Figure 1. Columbia Plateau Regional Aquifer System study area, Washington, Oregon, and 

Idaho.

Groundwater Resources of the Columbia Plateau Regional Aquifer System

watac15-1024_CPLAT-fig01

Yakim a River

Snake

River

Col umbi a River

Columbia

River

Banks

Lake Coeur

D'Alene

Lake

Palouse RiverPalouse River

Moses

Lake

Umatilla

River

Ellensburg

Umatilla Walla Walla

OREGON

WASHINGTON

IDAHO

Yakima

Heppner

MiltonFreewater

Pasco

Lewiston

Moscow

Spokane

The

Dalles

La Grande

122° 120° 118° 116°

46°

48°

47°

45°

0 20 40 60 80 MILES

0 20 40 60 80 KILOMETERS

WASHINGTON

OREGON IDAHO

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

Coordinate system: State Plane, Washington South, FIPS 4602: Projection: 

Lambert Conformal Conic. Horizontal datum: North American Datum 1983, 

Vertical datum: North American Vertical Datum of 1988.

What Is the Columbia Plateau?

The Columbia Plateau is a wide basalt 

plateau between the Cascade Range and the 

Rocky Mountains that covers parts of Washington, 

Oregon, and Idaho (fig. 1). The climate over 

much of the Columbia Plateau is semiarid with 

precipitation ranging from 7 to 15 in/yr in the 

central part (Vaccaro and others, 2015), yet the 

area supports a $6 billion per year agricultural 

industry, including the production of apples, corn, 

grapes, hops, mint, potatoes, stone fruit, and 

wheat. Groundwater pumpage and surface-water 

diversions supply water to irrigated croplands 

that account for about 5 percent of the Nation’s 

irrigated lands. Groundwater also is the primary 

source of drinking water for about 1.3 million 

people living on the plateau.

What Is the USGS Columbia Plateau 

Groundwater Availability Study?

The U.S. Geological Survey (USGS) 

conducts large-scale multidisciplinary regional 

studies of groundwater availability, such as this 

study of the Columbia Plateau Regional Aquifer 

System (CPRAS). This study provides citizens, 

communities, and natural resource managers 

with (1) improved information and knowledge 

of the status of the groundwater resources, 

(2) information about how the resources have 

changed over time, and (3) tools to forecast how 

the resources may change in the future.

Vineyard looking (south) toward Horse Heaven Hills, Washington. Photograph by Carmen Lee, 2011. 

Figure 1. Columbia Plateau Regional Aquifer System study area, Washington, Oregon, 

and Idaho.

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2015–3063

September 2015 Printed on recycled paper
Columbia

River

Snake River

Pivot irrigator, northeastern Washington. Photograph by 

David Beedon, 2003.

Basalt bluffs at Wallula Gap, Washington. Photograph by Lyn Topinka, 

U.S. Geological Survey, 2005.

What Is the Columbia Plateau Regional 

Aquifer System?

The CPRAS covers an area of about 44,000 square 

miles within the drainage of the Columbia River. The 

primary aquifers within the Columbia Plateau occur in 

basalts of the Columbia River Basalt Group (CRBG) and in 

places, overlying sediment. The CRBG consists of a series 

of lava flows that erupted from fissures and vents along 

the Washington/Oregon/Idaho border during the Miocene 

Age, 17 million to 6 million years ago. More than 350 flows 

have been identified and individual flows range in thickness 

from 10 to more than 300 feet; interbeds occur in places 

where sediment accumulated between flows.

Groundwater levels have declined over more than 

10,000 square miles (about 23 percent of the CPRAS) due 

to pumping at rates that exceed groundwater recharge. 

Areas with large and widespread declines are located in 

the central northern part of the study area (referred to as 

the Odessa Subarea), in parts of the Yakima River basin in 

Washington, in the Pullman-Moscow area in Washington 

and Idaho, and in parts of the Umatilla River basin in 

Oregon. These declines are in areas known to rely heavily 

on groundwater for irrigation and other uses. In contrast, 

about 5 percent of the CPRAS has experienced groundwater 

level rises due to the delivery and application of surface 

water for irrigation within the large Bureau of Reclamation 

irrigation projects (fig. 2).

What Kinds of Scientific Information 

Were Collected and Analyzed?

Many types of information 

were collected and analyzed as 

part of the CPRAS Study, some 

highlights are:

• Groundwater levels were measured 

in 1,752 wells during spring 2009 by the 

USGS and 10 other Federal, State, Tribal, 

and local agencies (Snyder and Haynes, 

2010). This information was used to map the 

generalized groundwater elevations for the 

CPRAS and to provide information on regional 

groundwater flow directions. Groundwater 

levels measured in 2009 in 470 wells also were 

compared to groundwater levels measured 25 years 

earlier in spring 1984. Water levels declined in 

83 percent of the measured wells, and declines greater 

than 25 feet were measured in 29 percent of the wells. 3-dimensional diagram of the Columbia Plateau Regional 

Aquifer System looking northeast, not to scale. Surface 

topography shown in blues and greens; subsurface basalt 

units shown in browns; approximate groundwater-flow 

directions indicated by arrows.
Figure 2. Long-term water level changes in selected wells, Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho. All 

hydrographs are at same scale. (modified from Snyder and Haynes, 2010).

