwater-level 

declines starting in the 1970s as observed with the eastern well 

groups (fig. 18). Since 1970, a groundwater mound has formed 

between Moses Lake, Potholes Reservoir, and the eastern 

flow path wells. Groundwater-levels in groups 6 (orange 

triangles) and 7 (pink triangles) generally are higher than in 

groups to the east and west, which consist of group 4 shallow 

(black circles), group 5 (dark blue triangles), group 8 (brown 

squares), and group 12 (gray squares) (figs. 15–18).

The southern well groups (groups 13 [red diamonds], 

14 [black diamonds and white diamonds], and 15 [yellow 

diamonds]) associated with the southern flow path (shown as 

diamonds in figure 15) show a hydraulic gradient from north 

to south toward the Snake River (fig. 19), which locally has an 

elevation of about 350 ft. The hydraulic behavior is complex, 

although the narrow spacing between adjacent well groups 

and dissimilar temporal changes indicates there may be some 

hydraulic barriers between these groups.
32 Groundwater Status and Trends for the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

Figure 17. Groups of wells corresponding to the eastern flow path near the Washington State Department of Ecology administrative 

areas in parts of the Palouse Slope/eastern Yakima Fold Belt and the Columbia Basin Ground Water Management Area, Washington. 

Hydrographs depict the winter median water levels for individual wells within each well group and the trend line representing the 

overall water-level trend of all wells in the group. Well locations and flow paths are shown in figure 15.

EXPLANATION

Group 8

Group 9

Group 10

Group 11 - shallow

Group 11 - deep

Group 12

1,100

1,200

1,300

1,400

1,500

1,600

1,700

1,800

1,900

600

700

800

900

1,000

Winter median water-level, in feet (NAVD88)

1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010

Year
Groundwater Status and Trends 33

Group 5

EXPLANATION

Group 6

Group 7

Winter median water-level elevation, in feet (NAVD88)

1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010

750

700

650

800

850

900

950

1,000

1,050

1,100

1,150

1,200

1,250

1,300

Year

Figure 18. Middle groups of wells near the Washington State Department of Ecology administrative areas in parts of the 

Palouse Slope/eastern Yakima Fold Belt and the Columbia Basin Ground Water Management Area, Washington. Hydrographs 

depict the winter median water levels for individual wells within each well group and the trend line representing the overall 

water-level trend of all wells in the group. Well locations are shown in figure 15.

The hydraulic behavior of group 12 (gray squares) is at 

the nexus of the eastern, middle, and southern well groups 

and apparently is influenced by each of the adjacent groups 

(figs. 17 and 19), although its component wells are not 

easily separated into the adjacent groups based on location. 

Hydraulic head values for group 12 wells commonly are 

intermediate between the other groups, with some wells 

exhibiting declines similar to group 8 (brown squares) wells 

from the eastern well groups (fig. 17), and other group 12 

water levels rising similarly to water levels in group 7 wells 

(pink triangles) from the middle well groups (fig. 18). No 

clear pattern of depth or location allows group 12 wells to be 

subdivided into the adjacent groups.
34 Groundwater Status and Trends for the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

tac12-0765_fig19

EXPLANATION

Group 12

Group 13

Group 14 - 1

Group 14 - 2

Group 15

Winter median water-level, in feet (NAVD88)

1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010

300

350

250

400

450

500

550

600

650

700

750

800

850

900

950

1,000

1,050

1,100

1,150

1,200

Year

Figure 19. Groups of wells corresponding to the southern flow path near the Washington State Department of Ecology 

administrative areas in parts of the Palouse Slope/eastern Yakima Fold Belt and the Columbia Basin Ground Water Management 

Area, Washington. An additional hydrograph for a proximal group of wells (group 12) is shown to illustrate the relation between 

well groups. Hydrographs depict the winter median water levels for individual wells within each well group and the trend line 

representing the overall water-level trend of all wells in the group. Well locations and flow paths are shown in figure 15.
Groundwater Status and Trends 35

Water levels on the Palouse Slope/eYFB were plotted 

against stratigraphic position relative to the simulated top 

of the Grande Ronde Basalt (fig. 20A–I). Analogous to the 

head distribution in the Umatilla area, heads are similar over 

large vertical intervals of basalt in the Palouse Slope/eYFB 

area indicating possible commingling of aquifers. Unlike the 

Umatilla area, even within a well group and for closely spaced 

wells, those wells constructed at the same elevation may have 

hydraulic heads hundreds of feet different from each other. 

This noisy data and the lack of apparent flow barriers between 

well groups, results in overlap of the hydrographs between 

groups. Even within well groups, division of hydrographs 

by approximate stratigraphic layer does not yield smoothlyvarying groundwater-level data that generates well-behaved 

potentiometric surface maps. This may be due to the complex 

connectivity between wells and commingled aquifers resulting 

in a range of composite head elevations (fig. 6).

