U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2017–3027

April 2017

Lake

Lowell

Lucky

Peak

Lake

Arrowrock

Reservoir

Black Canyon

Reservoir

Base from U.S. Geological Survey digital data, The National Map

Albers Equal-Area Conic projection, North American Datum of 1983

Boise River

Indian Creek

Payette River

Payette River

Snake River

OREGON

Approximate model boundary

IDAHO

ADA COUNTY

ELMORE

 COUNTY CANYON COUNTY

OWYHEE COUNTY

GEM COUNTY

PAYETTE

 COUNTY

MALHEUR

 COUNTY

BOISE FRONT

MOUNTAIN

HOME

PLATEAU

Adrian

Caldwell

Emmett

Eagle

Kuna

Horseshoe

Bend

Homedale

Garden City

Fruitland

Boise

Marsing

Melba

Wilder

Payette

Parma

Ontario

Nyssa

Notus

New

Plymouth

Nampa

Middleton

Meridian

BOISE COUNTY

0 4 8 12 16 MILES

0 6 12 18 KILOMETERS

117°10' 117°00' 116°50' 116°40' 116°30' 116°20' 116°10' 116°00' 115°50'

44°

00'

43°

50'

43°

40'

43°

30'

43°

20'

43°

10'

OREGON

IDAHO

Eastern Snake

 River Plain

Western Snake

 River Plain

.

King

Hill

Figure 1. Locations of communities, the boundary of approximate model extent, and other 

features, Treasure Valley, southwestern Idaho.

A Groundwater-Flow Model for the Treasure Valley and 

Surrounding Area, Southwestern Idaho

The U.S. Geological Survey (USGS), in partnership with the Idaho Department of Water 

Resources (IDWR) and Idaho Water Resource Board (IWRB), will construct a numerical 

groundwater-flow model of the Treasure Valley and surrounding area. Resource managers will use 

the model to simulate potential anthropogenic and climatic effects on groundwater for water-supply 

planning and management. As part of model construction, the hydrogeologic understanding of the 

aquifer system will be updated with information collected during the last two decades, as well as new 

data collected for the study.

The Treasure Valley

The Treasure Valley is “the 

agricultural area that stretches west 

from Boise into Oregon” (U.S. Board 

on Geographic Names, 2016), although 

it is commonly referred to as the lower 

Boise River Basin. The valley contains 

the three largest and sixth largest cities 

in Idaho—Boise, Meridian, Nampa, 

and Caldwell, respectively (fig. 1). The 

2016 population of the Treasure Valley 

was about 630,000, representing about 

37 percent of the total population of 

Idaho (SPF Water Engineering, 2016; 

U.S. Census Bureau, 2017). Except for 

the 30 percent of Boise’s municipal 

supply taken from the Boise River (Suez 

North America, 2017), groundwater 

withdrawals provide all domestic and 

municipal water in the valley.

SPF Water Engineering (2016) 

estimated that the population of the 

Treasure Valley will increase to about 

1.6 million by 2065, resulting in a 

corresponding increase in domestic, 

commercial, municipal, and industrial 

water demand. To address this 

anticipated demand for water, the Idaho 

Senate passed Concurrent Resolution 

137, which includes a request to 

“develop a ground water model, with 

all necessary measurement networks” 

for the Treasure Valley (Idaho Senate 

Resources and Environment 

Committee, 2016, p. 2).
View of Treasure Valley looking southwest from the Boise Front, Idaho. 

Photograph by James R. Bartolino, November 19, 2016.

The hydrologic boundaries of the Treasure Valley aquifer 

system are generally considered to be the Snake River to the 

south and west, the Mountain Home plateau to the east, and 

the uplands that divide the Boise and Payette River Basins to 

the north (fig. 1). The current study will extend the northern 

model boundary to the Payette River valley and the cities of 

Emmett, New Plymouth, and Fruitland because the surfacewater divide separating the Boise and Payette River Basins is not 

a continuous barrier to groundwater flow. This area is also the 

site of current development and proposed large-scale residential 

development.

Hydrology of the Treasure Valley

The Treasure Valley lies within the Snake River Plain, “an 

arcuate topographic and structural depression that extends 

across southern Idaho. Geology and hydrology of [the] eastern 

and western parts of the Snake River Plain are distinctly 

different; the west is predominantly sedimentary rocks, and 

the east is predominantly volcanic rocks” (Newton, 1991, p. 

