tac13-0810_fig01

Big Wood River

Big Wood River

Fox Creek

Quigley Creek

Croy Creek

Slaughterhouse Creek

Deer Creek

E. Fork Big Wood River

N. Fork 

Big Wood 

River

Baker Creek

Trail Creek

Indian Creek

Warm Springs Creek

Adams Gulch

Elkhorn Gulch

Oregon Gulch

Lake Creek

Eagle Creek

Silver Creek

Willow Creek

Seamans Creek

Big Wood River

IDAHO

Figure

location

13135500 13135520

13137000

Picabo

Hailey

Gannett

Ketchum

Bellevue

Sun Valley

IDAHO 75

PICABO HILLS

PRICE RD

GLENDALE 

RD

TIMMERMAN 

HILLS

Idaho 75

BASELINE RD

GANNETT RD

Poverty

 Flat

Hayspur Fish 

Hatchery

PERO RD

Wood River Valley

aquifer system boundary

Stanton 

Crossing

PUNKIN

 CENTER RD

Base modified from 

USGS 2009 1-Arc Second National Elevation Dataset;

roads from IGDC, 2006; hydrography from IDWR, 1996.

114°35' 114°25' 114°15' 114°5'

43°

45'

43°

35'

43°

25'

0

0 2 4 6 8 KILOMETERS

2 4 6 8 MILES

13140800

13141000 13150430

13139500

13137500

13138000

432042114163801

EXPLANATION

Streamgaging station

Well

Aquifer extent

Base modified from USGS 2009 1-Arc Second National Elevation Dataset; 

roads from IGDC, 2006; hydrography from IDWR, 1996.

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2013–3005

February 2013

Groundwater Resources of the Wood River Valley, Idaho: 

A Groundwater-Flow Model for Resource Management

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

(IDWR), will use the current understanding of the Wood River Valley aquifer system to construct a 

MODFLOW numerical groundwater-flow model to simulate potential anthropogenic and climatic 

effects on groundwater and surface-water resources. This model will serve as a tool for water rights 

administration and water-resource management and planning. The study will be conducted over a 

3-year period from late 2012 until model and report completion in 2015.

Figure 1. Locations of communities, selected U.S. Geological Survey 

streamgaging stations, and other features, Wood River Valley, south-central Idaho 

(modified from Bartolino and Adkins, 2012).

The Wood River Valley

The population of Blaine County in southcentral Idaho has nearly quadrupled from 1970 

to 2010; most of the growth has occurred in the 

Wood River Valley in the northern part of the 

county. Because the entire population of the 

valley depends on groundwater for domestic 

supply, from either domestic or municipal-supply 

wells, this growth has caused concern about 

the long-term sustainability of the groundwater 

resource (Bartolino and Adkins, 2012).

The upper Wood River Valley is more 

developed than the lower valley and contains 

the incorporated communities of Sun Valley, 

Ketchum, Hailey, and Bellevue (fig. 1). The 

lower Wood River Valley is dominated by farms 

and ranches (irrigated by groundwater and 

diverted surface water), and contains the small 

communities of Gannett and Picabo. A number 

of tributary canyons to the main valley have 

been developed over the last 50 years (Bartolino 

and Adkins, 2012).

The Aquifer System

The Wood River Valley aquifer system is 

composed primarily of Quaternary-age sediment 

and basalt. This material constitutes the three 

components of the aquifer system: a single 

unconfined aquifer underlying the entire valley, 

a deeper confined aquifer present to the south 

of Baseline Road (fig. 1), and a confining layer 

separating the two aquifers. The confining layer 

thickens toward the south and generally, as landsurface altitude decreases in the same direction, 

the water-level surface rises above land surface 

so that wells flow under artesian pressure. South 

and east of Gannett the confining unit thins and 

disappears over the basalt.

Printed on recycled paper
tac13-0810_fig02

Mud Creek

Cain Creek

Chaney Creek

niarD relhuB

Canal Number 45

Black Slough

BIG WOOD RIVER

Pattton Creek

Willow

Creek

Crystal Creek

Spring

Creek

Wilson Creek

Grove Creek

Silver Creek

Thompson Creek

Loving Creek

4,890

4,930

4,910

4,920

4,900

4,890

4,880

4,870

4,900

4,920

4,910

46

45

44

43

41

40

39

37

42

38

Base modified from USGS 2002 3-Arc Second National Elevation Dataset; 

hydrography from USGS National Hydrologic dataset, 1999.

0 1 0.5

0.5 0 1 2 3 KILOMETERS

2 MILES

114°18' 114°17' 114°16' 114°15' 114°14' 114°13' 114°12' 114°11' 114°10'

43°

20'

43°

21'

43°

22'

43°

19'

BASELINE RD

IDAHO 75

43

EXPLANATION

Wells completed in the confined aquifer with water-level measurement data 

Water-level contour—Shows elevation at which water level would have stood in tightly cased wells. 