Generalized potentiometric surfaces were developed 

for pre-development conditions (prior to 1951; fig. 21) 

and post-2000 conditions (fig. 22). Relatively high-error 

generalized surfaces were created using all CRBG wells 

without distinction between CRBG hydrogeologic unit 

because hydrographs within well groups show consistent 

general trends (figs. 16–19); however, few measurements 

within each group support the division of wells into 

hydraulically distinct zones vertically (fig. 20). Figures 21

and 22 were constructed by using median water levels for 

the defined periods for each CRBG well, and smoothing 

the data with LOESS local linear trend models (Cleveland 

and others, 1992). At any location, potential error of the 

simulated hydraulic head is high; the measured value could 

be hundreds of feet higher or lower than these generalized 

surfaces. Because trend models were used, fit is biased toward 

shallow data because few deep wells with significantly 

different water-levels were measured (fig. 20). However, when 

evaluating model fit, there is little spatial bias in the residuals 

between the measurement points and predicted surface; 

therefore, the resulting surfaces are good representations of 

the patterns in the hydrograph groups. Because of the large 

amount of smoothing of these potentiometric surfaces, smaller 

scale features associated with streams and structural barriers 

to flow, for example, the Frenchman Hills (fig. 1) are not well 

represented.

Groundwater flow during the pre-development 

period was from the uplands in the northeastern part of the 

GWMA toward the Columbia and Snake Rivers to the west 

and south, respectively (fig. 21). Although the locations 

where groundwater has the potential for local drainage to 

surface water features are shaded (fig. 21), the generalized 

potentiometric surface was not corrected for interaction with 

surface drainages because flow to local streams depends on 

the hydraulic head and local connection between the aquifers 

and the streams. Following the onset of large surface-water 

irrigation projects near Moses Lake (fig. 22) in the 1950s, 

water levels in the upper CRBG aquifers increased and 

formed a groundwater mound, resulting in a reversal of the 

hydraulic gradient toward the area of declines (compare 

figs. 14, 21, and 22). For wells examined, groundwater levels 

under surface-water irrigation areas typically rise about 50 ft, 

with larger rises occurring locally (for example, in the fault 

bounded valleys to the south of the Frenchman Hills and 

the Saddle Mountains). To the east, in the south central area 

of the GWMA where groundwater pumping is the primary 

source of irrigation, a trough-shaped depression sloping 

toward the south has formed, and groundwater flows toward it 

from east and west (fig. 22). The axis of the trough generally 

is coincident with the easternmost area of dense irrigation 

pumping of groundwater shown in the center of figure 22, 

much of which is occupied by well group 8 (brown squares) in 

figure 15.

Because most well groups do not exhibit pronounced 

persistent vertical gradients, the potential for commingling 

to contribute to hydraulic head declines in this area is not 

clear from the data. The limited data show that a significant 

downward hydraulic gradient is present in the eastern and 

western well groups (group 4 shallow [black circles] and 

deep [white circles] wells in figures 16 and 20B–C, group 

11 shallow [black squares] and deep [white squares] wells in 

figures 17 and 20E). Large vertical head differences apparently 

are abrupt in groups 4 and 11, while other well groups (for 

example group 10 [dark green squares] in figures 20E–F) 

show vertical gradients that are more continuous and exhibit 

considerable variability. The downward vertical gradients are 

consistent with the geologic model (Burns and others, 2011) 

that shows the deeper aquifers are exposed through erosion 

along the Snake and Columbia Rivers at elevations consistent 

with the lower hydraulic heads in these deeper units. In 

addition, hydraulic heads in the southern well groups are lower 

than in the trough-like cone of depression immediately to the 

north (fig. 22), indicating that natural or commingled well 

leakage might be allowing flow to pass through the possible 

horizontal flow barrier between group 12 (gray squares) and 

group 13 (red diamonds) (figs. 15 and 19). The complexity in 

flow near group 13 is further illustrated by the hydraulic head 

patterns of groups 13 and 14 (figs. 19 and 20G–I). Despite 

the fact that group 14 may be divided into two groups with 

apparently distinct behavior, groups 14-1 (white diamonds) 

and 14-2 (black diamonds) cannot be separated laterally or 

into shallow and deep groups, although each of these groups 

show similar post-1980 characteristics with various group 13 

wells (figs. 19 and 20H–I).
36 Groundwater Status and Trends for the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

Figure 20. Distance of the well bottom from the simulated top of Grande Ronde Basalt 

compared with water levels measured in wells near the Washington State Department 

of Ecology administrative areas in parts of the Palouse Slope/eastern Yakima Fold 

Belt and the Columbia Basin Ground Water Management Area, Washington. Palouse 

western well group for (A) 1968; (B) 1984; (C) 2006; Palouse eastern well group for (D) 

