Water-Resource Trends and Comparisons Between Partial 

Development and October 2006 Hydrologic Conditions, 

Wood River Valley, South-Central Idaho

Scientific Investigations Report 2007–5258

Prepared 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

U.S. Department of the Interior

U.S. Geological Survey
Cover:

Top: Silver Creek and the Bellevue fan looking northwest up the Wood River Valley 

from the Picabo Hills, Wood River Valley, south-central Idaho. 

(Photograph taken by James R. Bartolino, U.S. Geological Survey, November 19, 2004.)

Middle: The Big Wood River looking south (downstream) of the U.S. 20 bridge at 

Stanton Crossing, Wood River Valley, south-central Idaho. 

(Photograph taken by James R. Bartolino, U.S. Geological Survey, April 15, 2005.)

Bottom: The Big Wood River looking south (downstream) of the Glendale bridge, 

Wood River Valley, south-central Idaho.

(Photograph taken by James R. Bartolino, U.S. Geological Survey, June 21, 2006.)
Water-Resource Trends and Comparisons 

Between Partial-Development and 

October 2006 Hydrologic Conditions, 

Wood River Valley, South-Central Idaho

By Kenneth D. Skinner, James R. Bartolino, and Andrew W. Tranmer

Prepared 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

Scientific Investigations Report 2007–5258

U.S. Department of the Interior

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

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2007

For product and ordering information:

World Wide Web: http://www.usgs.gov/pubprod

Telephone: 1-888-ASK-USGS

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

natural hazards, and the environment:

World Wide Web: http://www.usgs.gov

Telephone: 1-888-ASK-USGS

Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

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, 30 p. 
iii

Contents

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

Introduction ....................................................................................................................................................1

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

Previous Work .......................................................................................................................................2

Description of Study Area ............................................................................................................................2

Hydrogeologic Setting and Framework ............................................................................................4

Ground Water...............................................................................................................................4

Surface Water..............................................................................................................................4

Climate.....................................................................................................................................................7

Population ............................................................................................................................................10

Methods and Data Sources .......................................................................................................................11

Ground-Water Level Trend Analysis................................................................................................11

Streamflow Trend Analysis ...............................................................................................................11

Ground-Water Levels .........................................................................................................................13

Partial Development Ground-Water Levels...........................................................................13

October 2006 Ground-Water-Level Measurement...............................................................14

Survey of Well Locations.................................................................................................14

Ground-Water-Level Maps.......................................................................................................14

Estimating Water-Level Change..............................................................................................15

Surface-Water Discharge Measurements.....................................................................................15

Trend Analysis ..............................................................................................................................................17

Ground-Water Level Trends..............................................................................................................17

Streamflow Trends..............................................................................................................................17

Partial Development and October 2006 Hydrologic Conditions ...........................................................21

Unconfined Aquifer.............................................................................................................................21

Partial-Development Water Table...........................................................................................21

October 2006 Water Table ........................................................................................................21

Changes in the Water Table, Partial Development to October 2006 .................................22

Confined Aquifer.................................................................................................................................22

Partial-Development Potentiometric Surface.......................................................................22

October 2006 Potentiometric Surface ....................................................................................22

Changes in the Potentiometric Surface, Partial Development to October 2006 .............22

Annual and Long-Term Fluctuation of Water Levels.....................................................................22

Surface-Water Discharge Measurements and Streamflow Gain/Loss.....................................23

Summary and Conclusions.........................................................................................................................24

Acknowledgments.......................................................................................................................................25

References Cited..........................................................................................................................................25

Appendix A. —Water-Level and Well Information Used to Construct Partial-Development

and October 2006 Water-Table and Potentiometric-Surface Maps, Wood River

Valley, South-Central Idaho..........................................................................................................29
iv

Figures

Figure 1. Map showing location of communities, continuous streamflow-gaging

stations, weather stations, and snow-survey sites, Wood River Valley,

south-central Idaho ........................................................................ 3

Figure 2. Graph showing Palmer drought severity index for Idaho climate zone 4 ............ 9

Figure 3. Graph showing Blaine County population by decennial census, 1900–2000,

and projected 2005 population, south-central Idaho .................................... 10

Figure 4. Map showing location of surface-water discharge measurements, Wood

River Valley, central Idaho, October 23–26, 2006 ....................................... 16

Figure 5. Graphs showing normalized residual plots of mean annual depth to water for

three wells and total annual precipitation at the Ketchum Ranger Station,

Wood River Valley, south-central Idaho ................................................ 18

Figure 6. Graphs showing trend analysis of annual 7- and 30-day low streamflows for

two gaging stations on the Big Wood River, south-central Idaho ..................... 19

Figure 7. Graph showing mean monthly discharge at Big Wood River near Bellevue,

south-central Idaho, water years 1915–96................................................ 20

Plates

Plate 1. Partial-development water-table map of the unconfined aquifer, Wood River Valley, 

south-central Idaho, 1952–86.

Plate 2. October 2006 water-table map of the unconfined aquifer, Wood River Valley, 

south-central Idaho, October 23–27, 2006.

Plate 3. Changes in the water-table altitude of the unconfined aquifer from partial 

development (1952–86) to October 2006, Wood River Valley, south-central Idaho.

Plate 4. Partial-development and October 2006 potentiometric surface and change maps of 

the confined aquifer, Wood River Valley, south-central Idaho.