G2). Prolific and unconnected aquifer systems underlie the two 

halves of the Snake River Plain; the eastern and western halves 

are separated by a hydrologic boundary in the area near King 

Hill. The western Snake River Plain was formed after volcanic 

activity led to faulting that caused the plain to subside relative to 

the surrounding mountains. The resulting basin filled with river 

and lake sediments that are interbedded with basalt in places; 

these deposits form the aquifer system underlying the Treasure 

Valley and immediate area. Geologic activity and glacial cycles 

during deposition created complexity in these deposits; thus, 

aquifer characteristics can change abruptly with small changes 

in depth or location. This complexity partitions the overall 

aquifer system into a number of interconnected aquifers with 

varying water levels, chemistry, and productivity. After basinfill deposition ended, subsequent erosion caused by the Snake, 

Boise, and Payette Rivers formed the current landscape of the 

Treasure Valley and surrounding area (Wood and Clemens, 2002).

Groundwater occurs under unconfined and confined 

conditions; some wells completed in the confined aquifers flow 

in areas where the potentiometric head is above land surface. In 

general, depth to groundwater varies from land surface or above 

to depths of more than 800 ft below land surface. The aquifer 

system can broadly be conceptualized as having three parts—a 

shallow water-table aquifer; a complex deep, underlying aquifer 

under confined conditions; and a lowermost confined geothermal 

aquifer. Groundwater flow in the shallow aquifer is generally 

from topographic highs to rivers and drains. 

The main, natural surface-water features of the Treasure 

Valley are the Snake River to the south, the Boise River in the 

central valley, and the Payette River to the north. The availability 

of water for agriculture diverted from the Boise and Payette 

Rivers was the primary reason for settlement in the area; a 

complex network of reservoirs, irrigation canals, laterals, ditches, 

and drains was constructed to support this irrigation (Stacy, 1993).

Prior to the late 1940s, groundwater use was limited to very 

shallow wells or to wells that flowed under artesian pressure. 

In 1891, artesian wells near the old State Penitentiary began 

supplying geothermal water for heating; by 1897, artesian wells in 

Hulls Gulch provided the municipal supply for Boise (Lindgren, 

1898; Wood and Burnham, 1983). Groundwater pumping 

increased steadily after the development of pumps capable of 

efficiently lifting water from greater depths in the late-1940s.

Authors of earlier water budgets for the area have identified 

the largest components of recharge to the aquifer system as 

seepage from the canal system (including Lake Lowell) and 

infiltration of applied irrigation water. The largest components 

of discharge from the aquifer system are seepage to rivers 

and drains, followed by pumping from wells (Newton, 1991; 

Petrich and Urban, 2004). Although these water budgets 

were comparable regarding the relative magnitudes of their 

components, they can be improved by collecting new, additional 

data (such as groundwater discharge to drains) and by applying 

new interpretive techniques (such as the determination of 

evapotranspiration rates using satellite imagery). In addition, 

previous efforts did not evaluate year-to-year and seasonal 

variations in the groundwater budget. In contrast, the present 

study involves developing a monthly water budget for a 30-year 

timeframe (1986–2015). 

Groundwater Modeling—A Tool for Understanding 

and Managing the Resource

In the most general terms, a model is a simplified 

representation of the appearance or operation of a real object 

or system. Groundwater-flow models attempt to reproduce, or 

simulate, the processes of a real aquifer system by solving a 

series of mathematical equations. Groundwater-flow models 

are usually constructed by representing the geology of the 

groundwater system as a series of rectangular, three-dimensional 

blocks or model cells surrounded by a hydrologic boundary 

(figs. 2 and 3).

A numerical computer model is a program containing 

equations that represent groundwater flow between the model 

cells. As the equations are solved, the program accounts for the 

flow of water through the model domain and for each cell; a 

groundwater-flow model calculates the volume of water flowing 

horizontally and vertically between the cells, and any changes 

in the volume of water stored in each cell. Groundwater-flow 

models often represent the best available tool for management 

decisions (Alley and others, 1999).
tac13-0810_fig02

Fault

Confining unit Confining unit

Unconfined

aquifer

Confined

aquifer

Figure 2. Block diagram of part of a hypothetical basin-fill 

groundwater system. The blue arrows show the direction of 

groundwater flow. Among the features shown are an unconfined 

aquifer overlying a confining unit and confined aquifer, a gaining 

stream, infiltration from irrigated agriculture, and mountain-front 

recharge. Modified from Leake (1997) and Reilly and McAda (2002).

tac13-0810_fig03

Fault

1

2

3

Layers

1 2 3 4 5

Columns

1 2 3 4 5

Rows

Figure 3. Block diagram of part of a hypothetical basin-fill 

groundwater system with some model cells shown superimposed. The 

model cells cover the entire groundwater system being simulated. 

Modified from Leake (1997) and Reilly and McAda (2002).