1968; (E) 1984; (F) 2006; and Palouse southern well group for (G) 1968; (H) 1984; and (I) 

2006. Locations of groups are shown in figure 15. 

tac12-0765_fig20a Winter median water-level elevation, in feet (NAVD88)

A. Palouse Western well groups for 1968

Stratigraphic position relative to top of the Grande Ronde Basalt, in feet

Well bottom in the Grande Ronde Basalt Well bottom above top of Grande Ronde Basalt Estimated top of Grande Ronde Basalt

EXPLANATION

Group 1

Well

Group 2

Group 3

Group 4 - shallow

700

800

900

1,000

1,100

1,200

1,300

1,400

-800 -600 -400 -200 0 200 400 600 800
Groundwater Status and Trends 37

tac12-0765_fig20b Winter median water-level elevation, in feet (NAVD88)

B. Palouse Western well groups for 1984

Stratigraphic position relative to top of the Grande Ronde Basalt, in feet

Well bottom in the Grande Ronde Basalt Well bottom above top of Grande Ronde Basalt Estimated top of Grande Ronde Basalt

Group 4 - Deep

Group 4 - Shallow

EXPLANATION

Group 1

Well

Group 2

Group 3

700

800

900

1,000

1,100

1,200

1,300

1,400

-800 -600 -400 -200 0 200 400 600 800

Figure 20.—Continued

Winter median water-level elevation, in feet (NAVD88)

C. Palouse Western well groups for 2006

Stratigraphic position relative to top of the Grande Ronde Basalt, in feet

Well bottom in the Grande Ronde Basalt Well bottom above top of Grande Ronde Basalt Estimated top of Grande Ronde Basalt

Group 4 - Deep

EXPLANATION

Well

Group 4 - Shallow

700

800

900

1,000

1,100

1,200

1,300

1,400

-800 -600 -400 -200 0 200 400 600 800
38 Groundwater Status and Trends for the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

tac12-0765_fig20d Winter median water-level elevation, in feet (NAVD88)

D. Palouse Eastern well groups for 1968

Stratigraphic position relative to top of the Grande Ronde Basalt, in feet

Well bottom in

the Grande

Ronde Basalt 

Well bottom above

top of Grande

Ronde Basalt

Estimated top of

Grande Ronde Basalt

700

800

900

1,000

1,100

1,200

1,300

1,400

1,500

1,600

1,700

1,800

1,900

-2,200 -2,000 -1,800 -1,600 -1,400 -1,200 -1,000 -800 -600 -400 -200 0 200 400 600

EXPLANATION

Well

Group 9

Group 10

Group 8

Group 11 - Shallow

Group 12

Figure 20.—Continued

Winter median water-level elevation, in feet (NAVD88)

E. Palouse Eastern well groups for 1984

Stratigraphic position relative to top of the Grande Ronde Basalt, in feet

Well bottom in

the Grande Ronde Basalt 

Well bottom above

top of Grande

Estimated top of

Ronde Basalt

Grande Ronde Basalt

700

800

900

1,000

1,100

1,200

1,300

1,400

1,500

1,600

1,700

1,800

1,900

-2,200 -2,000 -1,800 -1,600 -1,400 -1,200 -1,000 -800 -600 -400 -200 0 200 400 600

EXPLANATION

Well

Group 9

Group 10

Group 8

Group 11 - Deep

Group 11 - Shallow

Group 12
Groundwater Status and Trends 39

Figure 20.—Continued

tac12-0765_fig20f Winter median water-level elevation, in feet (NAVD88)

F. Palouse Eastern well groups for 2006

Stratigraphic position relative to top of the Grande Ronde Basalt, in feet

Well bottom in the 

Grande Ronde Basalt 

Well bottom above

top of Grande

Ronde Basalt

Estimated top of

Grande Ronde Basalt

700

800

900

1,000

1,100

1,200

1,300

1,400

1,500

1,600

1,700

1,800

1,900

-2,200 -2,000 -1,800 -1,600 -1,400 -1,200 -1,000 -800 -600 -400 -200 0 200 400 600

EXPLANATION

Well

Group 9

Group 10

Group 8

Group 11 - Shallow

Group 12

Winter median water-level elevation, in feet (NAVD88)

G. Palouse Southern well groups for 1968

Stratigraphic position relative to top of the Grande Ronde Basalt, in feet

Well bottom in the 

Grande Ronde Basalt 

Well bottom above top of 

Grande Ronde Basalt

Estimated top of Grande Ronde Basalt

300

400

500

600

700

800

900

1,000

1,100

1,200

-1,400 -1,200 -1,000 -800 -600 -400 -200 0 200 400 600

EXPLANATION

Well

Group 14 - 2

Group 15

Group 13
40 Groundwater Status and Trends for the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

tac12-0765_fig20h Winter median water-level elevation, in feet (NAVD88)