Tables

Table 1. Continuous record streamflow-gaging stations within a 25-mile buffer of

Wood River Valley, south-central Idaho ................................................ 5

Table 2. Mean annual and annual peak flow from selected streamflow-gaging

stations in and adjacent to Wood River Valley, south-central Idaho.................. 6

Table 3. National Weather Service stations, AgriMet stations, and snow-survey sites

in or near the Wood River Valley, south-central Idaho ................................. 7

Table 4. Summary of data from selected weather stations in and near the Wood River

Valley, south-central Idaho ............................................................... 9

Table 5. Surface-water trend analyses for three streamflow-gaging stations, Wood

River Valley, south-central Idaho ......................................................... 12

Table 6. Extremes of ground-water levels and annual variation for selected wells in

the Wood River Valley, south-central Idaho, 1954–2006 .............................. 20

Table 7. Streamflow measurements, Wood River Valley, south-central Idaho, October

23–26, 2006.................................................................................... 23
v

Conversion Factors, Datums, and Acronyms 

Conversion Factors

Multiply By To obtain

cubic foot per second (ft3

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

foot (ft) 0.3048 meter (m)

foot per mile (ft/mi) 0.1894 meter per kilometer (m/km)

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

mile (mi) 1.609 kilometer (km)

square mile (mi2

) 2.590 square kilometer (km2) 

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

°C=(°F-32)/1.8

Datums

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

(NAVD 88).”

Horizontal coordinate information is referenced to the “North American Datum of 1983 

(NAD 83).”

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

Acronyms

Acronym Definition

BFI Base Flow Index 

DEM digital elevation model

GIS geographic information system

GPS Global Positioning System

IDWR Idaho Department of Water Resources

NWIS National Water Information System

NWS National Weather Service

PDSI Palmer Drought Severity Index

RTK real-time kinematic

USGS U.S. Geological Survey

USEPA U.S. Environmental Protection Agency
vi

This page intentionally left blank.
Water-Resource Trends and Comparisons Between 

Partial-Development and October 2006 Hydrologic 

Conditions, Wood River Valley, South-Central Idaho

By Kenneth D. Skinner, James R. Bartolino, and Andrew W. Tranmer

Abstract 

This report analyzes trends in ground-water and surfacewater data, documents 2006 hydrologic conditions, and 

compares 2006 and historic ground-water data of the Wood 

River Valley of south-central Idaho. The Wood River Valley 

extends from Galena Summit southward to the Timmerman 

Hills. It is comprised of a single unconfined aquifer and an 

underlying confined aquifer present south of Baseline Road in 

the southern part of the study area. Streams are well-connected 

to the shallow unconfined aquifer. Because the entire 

population of the area depends on ground water for domestic 

supply, either from domestic or municipal-supply wells, rapid 

population growth since the 1970s has raised concerns about 

the continued availability of ground and surface water to 

support existing uses and streamflow. To help address these 

concerns, this report evaluates ground- and surface-water 

conditions in the area before and during the population growth 

that started in the 1970s. 

Mean annual water levels in three wells (two completed 

in the unconfined aquifer and one in the confined aquifer) with 

more than 50 years of semi-annual measurements showed 

statistically significant declining trends.

Mean annual and monthly streamflow trends were 

analyzed for three gaging stations in the Wood River Valley. 

The Big Wood River at Hailey gaging station (13139500) 

showed a statistically significant trend of a 25-percent increase 

in mean monthly base flow for March over the 90-year period 

of record, possibly because of earlier snowpack runoff. Both 

the 7-day and 30-day low-flow analyses for the Big Wood 

River near Bellevue gaging station (13141000) show a mean 

decrease of approximately 15 cubic feet per second since 

the 1940s, and mean monthly discharge showed statistically 

significant decreasing trends for December, January, and 

February. The Silver Creek at Sportsman Access near Picabo 

gaging station (13150430) also showed statistically significant 

decreasing trends in annual and mean monthly discharge for 

July through February and April from 1975 to 2005.

Comparisons of partial-development (ground-water 

conditions from 1952 to 1986) and 2006 ground-water 

resources in the Wood River Valley using a geographic 

information system indicate that most ground-water levels for 

the unconfined aquifer in the study area are either stable or 

declining. Declines are predominant in the southern part of 

the study area south of Hailey, and some areas exceed what is 

expected of natural fluctuations in ground-water levels. Some 

ground-water levels rose in the northern part of the study area; 

however, these increases are approximated due to a lack of 

water-level data in the area.

Ground-water level declines in the confined aquifer 

exceed the range of expected natural fluctuations in large 

areas of the confined aquifer in the southern part of the study 

area in the Bellevue fan. However, the results in this area are 

approximated due to limited available water-level data.

Introduction 

Blaine County in south-central Idaho experienced a 

40-percent population growth from 13,000 to more than 

20,000 people from 1990 to 2000. Between April 1, 2000, 

and July 1, 2005, the county population increased by about 

11.5 percent (U.S. Census Bureau, 2007). In addition to 

permanent residents, thousands of people annually visit 

Blaine County for winter and summer recreation. The entire 

population of the area depends on ground water for domestic 

supply, either from domestic or municipal-supply wells and 

surface water is used for recreation and irrigation supplies.