U.S. Geological Survey hydrographer making a stream-discharge measurement 

in Sand Run Gulch near Parma, Idaho. Photograph by James R. Bartolino, 

March 14, 2017. 

One of the keys to a successful groundwater-flow model 

that produces useful forecasts is the apt representation of 

important aspects of the physical system. The selection of these 

aspects depends, in part, on the objectives of the modeling 

project. These modeling objectives also influence the extent and 

depth of the modeled area, the size and shape of the model cells 

and layers, the methods used to represent the boundary conditions 

of the system, and the use of any specialized techniques or 

equations to address specific flow conditions or processes.

Eight groundwater-flow models have been published that 

include all or part of the Treasure Valley (Cosgrove, 2010; 

Johnson, 2013). These models were constructed with varying 

objectives, detail, and model extent. The USGS-IDWR Treasure 

Valley groundwater-flow model will incorporate new data 

and interpretations that were unavailable in previous efforts. 

As a transient model, it will be designed to advance the basic 

understanding of the aquifer system and, ultimately, to consider 

how changes in water use, recharge, or discharge may affect 

the groundwater system and its interaction with surface water. 

Additionally, by attempting to mathematically represent the 

groundwater-flow system, the model can be used to analyze the 

understanding of system components and which have the most 

effect on model forecasts. This analysis can then be used to 

guide the collection of additional data that will most improve the 

understanding of the Treasure Valley aquifer system. 

Collaborative USGS-IDWR Groundwater-Flow 

Model Project

The USGS established the first streamgage in the Treasure 

Valley in 1895, the same year that the Office of the State 

Engineer (a precursor agency to the IDWR) was established. 

Since then, the USGS and the State of Idaho have cooperated 

with each other, numerous local governments, and other entities 

to understand the water resources of the Treasure Valley. 

The USGS, in collaboration with the IDWR, will 

incorporate an improved understanding of the Treasure Valley 

aquifer system into a groundwater-flow model that will serve 

as a tool for water-rights administration and water-resource 

management and planning. The 5-year study is scheduled 

to begin in late 2016 and conclude in 2021. Additional data 

collection for the study, including streamflow measurements on 

drains, started in October 2016. 
A transient numerical groundwater-flow model will be 

constructed using a version of the public domain computer 

program, MODFLOW, to simulate potential human-caused and 

natural effects on groundwater and surface-water resources. The 

USGS is scheduled to publish an initial report in 2019 describing 

improvements in the hydrogeologic framework, conceptual 

water budget, and conceptual model. At the conclusion of the 

study in 2021, the USGS will publish the documented model and 

a final report to describe the groundwater-flow model construction 

and limitations, as well as results from one or two simulations 

that represent a range of potential anthropogenic activities 

and hydrologic conditions (formulated in consultation with 

stakeholders). The documented model will be published by the 

USGS and made publically available through USGS and 

IDWR websites.

A technical advisory committee (TAC) will be formed to 

provide transparency in model development and to facilitate 

stakeholder input. The IDWR will seek representation on the 

TAC from municipalities, irrigation entities, water-user groups, 

and other Treasure Valley stakeholders. This TAC will be similar 

to committees established for two previous USGS-IDWR 

collaborative groundwater-flow model development studies: the 

Wood River Valley Groundwater Flow Model Project (Fisher 

and others, 2016), and the Spokane Valley-Rathdrum Prairie 

Hydrologic Project (Hsieh and others, 2007).

References Cited

Alley, W.M., Reilly, T.E., and Franke, O.L., 1999, Sustainability of 

ground-water resources: U.S. Geological Survey Circular 1186, 79 

p. [Also available at http://pubs.er.usgs.gov/publication/cir1186.]

Cosgrove, D.M., 2010, Evaluation of ground water models in 

the Treasure Valley, Idaho Area: Idaho Falls, Western Water 

Consulting, Inc., 134 p. [Also available at http://www.idwr.idaho.

gov/Browse/WaterInfo/EADA/contractor_findings/.]

Fisher, J.C., Bartolino, J.R., Wylie, A.H., Sukow, Jennifer, and 

McVay, Michael, 2016, Groundwater-flow model for the 

Wood River Valley aquifer system, South-Central Idaho: U.S. 

Geological Survey Scientific Investigations Report 2016-5080, 

71 p., 6 appendices. [Also available at http://dx.doi.org/10.3133/

sir20165080.]

Hsieh, P.A., Barber, M.E., Contor, B.A., Hossain, M.A., Johnson, 

G.S., Jones, J.L., and Wylie, A.H., 2007, Ground-water flow

model for the Spokane Valley-Rathdrum Prairie Aquifer, Spokane

County, Washington, and Bonner and Kootenai Counties,

Idaho: U.S. Geological Survey Scientific Investigations Report

2007-5044, 79 p. [Also available at http://pubs.usgs.gov/

sir/2007/5044/.]