H. Palouse Southern well groups for 1984

Stratigraphic position relative to top of the Grande Ronde Basalt, in feet

Well bottom in the 

Grande Ronde Basalt 

Well bottom above top of 

Grande Ronde Basalt

Estimated top of Grande Ronde Basalt

300

400

500

600

700

800

900

1,000

1,100

1,200

-1,400 -1,200 -1,000 -800 -600 -400 -200 0 200 400 600

EXPLANATION

Well

Group 14 - 1

Group 13

Group 14 - 2

Group 15

Winter median water-level elevation, in feet (NAVD88)

I. Palouse Southern well groups for 2006

Stratigraphic position relative to top of the Grande Ronde Basalt, in feet

Well bottom in the 

Grande Ronde Basalt 

Well bottom above top of 

Grande Ronde Basalt

Estimated top of Grande Ronde Basalt

300

400

500

600

700

800

900

1,000

1,100

1,200

-1,400 -1,200 -1,000 -800 -600 -400 -200 0 200 400 600

EXPLANATION

Well

Group 14 - 1

Group 13

Group 14 - 2

Group 15

Figure 20.—Continued
Groundwater Status and Trends 41

tac12-0765_fig21

Pasco

FERRY STEVENS

OKANOGAN

CHELAN

DOUGLAS

SPOKANE GARFIELD

LINCOLN

GRANT

KITTITAS

WHITMAN

ADAMS

YAKIMA

FRANKLIN

BENTON

COLUMBIA

FRENCHMAN HILLS

SADDLE

MOUNTAINS

Moses

Lake

WALLA

WALLA

COLUMBIA

Banks Lake

RIVER

East Low Canal

McCarteney

Creek

West Canal

Lower Crab Creek

COLUMBIA

Palouse River

Snake River

Yakima River

Potholes

Reservoir

Crab Creek 1,700

2,400

2,400

2,400

400

500

400

500

500

2,300

600

2,200

1,900

2,100

700

1,800

800

2,000

1,700

900

1,600

1,000

1,500

1,100

1,200

1,400

1,300

EXPLANATION

EXPLANATION

Geologic model extent of Columbia River

Basalt Group

Columbia Basin Ground Water Management Area

Index contour

Land surface to 50 feet below land surface

Intermediate contour

Above land surface

Groundwater elevation contours of 

generalized potentiometric surface 

for conditions occurring before 1950, 

in feet, interval 100 feet (North 

American Vertical Datum of 1988) 

Potentiometric surface

900

1,000

120° 119°30' 119° 118°30' 118°

48°

47°

30'

47°

46°

30'

0 5 10 15 20 25 Miles

0 5 10 15 20 25 Kilometers

Base map modified from USGS and other digital data, various 

scales. Coordinate system: State Plane-Washington South FIPS 

4602, Projection: Lambert Conformal Conic; North American 

Datum of 1983.

Palouse Slope

Clearwater Embayment

Blue Mountains

Yakima Fold Belt

B

C

P

Y

Structural regions

OR ID

WA

B

C

P

Y

Figure 21. Generalized potentiometric surface for measured Columbia River Basalt Group aquifer conditions during the predevelopment period in parts of the Palouse Slope/eastern Yakima Fold Belt and the Columbia Basin Ground Water Management 

Area, Washington. Areas where potentiometric surface is above or near land surface indicate greater potential for groundwater 

discharge to coulees prior to substantial groundwater declines.
42 Groundwater Status and Trends for the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

tac12-0765_fig22

Pasco

FERRY STEVENS

OKANOGAN

CHELAN

DOUGLAS

SPOKANE GARFIELD

LINCOLN

GRANT

KITTITAS

WHITMAN

ADAMS

YAKIMA

FRANKLIN

BENTON

COLUMBIA

FRENCHMAN HILLS

SADDLE MOUNTAINS

Moses

Lake

WALLA

WALLA

COLUMBIA

Banks Lake

RIVER

East Low Canal

McCarteney

Creek

West Canal

Lower Crab Creek

COLUMBIA

Palouse River

Snake River

Yakima River

Potholes

Reservoir

Crab Creek

2,200

2,300

2,300

500

400

600

700

1,200

800

900

2,000

1,900

2,100

1,800

1,700

1,600

1,500

1,400

1,300

1,100

1,100

1,000

1,000

EXPLANATION

EXPLANATION

Geologic model extent of Columbia River

Basalt Group

Columbia Basin Ground Water Management Area

Index contour

Intermediate contour

Groundwater elevation contours of 

generalized potentiometric surface 

for conditions occurring during the 

period 2000–2009, in feet, interval 100 

feet (North American Vertical Datum 

of 1988) 

900

Estimated annual recharge in irrigated areas 

for 2006, in inches per year (Kahle and 

others, 2011)

72.7559

60

40

20

0

Irrigation source greater than 50 

percent groundwater (Kahle

and others, 2011)

1,000

120° 119°30' 119° 118°30' 118°

48°

47°

30'

47°

46°

30'

0 5 10 15 20 25 Miles

0 5 10 15 20 25 Kilometers

Base map modified from USGS and other digital data, various 

scales. Coordinate system: State Plane-Washington South FIPS 

4602, Projection: Lambert Conformal Conic; North American 

Datum of 1983.