Dashed where approximately located. Contour interval is 10 feet.

Boundary of study area

4,870

Canal/ditch

The sediment and basalt can be 

divided into three hydrogeologic units: 

a coarse-grained sand and gravel unit, 

a fine-grained silt and clay unit, and 

a basalt unit. Although the three units 

exist throughout the aquifer system, the 

two aquifers are primarily composed of 

coarse-grained sediment and basalt and 

the confining unit is mostly composed of 

the fine-grained sediment. The sediments 

are largely derived from two episodes of 

glaciation in the surrounding mountains 

and upper reaches of tributary canyons. 

The basalt unit contains two flows of 

different ages, and is limited to the 

southeastern part of the Wood River Valley.

In some areas, the underlying bedrock 

may be hydraulically connected to the 

sediment and basalt units; however, the 

bedrock likely contains a small percentage 

of available water in the aquifer system. 

These bedrock aquifers probably are 

separate from the Wood River Valley 

aquifer system.

Generally, groundwater movement 

through the Wood River Valley aquifer 

system is relatively straightforward. 

Groundwater under unconfined conditions 

moves down-valley to the south, where it 

either enters the deeper confined aquifer 

or remains in the shallow unconfined 

Figure 2. Groundwater levels in the confined aquifer, Wood River Valley aquifer system in October 2006 (modified from Skinner and 

others, 2007). 

aquifer; the two aquifers appear to hydraulically reconnect in the area south of Gannett. 

A groundwater budget by Bartolino (2009) indicates that recharge primarily is from 

precipitation or seepage from streams, and discharge primarily is through springs and 

seeps to streams, pumpage, or subsurface outflow from the aquifer system. The rerouting 

of surface water into a network of irrigation canals in the late 19th century, construction 

of groundwater wells, and increased demand have affected groundwater flow, but the 

overall direction of groundwater movement remains down the topographic gradient 

and toward the eastern outlet of the valley at Picabo and western outlet near Stanton 

Crossing (figs. 1 and 2).

Depth to groundwater in the upper valley commonly is less than 10 ft, and increases 

southward to approximately 90 ft; water levels in wells completed in the unconfined 

aquifer in the lower valley range from less than 10 ft to approximately 150 ft below land 

surface. Wells completed in the confined aquifer are under artesian pressure and flow 

where the water-level surface is above land surface (Skinner and others, 2007).

Hydrologic Trends

A USGS report by Skinner and others (2007) verified statistically significant 

declining trends in mean annual water levels in three wells that seem representative 

of general conditions in the aquifer system. Two of these wells are completed in the 

unconfined aquifer and one well is in the confined aquifer (fig. 3): all three have more than 

50 years of measurement data.

Skinner and others (2007) also analyzed streamflow trends for three streamgaging 

stations in the Wood River Valley (fig. 1). The findings included:

• The Big Wood River at Hailey streamgaging station (13139500) showed an increase 

in mean monthly base flow for March over the 90-year period of record, possibly 

because of earlier snowpack runoff.

• Low-flow analyses for the Big Wood River near Bellevue streamgaging station 

(13141000) showed a mean decrease of about 15 cubic feet per second since the 

1940s, whereas the mean monthly discharge showed decreasing trends for the winter 

months.
tac13-0810_fig03

0 

5 

10 

15 

20 

25 

30 

35 

40 

45 

50 

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 2015

Water level above land surface, in feet 

Year 

432042114163801 — 01S 18E 14AAB1 

Figure 3. Depth to water for well 432042114163801 completed in the confined aquifer 

of the Wood River Valley aquifer system, July 1954–February 2012. A water level above 

land surface indicates a flowing well.

Fault

Confining unit Confining unit

Unconfined

aquifer

Confined

aquifer

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

Figure 4. 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.

• The Silver Creek at Sportsman Access near Picabo 

streamgaging station (13150430) showed decreases in 

annual discharge, as well as mean monthly discharge for July 

through February and April, during the 1975–2005 period 

of record. Because Silver Creek and its tributaries are fed 

primarily by groundwater through seeps and springs, seasonal 

fluctuations in groundwater levels affect streamflow. 

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 

Fault

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

1

2

3

Layers

1 2 3 4 5

Columns

1 2 3 4 5

Rows

Figure 5. Block diagram of part of a hypothetical basin-fill 

groundwater system with some model cells shown superimposed. 

The model cells cover the entire simulated groundwater system.

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 boundary 

(figs. 4 and 5).

A numerical computer model, in this 

case MODFLOW, is a program containing 

a number of 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 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.