Water managers and private landowners are increasingly 

concerned about the effects of population growth on groundwater and surface-water supplies in the basins, including 

sustainability of ground-water resources and the effects of 

wastewater disposal on ground-water and surface-water 

quality. Development in recent years has been moving to 

tributary canyons of the upper Big Wood River basin, and 

residents in some canyon areas have reported declining 

ground-water levels. It is unclear at this time whether 

these declining ground-water levels are due to increased 

development or are a response to several years of drought 

conditions. In June 2005, Blaine County Commissioners 

approved an interim moratorium on selected development 

activities while the impacts of growth, including waterresource concerns, were evaluated.
2 Water-Resource Trends and Comparisons, Wood River Valley, South-Central Idaho

Although several technical reports have addressed 

specific areas and technical issues of the Wood River Valley, 

a current, holistic evaluation of water resources in these 

basins is needed to address concerns about current growth 

and development impacts and potential impacts of continued 

growth and development. In 2005, the U.S. Geological 

Survey (USGS) completed a project to compile and review 

existing information and data on the upper Big Wood River 

and Silver Creek basins, to identify gaps in information about 

water resources, and to prepare a workplan with priorities 

for data collection and interpretation to fill these gaps. This 

study is a part of that workplan, and it focuses on the effects 

of population growth on water resources in the Wood River 

Valley.

Purpose and Scope

The objectives of this study are (1) to provide data 

and interpretations about the water resources of the Wood 

River Valley to enable county and local governments to 

make informed decisions about those resources, and (2) to 

recommend additional data collection or studies necessary 

to furnish these data and interpretations to decision makers. 

This report analyzes trends in ground-water and surface-water 

data, documents 2006 hydrologic conditions, and compares 

2006 and historic ground-water data of the Wood River 

Valley of south-central Idaho. Maps of partial-development 

and October 2006 ground-water levels are included for the 

confined and unconfined unconsolidated aquifers, as well as 

of estimated changes in those water levels between the two 

periods.

Previous Work

Smith (1959) published the earliest ground-water level 

map of the Wood River Valley, including Hailey and the 

southern part of the aquifer system. Subsequent maps of 

this area were developed by Castelin and Chapman (1972), 

Moreland (1977), and Brockway and Grover (1978). Luttrell 

and Brockway (1984) mapped ground-water levels in the 

valley south of Oregon Gulch; Frenzel’s (1989) map included 

the valley north of Glendale Road. Small-scale ground-water 

level maps of all or part of the study area were compiled for 

regional studies by Mundorff and others (1964), Maupin 

(1992) Berenbrock and others (1995), and Briar and others 

(1996).

Surface waters of the Big Wood Valley have been 

examined and analyzed by Jones (1952), Smith (1960), 

Castelin and Chapman (1972), Moreland (1977), Brockway 

and Grover (1978), Luttrell and Brockway (1984), Frenzel 

(1989), Brockway and Kahlown (1994), and Wetzstein and 

others (1999). Brown (2000) summarized the information 

regarding ground water and surface water in Brockway and 

Kahlown (1994) and Wetzstein and others (1999). Many of 

these studies have recognized and emphasized the intimate 

association of the ground-water and surface-water systems. 

Description of Study Area

The Wood River Valley of south-central Idaho extends 

from Galena Summit southward to the Timmerman Hills 

(fig. 1). The valley can be separated into upper and lower parts 

along an east-west line immediately south of Bellevue: the 

upper valley is narrow, broadening downstream to a maximum 

width of 2 mi and the lower valley opens into a triangular fan 

(the Bellevue fan) about 9 mi across at the southern end. The 

study area of this report is the combined contributing area 

(about 912 mi2

) for the streamflow-gaging stations and the 

aquifer system within the Wood River Valley extending from 

the boundary with the Sawtooth National Forest southward to 

the Timmerman Hills. The part of the aquifer system evaluated 

was constrained by available data and comprises an area of as 

much as 86 mi2.

The Wood River Valley has a relatively flat bottom 

and land-surface elevations range from about 6,000 ft at the 

northern boundary of the study area to about 4,800 ft at the 

southern boundary. A number of tributary canyons intersect 

the valley, the largest of which are Warm Springs Creek, 

Trail Creek, East Fork Big Wood River, and Croy Creek. The 

main valley and the tributary canyons have steep sides and 

are surrounded by highlands with peaks ranging to more than 

11,000 ft in elevation.

In addition to their different physiographic 

characteristics, the upper and lower valleys also differ in land 

use. The upper Wood River Valley is more developed and 

contains the communities of Sun Valley, Ketchum, Hailey, 

and Bellevue. Land use in the upper valley is predominantly 

housing, with many large homes situated on landscaped 

acreage. The lower Wood River Valley is primarily irrigated 

farms and ranches (irrigated by ground water and diverted 

surface water), and contains the small communities of Gannett 

and Picabo. Although some of the tributary canyons in the 

upper valley, such as Trail Creek and Warm Springs Creek 

have supported development for more than 50 years, more 

recent development has expanded into the valley’s other 

tributary canyons. Three wastewater-treatment plants in the 

study area discharge to the Big Wood River; however, many 

homes rely on septic systems for waste disposal.