Palouse Slope

Clearwater Embayment

Blue Mountains

Yakima Fold Belt

B

C

P

Y

Structural regions

OR ID

WA

B

C

P

Y

Figure 22. Generalized potentiometric surface for measured Columbia River Basalt Group aquifer conditions during 2000–2009 in 

parts of the Palouse Slope/eastern Yakima Fold Belt and the Columbia Basin Ground Water Management Area, Washington.
Summary and Conclusions 43

The central Palouse Slope shows a general lack of 

geologic structure or other evidence for strong horizontal 

flow barriers to the east, but the east-west trending faults 

associated with the ridges between the western valleys (the 

Frenchman Hills and the Saddle Mountains) in the eastern 

Yakima Fold Belt are likely barriers to flow. Luzier and 

Burt (1974) identified a “groundwater dam” on the Palouse 

Slope extending from the northwest to the southeast through 

the junction of Adams, Grant, and Lincoln Counties that is 

associated with an apparent change in hydraulic gradient 

(this feature is only partially represented because of the 

general nature of the smoothed potentiometric maps shown in 

figures 21 and 22). This steeper hydraulic gradient also was 

identified in this analysis at the boundary between group 9 

(orange squares) and group 8 (brown squares) of the eastern 

well groups (figs. 15 and 17) and was present at least as 

early as 1940 prior to substantial water deliveries from the 

Columbia Basin Project, which began in the 1950s. One of 

the few locations with persistent bias in the residuals for 

the pre-development potentiometric surface was in the area 

where group 9 hydraulic heads were simulated as too high, 

indicating a local steepening of the hydraulic gradient to the 

east, and a flattening to the west. The flattening of water levels 

is evident in group 8 in 1968 (fig. 20D). As an alternative to 

the groundwater dam proposed by Luzier and Burt (1974), 

this steepened hydraulic gradient is hypothesized to result 

from a complex discharge boundary where the upper aquifers 

are intersected at land surface by incised canyons, which 

allows water to flow into the sediment-filled coulees. The 

potential for this outflow is indicated by the pre-development 

generalized potentiometric surface above or near the land 

surface in the coulees (fig. 21). Incision of the coulees into the 

Wanapum Basalt geologic unit (fig. 2) is sufficient that several 

lava flows and potentially several aquifers in the Wanapum 

Basalt may be intersected (Burns and others, 2011), possibly 

forming pathways for groundwater flow that would result in 

complex head patterns (not shown in figure 21) and apparent 

alterations in the regional hydraulic gradient. Most late-time 

(2000–2010) hydraulic heads are now below the elevation 

of the coulees, indicating that groundwater discharge to the 

coulees has declined over time. Two additional pieces of 

evidence support the hypothesis that groundwater historically 

discharged in this location. First, because shallow aquifers 

tend to drain into the coulees possibly dewatering parts of 

these aquifers, fewer wells are completed in the aquifers 

intersecting the coulees along a north-south swath in this area. 

Second, the apparently random highly variable heads about 

300–400 ft above the Grande Ronde Basalt geologic model 

unit (approximate elevation of Wanapum Basalt intersected 

by the coulees) and flattening of the water levels below this 

stratigraphic elevation are characteristic of a highly variable 

flow field where groundwater is flowing to local drainage 

features (compare group 8 [brown squares] wells in figure 20D

to hypothetical well groups in figure A2B and the associated 

discussion). The lowest land-surface elevation in the coulees 

where the potentiometric surface is above land surface is the 

controlling aquifer drainage elevation. This elevation is about 

1,180 ft, which is the approximate inflection point of group 8 

wells in 1968 (fig. 20D).

Summary and Conclusions

The Columbia Plateau Regional Aquifer System 

(CPRAS) covers an area of about 44,000 square miles in a 

structural and topographic basin within the drainage of the 

Columbia River in Washington, Oregon, and Idaho. The 

primary aquifers occur in basalts of the Columbia River 

Basalt Group (CRBG) and in places, overlying sediment. 

The system consists of a series of productive basalt aquifers 

within permeable interflow zones separated by less permeable 

flow interiors, overlaid locally by aquifers within sedimentary 

deposits. Groundwater levels have declined throughout much 

of the CPRAS since the 1970s.

Information was compiled from about 60,000 wells and 

450,000 water-level measurements from wells in the CPRAS. 

Data are from an inventory of published and unpublished 

well data from many agencies. A subset of these data for the 

CRBG aquifer wells were used to develop a simple linear 

groundwater-level trend map for 1968–2009, which illustrates 

a persistent pattern of widespread groundwater-level declines. 