By applying the basic laws of physics and reasonably 

representing the actual groundwater system in the model 

cells and boundaries, a groundwater-flow model can provide 

an accurate, quantitative depiction of the relations between 

groundwater flow-system stresses (such as pumpage) and 

responses (such as water-level declines). This understanding 

enables forecasts of future hydrologic conditions in response 

to changes in recharge, discharge, or varying management 

scenarios. Such forecasts inherently contain some uncertainty 

because of sparse or inaccurate data, errors in scientists’ 

understanding of the system, and poor estimates of future 

conditions. Despite such uncertainties, groundwater-flow 

models often represent the best available tool for management 

decisions (Alley and others, 1999).
Publishing support provided by the 

U.S. Geological Survey

Tacoma Publishing Service Center

Authors: James Bartolino and Sean Vincent

For Information Contact:

Director, Idaho Water Science Center

U.S. Geological Survey

F.H. Newell Federal Building

230 Collins Road

Boise, ID 83702

208-387-1300

http://id.water.usgs.gov 

One of the keys to a successful groundwater-flow model that 

produces accurate forecasts is the appropriate 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.

The Wood River Valley groundwater-flow model will be 

designed to further the basic understanding of the aquifer system, 

and ultimately to examine effects on the groundwater system 

and its interaction with the Big Wood River due to changes in 

water use, recharge, or discharge. Additionally, by virtue of 

the attempt to mathematically represent the groundwater-flow 

system, the model can be used to evaluate how well components 

of the system are understood and which components have the 

most effect on calculations. This analysis then can be used to 

guide the collection of additional data that will most improve the 

understanding of the Wood River Valley aquifer system.

The Collaborative USGS-IDWR 

Groundwater-Flow Model Project

The USGS began cooperative groundwater studies in the 

Wood River Valley in 1928 with one of the precursor agencies 

to the IDWR (Stearns and others, 1936). Since then, the 

USGS and IDWR have cooperated with each other, numerous 

local governments, and other entities to understand the water 

resources of the valley. The latest effort began in 2004 when 

the USGS, in cooperation with Blaine County, City of Hailey, 

City of Ketchum, The Nature Conservancy, City of Sun Valley, 

Sun Valley Water and Sewer District, Blaine Soil Conservation 

District, City of Bellevue, and Citizens for Smart Growth 

undertook a four-phase, multiyear effort to better understand the 

groundwater system and provide information for scientificallyinformed decisions.

The USGS, in collaboration with the IDWR, will incorporate 

this improved understanding of the Wood River 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 3-year study will be from late 2012 through 

2015. Additional data collection, including water-level 

monitoring and streamflow measurements, will be done in 2013.

The numerical groundwater-flow model will be constructed 

using MODFLOW to simulate potential anthropogenic and 

climatic effects on groundwater and surface-water resources. 

A USGS report will be published to describe numerical model 

construction and limitations, as well as results from several 

simulations that represent a range of potential anthropogenic 

activities (formulated in consultation with stakeholders) and 

hydrologic conditions. The documented model will be published by 

the USGS and made publically available through a 

USGS website.

A Technical Advisory Committee is planned to provide for 

transparency in model development and to serve as a vehicle for 

stakeholder input. Technical representation will include interested 

parties such as water-user groups and current USGS cooperating 

organizations in the Wood River Valley.

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.)

Bartolino, J.R., 2009, Ground-water budget for the Wood River Valley aquifer 

system, south-central Idaho: U.S. Geological Survey Scientific Investigations Report 2009-5016, 36 p. (Also available at http://pubs.usgs.gov/

sir/2009/5016/.)

Bartolino, J.R., and Adkins, C.B., 2012, Hydrogeologic framework of the Wood 

River Valley aquifer system, south central Idaho: U.S. Geological Survey 

Scientific Investigations Report 2012–5053, 46 p., 1 plate in pocket. (Also 

available at http://pubs.usgs.gov/sir/2012/5053/.)

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/.)

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.)

Skinner, K.D., Bartolino, J.R., and Tranmer, A.W., 2007, Water-resource trends 

and comparisons between partial development and October 2006 hydrologic 

conditions, Wood River Valley, south-central Idaho: U.S. Geological Survey 

Scientific Investigations Report 2007-5258, 31 p., 4 plates, 1 appendix. (Also 

available at http://pubs.er.usgs.gov/usgspubs/sir/sir20075258.)

Stearns, H.T., Crandall, Lynn, and Steward, W.G., 1938, Geology and groundwater resources of the Snake River Plain in southeastern Idaho: U.S. Geological Survey Water-Supply Paper 774, 268 p., 6 plates in pocket. (Also available 

at http://pubs.er.usgs.gov/usgspubs/wsp/wsp774.)

Silver Creek on the Nature Conservancy Silver Creek Preserve, Idaho. View 

is from the Picabo Hills looking north up the Wood River Valley. The low hills 

to the right (east) are the southeastern edge of the Pioneer Mountains, the 

snow covered peaks in the background are in the Smokey Mountains. The 

valley bottom visible in the medium to far is the Bellevue Fan. Photograph taken 

November 19, 2004.