Most of the Wood River Valley is drained by the Big 

Wood River or its tributaries, except for the southeastern 

part of the Bellevue fan, which is drained by Silver Creek, 

a tributary to the Little Wood River. Several of the tributary 

canyons to the Big Wood River have perennial streams, 

although most of the streams flow only in response to 

precipitation or snowmelt.
Description of Study Area 3

Figure 1. Location of communities, continuous streamflow-gaging stations, weather stations, and snow-survey sites, Wood River 

Valley, south-central Idaho.

Big Wood River

Little Wood River

Silver

Creek

Big Wood R.

Magic

Lake

East Fk Big Wood River

Warm Springs

Croy Creek

N Fk Big Wood R

Trail Cr

Creek

Little Wood

R

River

Snake

20

20

26

93

26

93

75

75

84

93

Baseline

Rd Glendale Rd

BLAINE

CUSTER

CAMAS

GOODING

ELMORE

ELMORE

BUTTE

BOISE

LINCOLN

Ketchum

Gooding

Gannett Fairfield

Picabo

Hailey

Galena

Richfield

Sun Valley

Ketchum

Gooding Shoshone

Gannett

Bellevue

Fairfield

Picabo

Hailey

Stanton

Crossing

Galena

Richfield

Sun Valley

115° 114°45' 114°30' 114°15' 114° 113°45'

43°

45'

44°

43°

30'

43°

43°

15'

0 5 10 15 20 MILES

0 5 10 15 20 KILOMETERS

River

Malad

13150430

13140500

Sun Valley

Richfield

EXPLANATION

Active stream gaging station

Inactive stream gaging station

Agrimet

Snow course

SNOTEL

Sawtooth National Forest

Study Area Boundary

Upper Wood River and Silver

 Creek Basins Boundary

Active weather station

Inactive weather station Picabo

Ketchum RS

Chocolate Gulch

Projection information: Transverse Mercator, false easting = 

2,500,000, false northing = 1,200,000, central meridian = -114.0, 

scale factor = 0.9996, latitude of origin = 42.0, datum = NAD83; 

roads from Alpine Enterprises, 2004, hydrography from USGS 

NHD, 1999, 1:100,000; shaded relief from USGS 90-m grid, 2002.

13297500

13292380

13150430

13148500

13148200

13147900

13141500

13120500

13120000

13292280

13142500

13142000

13140800

13139500

13297597

13297450

13293350

13293000

13292500

13150500

13150000

13149500 13148000

13146000

13145500

13145000

13144500

13144000

13143000

13141300

13141000

13140500

13138500

13137000

13135500

13151000

13149700

13149000

13143500

13139510

13139000

13138000

13137500

13136500 13136000

Muldoon

Mud

Spring

Ketchum RS

Mount

Baldy

Telfer

Ranch

Copper

Basin

Fishpole

Lake

Couch

Summit #2

Galena

Swede

Peak

Soldier RS

Garfield RS

Bear

Canyon

Stickney

Mill

Hyndman

Creek

Galena

Summit

Smiley

Mountain

Chocolate Gulch

Lost-Wood

Divide

Dollarhide

Summit

Vienna

Mine

Picabo

Agrimet

Fairfield

Agrimet

Carey

Hailey

3NNW

Carey

3 NNW

Fairfield

3SE

Bald Mountain Ketchum

17 WSW

Magic Lake Resort

Galena

Carey

2S

Sun Valley

Galena

Summit

Soldier

Creek

Adm Site

Richfield

Magic Dam

Outflow

Hailey

Friedman

Memorial

Airport

Picabo

Hailey

Ketchum RS

Fairfield RS

Hailey 2

Vienna

Mine

Upper Wood River

and Silver Cr. Basins

IDAHO

Upper Salmon

River Basin

Big Lost

River Basin
4 Water-Resource Trends and Comparisons, Wood River Valley, South-Central Idaho

Hydrogeologic Setting and Framework

The Wood River Valley lies within the Northern Rocky 

Mountain physiographic province (Fenneman, 1931). 

Bedrock highlands of Precambrian metamorphic, Mesozoic 

sedimentary, and Tertiary intrusive and volcanic rocks 

surround the valley. The valley itself is filled with interbedded, 

Quaternary basalts and lacustrine, fluvial, and proglacial 

sediments deposited during late-Pleistocene glaciation. 

Sediments underlying the valley floor in the southern part of 

the Wood River Valley were largely deposited as an alluvial 

fan (the Bellevue fan) with the Big Wood River continually 

shifting and depositing sediment across its surface. Episodic 

volcanic activity disrupted the surface-water drainage pattern; 

after one such eruption created a lava dam, a lake formed over 

the Bellevue fan, depositing fine-grained lacustrine sediments. 

After the dam was breached, deposition of alluvial sediments 

continued until post-glacial climate change caused the Big 

Wood River to incise about 30 ft; resulting in its current 

appearance. However, glaciofluvial sediments deposited 

in the tributary canyons have been largely unaffected by 

the Holocene climate; thus they preserve their Pleistocene 

form. Schmidt (1962) provides a detailed discussion of the 

depositional history of the sediments that comprise the aquifer. 