Overall declines from the analysis of data from 761 wells in 

the CRBG aquifers were measured in 72 percent of the wells. 

The mean of the trends was a decline of 1.9 feet per year 

(ft/yr). Rates of declines greater than 1.0 ft/yr were measured 

in 50 percent of wells, declines greater than 2.0 ft/ yr 

in 38 percent of wells, declines greater than 4.0 ft/yr in 

29 percent of wells, declines greater than 6.0 ft/yr in 9 percent 

of wells, and rates of decline greater than 8.0 ft/yr in 4 percent 

of wells.

The groundwater-level data also were used to identify 

groups of wells with similar hydraulic heads and temporal 

trends within limited subregions. Comparisons between 

adjacent well groups were used to define sets of well groups 

that delineate areas of overall similar groundwater-flow 

conditions. Discontinuities in hydraulic head between the 

sets of well groups were used to help infer the presence of 

barriers to groundwater flow. These barriers can include 

geologic features that influence the hydraulic properties of 

the aquifer, such as changes in lithology or the occurrence of 

folds and faults. In areas without flow barriers, dissimilarities 

in response of well groups over time result from the formation 

of irrigation-derived groundwater mounds or pumping 

induced regions of decline. The areas of focus for this analysis 

included the Umatilla area, Oregon, and the Palouse Slope/

eastern Yakima Fold Belt (eYFB) in the Columbia Basin 

Ground Water Management Area (GWMA) consisting of 

Adams, Franklin, Grant, and Lincoln Counties, Washington.
44 Groundwater Status and Trends for the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

In the Umatilla area, 286 wells were divided into 

groups with similar water levels and trends, and large 

hydraulic gradients between nearby groups defined barriers to 

groundwater flow. The barriers divide the groundwater-flow 

system in the Umatilla area into several leaky compartments 

that occur in vertical and horizontal directions. However, the 

degree of leakiness is highly variable. Significant findings for 

the Umatilla area include:

• Most CRBG aquifers are declining, except for a few 

shallow aquifers. Cumulative declines range from 

100 to 300 feet since 1970.

• High horizontal hydraulic gradients and differences 

in temporal trends in water levels between adjacent 

well groups define horizontal flow barriers that 

generally correspond to mapped geologic structural 

features. Horizontal hydraulic gradients increase 

from north to south, which generally corresponds 

to an increase in structural complexity. This implies 

that recharge from the uplands into the heavily 

developed areas may be impeded.

• Significant vertical hydraulic gradients have been 

documented in a relatively small part of the Umatilla 

area, where the shallow aquifer system is distinct 

from the deep system. Since the 1970s, downward 

vertical gradients in these areas have been increasing 

as water-level declines have occurred in deeper 

wells due to pumping and commingling of these 

aquifers. Because the geology generally is conducive 

to creating vertical hydraulic gradients, the absence 

of vertical gradients over much of the area may be 

a consequence of flow through commingling wells 

resulting in the equilibration of the heads in the 

aquifers.

In the Palouse Slope of the central GWMA, the largest 

declining groundwater-level trends (1968–2009) follow a 

general north-south line through the middle of the GWMA. 

An analysis of 1,195 wells along major flow paths and through 

the area of persistent groundwater-level declines indicates 

that barriers to flow are not as evident in this area as near 

Umatilla, but well groups were still identifiable based on 

similar hydraulic heads and response to hydraulic stresses. 

This is consistent with the geologic interpretation of the 

Palouse Slope as being a gently folded structure created during 

subsidence when CRBG lavas were deposited in voluminous 

sheet flows. In this area, hydrographs of well groups were 

viewed along general flow paths that have developed as the 

result of irrigation stresses on the aquifer system. 

Significant findings for the Palouse Slope/eYFB include: 

• Groundwater levels in CRBG aquifers have risen 

since the 1950s in areas heavily irrigated with 

surface water and have declined since the 1970s in 

areas commonly irrigated with groundwater. For 

wells examined, typical rises in water level under 

surface-water irrigation areas are about 50 feet, 

with larger rises occurring locally (for example, 

in the fault bounded valleys to the south of the 

Frenchman Hills and the Saddle Mountains). 

Cumulative declines of 200 feet or greater are 

common in areas where pumping groundwater is 

the dominant source of irrigation water.

• Horizontal flow barriers are less apparent in this 

area than in the Umatilla area.

• Data indicate that significant vertical hydraulic 

gradients still exist, although much of the aquifer 

thickness is affected by commingling of wells.

• Prior to development, groundwater flow was 

from the uplands toward the Columbia and Snake 

Rivers. Late-time (2000–2010) flow patterns 

in the area are substantially altered by largescale irrigation recharge and pumping patterns, 

and in some cases are reversed relative to predevelopment conditions.