watac15-1024_CPLAT-fig04

Snake

River

Columbia

River

Banks

Lake

Palouse River Moses

Lake

Walla Walla

Pendleton

OREGON

WASH INGTON

IDAHO

Yakima

Pasco

Spokane

The

Dalles

Pullman

122° 120° 118° 116°

46°

48°

47°

45°

0 20 40 60 80 MILES

0 20 40 60 80 KILOMETERS

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

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

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

EXPLANATION

Columbia Basin Project

Other Bureau of Reclamation

 irrigation projects

Boundary of Columbia 

 Plateau Regional 

 Aquifer System

Note: Water level values are in

feet below land surface.

Well location

Well identification No. 453934118491701

Depth: 1,500 feet

Average rate of change:-2.2 feet per year

1960 1970 1980 1990 2000 2010

100

150

200

250

300

Water level

C

C

Well identification No. 

464028119101601

Depth: 498 feet

Average rate of change: 

+6.2 feet per year

0

1960 1970 1980 1990

50

100

150

200

250

300

Water level

B

B

1940 1950 1960 1970 1980 1990 2000 2010

0

50

100

150

Water level

Well identification No.

PBAC WSU Test Well

Depth: 145 feet 

Average rate of change: -1.3 feet/year

D

D

Well identification No. 

454031121224001

Depth: 553 feet

Average rate of change: 

-4.0 feet per year

1970 1980 1990 2000 2010

-100

-50

0

50

100

Water level

A

A

A

Figure 2. Long-term water level changes in selected wells, Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho. 

All hydrographs are at same scale. (Modified from Snyder and Haynes, 2010.)
Wine grapes near George, Washington. Photograph by Pearl Kahle, 2015.

• A digital 3-dimensional geologic model was constructed for 

the CPRAS to define the general aquifer system geometry 

(Burns and others, 2011) for use in the regional numerical 

groundwater-flow model. Data for 13,226 wells were 

used to construct digital surfaces that represent upper and 

lower subsurface unit boundaries. A web interface tool 

was developed to allow users to explore the 3D geologic 

framework of the CPRAS by drawing diagrams of ‘well 

logs’ at any site, or by developing geologic cross sections 

between multiple sites (http://or.water.usgs.gov/proj/cpras/

index.html).

• A new Simplified Surface Energy Balance (SSEB) method 

that uses satellite data was developed to estimate monthly 

evapotranspiration (ET) (Senay and others, 2007, 2008; 

Kahle and others, 2011). ET is a large component of the 

water budget; it can account for 100 percent of the annual 

precipitation in the arid areas and 45–70 percent in the more 

humid uplands, and historically, had not been estimated. 

• A spatially distributed soil-water balance model was 

developed to use relations among climate, soils, land 

cover, and irrigation data to compute monthly irrigation 

requirements and surplus moisture available for 

groundwater recharge in irrigated areas (Kahle and others, 

2011). Estimates of groundwater pumpage and surfacewater diversions needed for irrigation and groundwater 

recharge resulting from irrigation were then calculated with 

ET from the SSEB driving the calculations. 

What Is the Groundwater-Flow Model of the System?

A major product of this assessment is a numerical 

groundwater-flow model of the CPRAS (Ely and others, 

2014). A groundwater-flow model is a mathematical 

representation of groundwater flow through an aquifer 

system that uses information about aquifer properties, 

groundwater recharge, discharge, and groundwater levels, 

as well as physical processes governing groundwater flow. 

Models are useful tools for investigating variations in water 

supply, water demand, management strategies, groundwater/

surface-water exchanges, and potential effects of changing 

climate on a hydrologic system.

The CPRAS model domain was subdivided into a 

horizontal grid of 126 rows and 131 columns using a 

uniform 3-kilometer (9,842-feet) grid. Each of the five 

hydrogeologic units represented in the model was subdivided 

into approximately 100-foot-thick layers, yielding 100 model 

layers. The detailed vertical discretization improved the 

understanding of the important vertical flow component in 

the CPRAS.

The model was used to estimate annual water budgets 

for the system, flow between hydrogeologic units, changes in 

the system from predevelopment conditions, likely changes 

in the system if the conditions in 2007 existed through 2050, 

and potential effects in 2050 of increased irrigation demand 

owing to climate warming. The model results show the 

potential usefulness of this tool in providing information for 

short- to long-term management planning and decisions by 

numerous stakeholders.

Among the important findings of this new 

groundwater‐flow model of the CPRAS are: 

• The use of many thin layers improves representation of 

vertical groundwater gradients and connectivity with 

surface-water features.

• Groundwater pumping has increased substantially since 

the 1970s–1980s and this increase has resulted in declining 

water levels at depth and decreasing stream base flows over 

much of the study area; and

• During dry to average precipitation years, groundwater 

pumping causes a net loss of groundwater in storage. 

Groundwater pumping presently exceeds groundwater 

recharge during all but the wettest of years. USGS hydrologic technician measuring the water level in a well near 

Yakima. Photograph by Karen Payne, U.S. Geological Survey, March 2009.
What Are the Key Points Regarding Water Resources?