Ground Water

The aquifer system in the study area, which includes 

unconfined and confined aquifers, is comprised primarily of 

the Quaternary sediments of the Wood River Valley. Because 

the aquifer system is sufficiently productive at shallow depths, 

few wells in the main valley have been drilled through these 

deposits to bedrock. Cross sections by Moreland (1977) 

suggest that the thickness of the Quaternary deposits approach 

500 ft in places, however, in most of the study area the 

deposits are much thinner. In the vicinity of Ketchum, bedrock 

is at a depth of about 100 ft. Castelin and Winner (1975) 

performed a surficial geophysical survey across the Big Wood 

River canyon about 1 mi upstream of its confluence with the 

North Fork Big Wood River, and they concluded that bedrock 

was at a depth of 22–32 ft. Thickness of the unconsolidated 

sediment in the tributary canyons may be as little as 30 ft in 

the Warm Springs Creek drainage. Schmidt (1962) noted that 

“The thickness of the valley-bottom deposits 

averages about 8 feet where tested by drilling on 

lower Rock, Reed, and Brock Creeks.” 

The upper unconfined aquifer is present throughout the 

entire study area and lies directly on bedrock in the upper 

Wood River Valley and on Quaternary lacustrine deposits 

south of Baseline Road. Depth to water in the unconfined 

aquifer is commonly less than 10 ft in the upper valley, 

increasing to about 90 ft southward; in the lower valley they 

range from less than 10 to about 150 ft below land surface.

The lower confined aquifer is only present south of 

Baseline Road and is separated from the overlying unconfined 

aquifer by fine-grained lacustrine deposits present at a 

depth of approximately 150 ft (Moreland, 1977). These 

lacustrine sediments thicken toward the south thus creating a 

confining layer beneath which ground water is under confined 

conditions. In general, as land surface elevation decreases to 

the south the potentiometric surface rises above land surface 

and wells have the ability to flow under artesian pressure. 

Locally (primarily in tributary canyons), the less 

permeable igneous, sedimentary, or metamorphic rocks that 

comprise the bedrock underlying the Quaternary sediments 

of the confined and unconfined aquifers provide water to 

wells either through hydraulic connection or through well 

completions directly in these rocks. Currently (2006) little is 

known about the water-bearing characteristics of these units 

because relatively few wells have been completed in them. 

As additional wells are drilled into these units and more 

information becomes available further assessment will be 

possible. However, some of these rocks are the source of water 

to the geothermal springs located on the west side of the Wood 

River Valley in tributary canyons (Street, 1990); consequently 

water quality may be poor.

Because the ground-water and surface-water systems 

are closely linked in the Wood River Valley, a ground-water 

level map alone cannot describe ground-water conditions. 

Additional information is required, such as locations of 

seepage to and from streams in the Wood River Valley.

Brockway and Grover (1978) used the terms “Big WoodSilver Creek aquifer system” and “Big Wood-Silver Creek 

aquifer” interchangeably in referring to the unconfined and 

confined aquifers within the valley fill, although they did not 

explicitly define either term. This nomenclature has not been 

used subsequently; instead the aquifer typically has been 

referred to by the hydrologic condition in a given location 

(confined or unconfined). Thus, for clarification, the aquifer 

system of the Wood River Valley is here informally named the 

Wood River Valley aquifer system. It includes the Quaternary 

sediments of the Wood River Valley and its tributaries and 

locally, underlying igneous, sedimentary, or metamorphic 

rocks where they are hydraulically connected and used for 

water supply. The Wood River Valley aquifer system is 

comprised of a single unconfined aquifer and an underlying 

confined aquifer present south of Baseline Road. 

Surface Water

Most of the Wood River Valley is drained by the Big 

Wood River or its tributaries, except for the southeastern part 

of the Bellevue fan, which is drained by Silver Creek. The 

Big and Little Wood Rivers meet near Gooding, about 35 mi 

southwest of the study area, where they become the Malad 

River, a tributary to the Snake River. The Big Wood River 
Description of Study Area 5

originates near Galena Summit, about 20 mi northwest of 

Ketchum, and it gains flow from a number of perennial and 

intermittent tributaries. It meanders across the narrow upper 

valley until Bellevue, where it flows along the western side of 

the Bellevue fan, finally exiting the valley at Stanton Crossing. 

Fed by springs and seeps, Silver Creek and its tributaries 

originate on the Bellevue fan and flows out of the valley at 

Picabo. Most of the tributary canyons to the Big Wood River 

are intermittent and flow only in response to precipitation or 

snowmelt, however, Trail Creek, North Fork Big Wood River, 

East Fork Big Wood River, Warm Springs Creek, Croy Creek, 

and Deer Creek typically flow into the Big Wood River yearround. Streams in some of the tributary canyons are perennial 

in their upper reaches—some of this flow likely infiltrates 

directly into the Wood River Valley aquifer system or reaches 

the Big Wood River by subsurface flow through streambed 

gravels. A network of irrigation canals and drains exists 

throughout the study area; most of the Wood River Valley was 

under irrigation by 1900 (Jones, 1952). The irrigation system 

complicates the interpretation of streamflow measurements on 

the Big Wood River because most of the numerous irrigation 

diversions and returns between stream gages are ungaged; 

seepage into and out of irrigation canals and ditches further 

affects such interpretation. 

The USGS has operated 44 continuous-record 

streamflow-gaging stations within a 25-mi buffer of the study 

area, including 9 stations in the Upper Salmon River basin 

and 2 stations in the Big Lost River basin; the remaining 33 

stations are within the Wood River basin (fig. 1). Of these 

44 stations, 13 were active in 2006. Within or immediately 

adjacent to the study area itself there have been 13 stations, 

three of which were active in 2006: Big Wood River at 

Hailey (13139500), Big Wood River at Stanton Crossing near 

Bellevue (13140800), and Silver Creek at Sportsman Access 

near Picabo (13150430) (table 1). Of the 13 stations within the 

study area, 6 were operated only during water years 1920–21. 