Idaho Senate Resources and Environment Committee, 2016, Senate 

Concurrent Resolution 137: Legislature of the State of Idaho, 

63rd, 2nd session, 2 p. [Also available at https://legislature.idaho.

gov/legislation/2016/SCR137Bookmark.htm.]

Johnson, Jennifer, 2013, Development of a transient groundwater 

model of the Treasure Valley aquifer, Idaho: Boise, Bureau of 

Reclamation, July 2013, 43 p.

Leake, S.A., 1997, Modeling ground-water flow with MODFLOW 

and related programs: U.S. Geological Survey Fact Sheet 121–97,

4 p. [Also available at http://pubs.usgs.gov/fs/FS-121-97/.]

Lindgren, Waldemar, 1898, Boise folio, Idaho: U.S. Geological 

Survey Folios of the Geologic Atlas 45, 7 p., 4 pl. [Also available 

at https://pubs.er.usgs.gov/publication/gf45.]

Newton, G.D., 1991, Geohydrology of the regional aquifer system, 

western Snake River Plain, southwestern Idaho: U.S. Geological 

Survey Professional Paper 1408-G, 52 p., 1 plate in pocket. [Also 

available at http://ngmdb.usgs.gov/Prodesc/proddesc_4859.htm.

Petrich, C.R., and Urban, S., 2004, Characterization of ground water 

flow in the lower Boise River basin: Moscow, University of Idaho 

Water Resources Research Institute, Research Report IWRRI2004-01, 148 p. [Also available at http://www.idwr.idaho.gov/files/

projects/treasure-valley/TVHP-Characterization.pdf.]

Reilly, T.E., and McAda, D.P., 2002, Ground-water-flow models and 

how they are used to study the basin, in Bartolino, J.R., and Cole, 

J.C., 2002, Ground-water resources of the middle Rio Grande

Basin, New Mexico: U.S. Geological Survey Circular 1222,

p. 102–103. [Also available at http://pubs.er.usgs.gov/ usgspubs/

cir/cir1222.]

SPF Water Engineering, 2016, Treasure Valley DCMI waterdemand projections (2015–2065): Boise, SPF Water Engineering, 

131 p., 3 appendices. [Also available at http://www.idwr.idaho.

gov/files/publications/20160808-OFR-Treasure-Valley-WaterDemand-(2015-2065).pdf.] 

Stacy, S.M., 1993, When the river rises—Flood control on the Boise 

River 1943–1985: Boulder, University of Colorado Program on 

Environment and Behavior Special Publication 27, 187 p.

Suez North America, 2017, About your water: Suez North America 

website, accessed February 13, 2017, at http://www.mysuezwater.

com/idaho/water-in-my-area/about-my-water.

U.S. Board on Geographic Names, 2016, Feature detail report for 

Canyon County: U.S. Geological Survey website, accessed July 20, 

2016, at http://geonames.usgs.gov/apex/f?p=gnispq:3:::NO::P3_

FID: 399406.

U.S. Census Bureau, 2017, QuickFacts, Idaho: U.S. Census Bureau

database, accessed March 7, 2017, at https://www.census.gov/

quickfacts/table/PST045215/16.

Wood, S.H., and Burnham, W.L., 1983, Boise, Idaho geothermal 

system: Geothermal Resources Council, Transactions, v. 7, 

p. 215–223.

Wood, S.H., and Clemens, D.M., 2002, Geologic and tectonic 

history of the western Snake River Plain, Oregon and Idaho, 

in Bonnichsen, B., White, C.M., and McCurry, M., eds., 2004, 

Tectonic and magmatic evolution of the Snake River Plain 

Volcanic Province: Moscow, Idaho: Geological Survey Bulletin 

30, p. 69–103. [Also available at http://www.idahogeology.org/

Products/reverselook.asp?switch=title&value=Tectonic_and_

Magmatic_Evolution_of_the_Snake_River_Plain_Volcanic_

Province.]

Authors: 

James R. Bartolino, U.S. Geological Survey 

Sean Vincent, Idaho Department of Water Resources

For more information contact:

Director, Idaho Water Science Center 

U.S. Geological Survey 

F.H. Newell Federal Building, 230 Collins Road, Boise, ID 

83702 https://id.water.usgs.gov

Publishing support provided by the U.S. Geological Survey 

Science Publishing Network 

Tacoma Publishing Service Center

ISSN 2327-6916 (print)

ISSN 2327-6932 (online)

https://doi.org/10.3133/fs20173027