• The effect of commingling of water from 

multiple aquifers and its relative contribution 

to groundwater-level declines is not clear from 

these data; however, groundwater-flow modeling 

currently under development as part of the overall 

study may aid in quantifying these effects.

• The presence of a groundwater discharge 

boundary, where the upper aquifers are 

intersected at land surface by incised canyons 

and groundwater flows into the sediment-filled 

coulees, is an alternative to the hypothesis of a 

groundwater dam to explain local steepening of 

the hydraulic gradient along the Palouse Slope.
Selected References 45

The use of stratigraphic coordinates to identify aquifers 

with similar water levels and trends worked well in the 

analysis of the groundwater conditions for the Umatilla area 

and Palouse Slope/eYFB. This suggests a way to advance the 

understanding of the geometry of local and regional aquifers 

by using the elevations of well bottoms for groups of wells 

with similar water levels and trends. This technique may 

be used to help identify the influence (or lack of influence) 

of structural features on groundwater flow or even to infer 

where previously unmapped structural features may exist. 

Trend surfaces representing aquifers and lava flow top 

elevations may be mapped in many areas across the Columbia 

Plateau, refining the current understanding of major geologic 

formations into smaller hydraulically important units.

Acknowledgments

The authors thank the many people who contributed 

their time and knowledge to help complete this study. The 

following organizations are noted for their special assistance 

with the collection of data: Columbia Basin Ground Water 

Management Area, Confederated Tribes of the Umatilla 

Indian Reservation, GSI Water Solutions, Inc., Oregon 

Water Resources Department, U.S. Department of Energy, 

Walla Walla Basin Watershed Council, and Washington State 

Department of Ecology. The following USGS employees are 

gratefully acknowledged for their contributions to the study: 

Danial Polette and Leonard Orzol assisted with analysis of the 

data; Fred Tillman developed and modified Excel® spreadsheet 

tools that greatly simplified the processing of the data; Sue 

Kahle and Marshall Gannett provided guidance and insights; 

and the many other USGS employees whose everyday efforts 

go to support this and many other studies. 

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4 sheets. (Also available at http://pubs.er.usgs.gov/

usgspubs/wri/wri864046.)

Whiteman, K.J., Vaccaro, J.J., Gonthier, J.B., and Bauer, 

H.H., 1994, Hydrogeologic framework and geochemistry 

of the Columbia Plateau regional aquifer system: U.S. 

Geological Survey Professional Paper 1413-B, 73 p. 

(Also available at http://pubs.er.usgs.gov/usgspubs/pp/

pp1413B.)
48 Groundwater Status and Trends for the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

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Appendix A 49

Appendix A. Interpretation Guide for Graphs of Stratigraphic Position

Compared with Hydraulic Head for Wells in the Columbia River Basalt Group, 

Washington, Oregon, and Idaho

After wells have been grouped into spatial clusters 

based on similar values of head and similar trends, the 

variability within each group may be examined to evaluate 

vertical hydraulic gradients. This is accomplished by plotting 

all available head data collected during a short period 

(relatively unaffected by temporal trends) as a function of the 

stratigraphic position of the aquifer being measured. If the 

vertical head gradients are large relative to the slope of the 

potentiometric surface within each aquifer, then the vertical 

gradient is apparent.

The assignment of hydraulic head values to a given 

stratigraphic position is fraught with complications that may 

affect the accuracy of any single data point. For Columbia 

River Basalt Group (CRBG) wells, it is assumed that the well 

terminates in a productive aquifer, and that the measured head 

in that well is representative of an aquifer at that stratigraphic 

position. Stratigraphic position is computed as the difference 

between the elevation of the bottom of the well and an 

estimate of the elevation of a geologic horizon that originally 

was flat. Use of stratigraphic position removes the influence of 

dipping, folding, and faulting when correlating strata that can 

act as laterally continuous aquifers. Conversely, in areas where 

CRBG units are discontinuous or exhibit significant thinning 

or thickening, stratigraphic position may have a spatial trend. 

As long as the group of wells being examined does not 

cover too large an area, spatial bias of stratigraphic position 

generally will be small. In addition to this complication, well 

commingling can result in hydraulic head measurements that 

are not representative of the aquifer at the well bottom (for 

example, well 1 in fig. 6).

Despite the potential complications, combinations of 

persistent patterns in stratigraphic position and hydraulic head 

may be interpreted hydrologically (fig. A1). The clustering 

of wells at distinct stratigraphic positions is evidence that 

distinct aquifers are locally important, and when hydraulic 

heads are distinctively different between aquifers, downward 

(fig. A1A) and upward (fig. A1B) vertical gradients exist. 