Thus, only 7 stations have at least 10 years of streamflow data. 

Warm Springs Creek at Guyer Hot Springs near Ketchum 

(13136500) is the only tributary to the Big Wood River gaged 

for more than 2 years.

Differing periods of stream gage operation, along with 

irrigation diversions and returns, complicate the comparison 

of streamflow between gaging stations. For instance, the Big 

Wood River at Stanton Crossing near Bellevue gaging station 

(13140800) replaced the Big Wood River near Bellevue 

gaging station (13141000) in 1996, however, the two “are 

not equivalent because of inflow between sites” (Brennan 

and others, 2005). Thus, caution was exercised in comparing 

streamflow statistics for the 8 gaging stations with at least 

10 years of streamflow data (table 2). 

Table 1. Continuous record streamflow-gaging stations within a 25-mile buffer of Wood River Valley, south-central Idaho.

[Station numbers shown in bold type were evaluated for the study; –, not applicable]

Gaging 

station No. Gaging station name Latitude Longitude Period of record (water years)

13120000 North Fork Big Lost River at Wild Horse, near Chilly 43°55'58"N 114°06'50"W 1944–present

13120500 Big Lost River at Howell Ranch, near Chilly 43°59'54"N 114°01'16"W 1904–14, 1920-present

13135500 Big Wood River near Ketchum 43°47'11"N 114°25'27"W 1948–72

13136000 Big Wood River at Ketchum 43°41'19"N 114°22'24"W 1920–21

13136500 Warm Springs Creek at Guyer Hot Springs, near Ketchum 43°41'00"N 114°24'37"W 1941–58

13137000 Warm Springs Creek near Ketchum 43°40'58"N 114°24'24"W 1920–21

13137500 Trail Creek at Ketchum 43°40'40"N 114°21'40"W 1920–21

13138000 East Fork Big Wood River at Gimlet 43°36'12"N 114°20'21"W 1920–21

13138500 Big Wood River at Gimlet 43°35'51"N 114°20'49"W 1920–21

13139000 Big Wood Slough at Hailey 43°31'05"N 114°19'12"W 1915–74

13139500 Big Wood River at Hailey 43°31'02"N 114°19'18"W 1889, 1915-present

131395101 Combination Big Wood River/Slough at Hailey 43°31'05"N 114°19'10"W –

13140500 Big Wood River at Glendale Bridge, near Bellevue 43°25'35"N 114°15'39"W 1920–21

13140800 Big Wood River at Stanton Crossing, near Bellevue 43°19'45"N 114°19'09"W 1996–present

13141000 Big Wood River near Bellevue 43°19'41"N 114°20'30"W 1911–96

13141300 Mormon Reservoir near Fairfield 43°16'50"N 114°48'02"W 1985–87

13141500 Camas Creek near Blaine 43°19'58”N 114°32'31"W 1912–21, 1923–25, 

1926–present

13142000 Magic Reservoir near Richfield 43°15'19"N 114°21'29"W 1909–present
6 Water-Resource Trends and Comparisons, Wood River Valley, South-Central Idaho

Table 2. Mean annual and annual peak flow from selected streamflow-gaging stations in and adjacent to Wood River Valley, southcentral Idaho.

[Location of gaging stations is shown in figure 1. Abbreviations: ft3/s, cubic foot per second]

Gaging 

station No. Gaging station name Water years

Mean annual flow (ft3

/s) Annual peak flow (ft3/s)

Maximum Year Minimum Year Mean Maximum Year Minimum Year Mean

13135500 Big Wood River near Ketchum 1948-72 270 1965 86 1961 167 357 1956 181 1941 261

13136500 Warm Springs Creek at Guyer 

Hot Springs, near Ketchum

1941-58 139 1943 55 1955 87 961 1958 275 1945 521

13139000 Big Wood Slough at Hailey 1915-74 190 1921 4 1972 78 419 1921 14 1973 168

13139500 Big Wood River at Hailey 1889, 

1915-present

842 1983 70 1931 403 6,150 1983 545 1934 2,527

13140800 Big Wood River at Stanton Crossing, 

near Bellevue

1996-present 723 1997 47 2001 272 4,670 1997 287 2001 2,524

13141000 Big Wood River near Bellevue 1915-96 763 1983 49 1992 300 5,310 1983 119 1934 1,861

13150430 Silver Creek at Sportsman Access, 

near Picabo

1974-present 222 1983 106 2003 150 566 1985 158 1988 325

13150500 Silver Creek at Highway 20, 

near Picabo

1920-62 192 1952 110 1961 159 357 1956 181 1941 261

Gaging 

station No. Gaging station name Latitude Longitude Period of record (water years)

13142500 Big Wood River below Magic Dam, near Richfield 43°14'53"N 114°21'20"W 1911–present

13143000 Lincoln Canal near Richfield 43°10'27.56"N 114°19'07.5"W 1925–48

13143500 Lincoln Canal near Shoshone 43°04'49.15"N 114°18'33.32"W 1925–48

13144000 Big Wood River above North Gooding Canal near Shoshone 43°05'27.54"N 114°17'46.19"W 1921–25, 1927, 1932–33, 1938