Groundwater availability is critical to managing water 

resources in the CPRAS because of the high water demand 

for agriculture, economic development, and ecological needs 

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

availability include:

• Widespread water-level declines associated with 

development of groundwater resources for irrigation and 

other uses;

• Decrease in base flow to rivers and associated effects on 

water temperature and quality;

• Limited availability of non-appropriated surface water;

• Potential capture of surface water, which was appropriated 

through senior water rights by pumpage of groundwater, 

which was appropriated through junior water rights; and

• Current and projected effects of climate change and 

variability on increasing pumping demand, groundwater 

recharge, base flow in rivers, and ultimately, sustainable 

groundwater yields. 

Ongoing activities in the region for enhancement of 

fisheries and obtaining additional water for agricultural, 

municipal, and domestic use may be affected by groundwater 

withdrawals and rules implemented under the Endangered 

Species Act for numerous stocks of salmonids. The study 

addressed some of these groundwater availability issues by 

improving the understanding of the hydrogeologic system, 

the status and trends of the groundwater system, the general 

relation between groundwater and surface waters, current 

water use, and the water budget for the CPRAS.

Agriculture in the Quincy Valley. Photograph by David Putnam, July 2006.

Fuji apples east of Moses Lake, Washington. Photograph by 

Sue Kahle, 2015.
ISSN 2327-6932 (online)

ISSN 2327-6916 (print)

http://dx.doi.org/10.3133/fs20153063

Authors: Sue C. Kahle and John J. Vaccaro

For more information:

Director, Washington Water Science Center

U.S.Geological Survey

934 Broadway, Suite 300

Tacoma, Washington 98402

http://wa.water.usgs.gov/

Project web page at: 

http://wa.water.usgs.gov/projects/cpgw/

Publishing support provided by the 

U.S. Geological Survey, 

Science Publishing Network, 

Tacoma Publishing Service Center

Additional Information

This USGS Fact Sheet is a brief summary of USGS 

Professional Paper 1817, “Groundwater availability of the 

Columbia Plateau Regional Aquifer System, Washington, 

Oregon, and Idaho” by J.J. Vaccaro, S.C. Kahle, D.M. 

Ely, E.R. Burns, D.T. Snyder, J.V. Haynes, T.D. Olsen, 

W.B. Welch, and D.S. Morgan. For additional information 

regarding this study, readers are referred to the Professional 

Paper for more in-depth descriptions of the CPRAS study 

and results of the investigation.

References Cited

Burns, E.R., Morgan, D.S., Peavler, R.S., and Kahle, S.C., 2011, 

Three-dimensional model of the geologic framework for the 

Columbia Plateau Regional Aquifer System, Idaho, Oregon, and 

Washington: U.S. Geological Survey Scientific Investigations 

Report 2010-5246, 44 p., accessed March 1, 2011, at http://pubs.

usgs.gov/sir/2010/5246/.

Ely, D.M., Burns, E.R., Morgan, D.S., and Vaccaro, J.J., 2014, 

Numerical simulation of groundwater flow in the Columbia 

Plateau Regional Aquifer System, Idaho, Oregon, and 

Washington: U.S. Geological Survey Scientific Investigations 

Report 2014-5127, 90 p., http://dx.doi.org/10.3133/sir20145127.

Kahle, S.C., Morgan, D.S., and Welch, W.B., 2011, Hydrogeologic 

framework and hydrologic budget components of the Columbia 

Plateau Regional Aquifer System, Washington, Oregon, and 

Idaho: U.S. Geological Survey Scientific Investigations Report 

2011-5124, 66 p., accessed August 26, 2011, at http://pubs.usgs.

gov/sir/2011/5124/.

Senay, G.B., Budde, M.E., Verdin, J.P., and Melesse, A.M., 2007, 

A coupled remote sensing and simplified surface energy balance 

approach to estimate actual evapotranspiration from irrigated 

fields: Sensors, v. 7, p. 979–1,000.

Ice fishing on Moses Lake, Washington. Photograph by Karen Payne, U.S. Geological Survey, 2011.

Senay, G.B, Verdin, J.P., Lietzow, R., and Melesse, A.M., 2008, 

Global reference evapotranspiration modeling and evaluation: 

Journal of American Water Resources Association, v. 44, no. 4, 

p. 969–979.

Snyder, D.T., and Haynes, J.V., 2010, Groundwater conditions 

during 2009 and changes in groundwater levels from 1984 to 

2009, Columbia Plateau Regional Aquifer System, Washington, 

Oregon, and Idaho: U.S. Geological Survey Scientific 

Investigations Report 2010-5040, 12 p., accessed October 27, 

2010, at http://pubs.usgs.gov/sir/2010/5040/.

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

Haynes, J.V., Olsen, T.D., Welch, W.B., and Morgan, D.S., 

2015, Groundwater availability of the Columbia Plateau 

Regional Aquifer System, Washington, Oregon, and Idaho: U.S. 

Geological Survey Professional Paper 1817, 87 p., http://dx.doi.

org/10.3133/pp1817.