Similarly, there may be multiple distinct aquifers with no 

vertical gradient (fig. A1C). For the CRBG, significant vertical 

distance between two distinct aquifers implies the presence 

of a potential confining unit and the likelihood that a vertical 

hydraulic gradient should exist. However, aquifers may be 

hydraulically connected naturally through complex CRBG 

geometry, or they may be connected through commingling 

wells. If historical evidence supports a significant 

vertical gradient (fig. A1A–B), but recent measurements 

are consistently similar for each aquifer (fig. A1C), then 

commingling wells are the likely cause.

For many areas in the Columbia Plateau Regional Aquifer 

System (CPRAS), the repeating sequence of CRBG interflows 

and flow interiors results in a continuum of stratigraphic 

position (fig. A1D). In this case, there is either a single aquifer 

or many thin aquifers, and there can be downward, upward, or 

no significant vertical gradient present. For most areas within 

the CPRAS, vertical gradients are expected, so no vertical 

gradient over several hundred feet of stratigraphic position 

(fig. A1D) indicates that aquifers likely are hydraulically 

connected either naturally or through commingling wells.
50 Groundwater Status and Trends for the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

Figure A1. Expected patterns of hydraulic head as a function of stratigraphic position. (A) Two distinct aquifers with a significant 

downward hydraulic gradient; (B) two distinct aquifers with a significant upward gradient; (C) two distinct aquifers with no 

significant vertical gradient (D) a single thick aquifer or multiple thin aquifers with no significant vertical gradient; (E) a single aquifer 

with (apparently) two distinctly different hydraulic heads; and (F) a single aquifer with an apparent continuum of heads.

tac12-0765_fig_AppA_01 Hydraulic head: Increasing 

→

A

Stratigraphic position: Increasing →

Hydraulic head: Increasing 

→

D

Stratigraphic position: Increasing → Stratigraphic position: Increasing → Hydraulic head: Increasing 

→

B

Stratigraphic position: Increasing →

Hydraulic head: Increasing 

→

C

Stratigraphic position: Increasing →

Hydraulic head: Increasing 

→

E

Stratigraphic position: Increasing →

Hydraulic head: Increasing 

→

F
Selected References 51

Hydraulic head: Increasing 

→

A

Stratigraphic position: Increasing → Stratigraphic position: Increasing → Hydraulic head: Increasing 

→

B

Figure A2. Combinations of expected patterns of hydraulic head as a function of 

stratigraphic position. (A) No vertical gradients exist for a thick part of the upper 

aquifer system (consisting of multiple aquifers), although a significant downward 

gradient exists in the lower aquifer system; (B) an apparent continuum of heads exists 

in the upper aquifers, and no vertical gradients exist in the lower aquifers.

During the process of creating spatial clusters of wells, 

two distinctly different heads at the same stratigraphic position 

(fig. A1E) potentially signals the presence of a horizontal 

barrier to groundwater flow. If all higher head wells are in a 

separate location from the lower head wells, then some barrier 

to flow may exist, and the wells would be divided into separate 

groups. If the high and low head wells appear to be randomly 

mixed spatially, then well construction, pumping from 

adjacent wells, or geologic heterogeneity are the likely causes 

of the pattern (fig. 6). If a continuum of heads exist within a 

single aquifer (fig. A1F), then additional explanations may 

include a steep regional gradient of the potentiometric surface, 

or local perturbations to the potentiometric surface that may 

occur in the presence of groundwater discharge boundaries 

(such as incised canyons with springs and seeps). These 

local and regional effects may be separated by removing the 

regional trend from the potentiometric surface and examining 

the range of the residual heads.

Combinations of expected patterns of hydraulic head as 

a function of stratigraphic position (figs. A1A–F). are frequent 

in the real data for CRBG wells as illustrated in figures 12A–F

and 20A–I. Over time, new deep wells have increasingly been 

required to case and seal to the aquifer, hydraulically isolating 

these wells from the upper commingled aquifers. Whereas the 

upper aquifers frequently have fairly uniform hydraulic heads, 

the deeper aquifers have a distinctly different head (compare 

fig. A2A to group 4 shallow and deep wells [black circles and 

white circles] shown in figure 20B–C and to group 11 shallow 

and deep wells [black squares and white squares] in fig. 20E). 

A continuum of heads associated with local groundwater 

drainage of upper aquifers may be combined with a uniform 

vertical gradient for deep aquifers (fig. A2B) to provide the 

observed pattern of group 8 wells (brown squares) during predevelopment prior to lowering of the potentiometric surface 

below the land surface (fig. 20D).
52 Groundwater Status and Trends for the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho

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Publishing support provided by the U.S. Geological Survey 

Publishing Network, Tacoma Publishing Service Center

For additional information contact:

Director, Oregon Water Science Center

U.S. Geological Survey

2130 SW 5th Avenue

Portland, Oregon 97201

http://or.water.usgs.gov
Burns and others— Groundwater Status and Trends for the Columbia Plateau Regional Aquifer System—Scientific Investigations Report 2012–5261