13144500 Big Wood River below North Gooding Canal near Shosone 43°04'12.25"N 114°18'28.04"W 1921–28, 1930, 1932–33, 1938

13145000 Big Wood River near Shoshone 42°59'36.58"N 114°27'16.61"W 1908–13

13145500 Big Wood River above Thorn Creek near Gooding 43°00'39"N 114°36'31.75"W 1926–27

13146000 Thorn Creek spillway near Gooding 43°01'27.17"N 114°36'34.16"W 1928–48

13147900 Little Wood River at High Five Creek, near Carey 43°29'30"N 114°03'30"W 1958–74, 1979–present

13148000 Little Wood River at Campbell Ranch, near Carey 43°27'40"N 114°02'50"W 1920–26, 1937–38, 1941–43, 

1944–58

13148200 Little Wood River Reservoir, near Carey 43°25'31"N 114°01'38"W 1955–present

13148500 Little Wood River near Carey 43°23'24"N 113°59'59"W 1904–05, 1926–42, 

1943–present

13149000 Fish Creek at Fish Creek Dam, near Carey 43°26'12"N 113°48'29"W 1920–39

13149500 West Fork Fish Creek near Carey 43°26'44"N 113°49'54"W 1920–29

13149700 Fish Creek Reservoir near Carey 43°25'21"N 113°49'36"W 1985–87

13150000 Fish Creek near Carey 43°25'12"N 113°49'45"W 1919–20, 1923–39

13151000 Little Wood River near Richfield 43°02'51.21"N 114°07'55.2"W 1911–73

13150430 Silver Creek at Sportsman Access, near Picabo 43°19'20"N 114°06'24"W 1974–present

13150500 Silver Creek at Highway 20, near Picabo 43°17'01"N 114°01'04"W 1920–62

13292280 Salmon River at Pole Creek Road, above diversions, 

near Obsidian

43°54'08"N 114°47'25"W 2003–present

13292380 Pole Creek below Pole Creek Ranger Station, near Obsidian 43°54'36"N 114°45'23"W 2003–present

13292500 Salmon River above Lost Creek, near Obsidian 43°57'57"N 114°48'01"W 1941–53

13293000 Alturas Lake Creek near Obsidian 43°56'55"N 114°50'00"W 1941–52

13293350 Fourth of July Creek above diversions, near Obsidian 44°02'26"N 114°45'21"W 2003–04

13297450 Little Boulder Creek near Clayton 44°05'57"N 114°26'50"W 1970–86

13297500 Big Boulder Creek near Clayton 44°06'55"N 114°26'29"W 1926–30, 2005

13297597 Herd Creek below Trail Gulch, near Clayton 44°08'50"N 114°17'28"W 1980–85

1

This station was renamed Big Wood River at Hailey (13139500).

Table 1. Continuous record streamflow-gaging stations within a 25-mile buffer of Wood River Valley, south-central Idaho.—Continued

[Station numbers shown in bold type were evaluated for the study; –, not applicable]
Description of Study Area 7

Climate

The Wood River Valley has a mild, arid climate during 

the summer months, but cold, wet conditions in the winter. 

About 60 percent of the total annual precipitation falls 

between the first of November and the end of March mostly as 

snow. The growing season in the study area varies widely with 

elevation, ranging from about 3 months at Sun Valley to about 

5 months in the lower valley.

The entire study area is classified as Dsb under a 

modified Köppen system in which D indicates a mean 

temperature of the warmest month greater than 10°C (40°F) 

and a mean temperature of the coldest month 0°C (32°F) or 

below; s indicates that precipitation in the driest month is less 

than 1.6 in. (40 mm) and less than one-third of the amount in 

the wettest month; and b indicates that the mean temperature 

of each of the four warmest months is 10°C (40°F) or greater 

and the mean temperature of the warmest month is below 

22°C (72°F) (Critchfield, 1983; Godfrey, 2000).

The National Weather Service (NWS) has 21 weather 

stations within a 25-mi buffer of the study area, although 

only 8 are active (table 3, fig. 1). Of these 8 stations, only 2 

are within or immediately adjacent to the study area and have 

sufficient long-term data for climatic calculations: Ketchum 

Ranger Station and Picabo. As of 2006, there were two active 

AgriMet stations located in or within 25 mi of the study area: 

Picabo, Idaho (PICI) and Fairfield, Idaho (FAFI) (Bureau of 

Reclamation, 2006). In addition, 21 active Natural Resources 

Conservation Service snow-survey sites are located in or 

within 25 mi of the study area; 13 of these are instrumented 

SNOTEL sites and 8 are snow courses (Natural Resources 

Conservation Service, 2006).

Table 3. National Weather Service stations, AgriMet stations, and snow-survey sites in or near the Wood River Valley, 

south-central Idaho.

[Western Regional Climate Center (2006); Bureau of Reclamation (2006); Natural Resources Conservation Service (2006); Abbreviations:

NWS, National Weather Service; RS, Ranger Station; –, to be measured in 2007; **, unknown]

Station name Station or 

identification No. 

Elevation 

(feet above 

sea level)

Latitude Longitude

Dates in operation

Starting date Ending date

NWS stations

Bald Mountain 100540 8,701 43°39'N 114°24'W 12-01-53 02-17-77

Carey 101472 4,783 43°17'N 113°56'W 09-01-68 11-13-84

Carey 2S 101461 4,764 43°17'N 113°57'W 07-01-56 04-30-58

Carey 3 NNW 101466 4,843 43°21'N 113°57'W 10-23-62 09-20-68

Fairfield 3SE 103110 5,020 43°19'N 114°45'W 10-01-78 08-19-98

Fairfield RS 103108 5,070 43°21'N 114°48'W 10-01-48 –

Galena Summit 103422 8,806 43°53'N 114°42'W 09-01-63 09-30-76

Galena 103417 7,300 43°52'N 114°39'W 09-01-63 03-31-96

Hailey 103940 5,312 43°31'N 114°18'W 01-12-05 –

Hailey 2 103947 5,295 43°31'N 114°19'W 03-01-66 –

Hailey 3NNW 103942 5,420 43°34'N 114°20'W 08-01-48 11-31-88

Hailey Friedman Memorial Airport 94161 5,306 43°30'N 114°18'W 06-01-53 –

Ketchum 17 WSW 104840 8,425 43°37'N 114°41'W 08-01-54 03-31-75

Ketchum RS 104845 5,890 43°41'N 114°21'W 05-01-73 –

Magic Dam Outflow 105513 4,665 43°15'N 114°22'W 08-01-76 –

Magic Lake Resort 105510 4,803 43°17'N 114°23'W 06-01-66 10-10-75

Picabo 107040 4,880 43°18'N 114°04'W 04-22-58 –

Richfield 107673 4,310 43°04'N 114°09'W 08-01-48 –

Sun Valley 108906 5,980 43°42'N 114°21'W 08-01-48 12-31-72

AgriMet stations

Picabo PICI 4,900 43° 19'N 114°10'W 04-21-93 –

Fairfield FAFI 5,038 43° 18'N 114°50'W 06-25-87 –
8 Water-Resource Trends and Comparisons, Wood River Valley, South-Central Idaho

Eight active and inactive NWS stations provide sufficient 

data for calculation of long-term means (table 4). The 

remaining 13 stations were not included because of partial, 

missing, or incomplete data or because only precipitation data 

were collected. Mean annual air temperatures at the 8 included 

stations range from 35.0°F at Galena to 45.6°F at Richfield 

(table 4). The coldest month in the area is January, with mean 

low air temperatures ranging from 0.3°F at Sun Valley to 

13.2°F at Richfield. The warmest month typically is July, with 

mean high air temperatures ranging from 76.3°F at Galena to 

87.1°F at Richfield. Mean first-freeze dates range from August 

21 at Galena to October 15 at Richfield; mean last-freeze 

dates range from July 1 at Galena to April 27 at Richfield. 

Mean annual precipitation ranges from 11 in. at Richfield 

to about 25 in. at Galena. July and August typically are the 

driest months; December and January are the wettest. The 

greatest snow depth typically is in February, ranging from 7 in. 

at Richfield to 45 in. at Galena (Western Regional Climate 

Center, 2006.)

In December 1972, the Sun Valley station was 

discontinued; data collection was begun at the Ketchum 

Ranger Station in May 1973 (table 3). For the statistical 

analysis of trends described in section, “Ground-Water Level 

Trends,” precipitation from these two stations was combined 

to create a continuous series dating from 1948 to 2006. The 

NWS follows this practice in precipitation data retrieved from 

the National Climatic Data Center. 

Station name Station or 

identification No. 

Elevation 

(feet above 

sea level)

Latitude Longitude

Dates in operation

Starting date Ending date

Snow survey sites

Chocolate Gulch1 14F19s 6,440 43o46'N 114o25'W 1994 –

Couch Summit #22 14F18 6,840 43o31'N 114o48'W 1961 –

Dollarhide Summit1 14F08s 8,420 43o36'N 114o40'W 1982 –

Galena1 14F17s 7,470 43o53'N 114o40'W 1982 –

Galena Summit1 14F12s 8,780 43o52'N 114o43'W 1982 –

Garfield RS1 13F04s 6,560 43o37'N 113o56'W 1981 –

Hyndman Creek1 14F16s 7,440 43o43'N 114o10'W 1981 –

Ketchum RS2 14F21 5,890 43o45'N 114o21'W ** –

Lost-Wood Divide1 14F03s 7,900 43o49'N 114o16'W 1982 –

Mount Baldy2 14F09 8,900 43o40'N 114o24'W 1949 –

Muldoon2 13F05 6,320 43o34'N 113o55'W 1953 –

Soldier RS1 14F11s 5,740 43o29'N 114o50'W 1987 –

Stickney Mill1 14F02s 7,430 43o52'N 114o13'W 1981 –

Swede Peak1 13F09s 7,640 43o38'N 113o58'W 1982 –

Vienna Mine1 14F04s 8,960 43o48'N 114o51'W 1981 –

1

SNOTEL site.

 2Snow-course site.

Table 3. National Weather Service stations, AgriMet stations, and snow-survey sites in or near the Wood River Valley, 

south-central Idaho.—Continued.

[Western Regional Climate Center (2006); Bureau of Reclamation (2006); Natural Resources Conservation Service (2006); Abbreviations:

NWS, National Weather Service; RS, Ranger Station; –, to be measured in 2007; **, unknown]

Although drought can be defined in many different 

ways (meteorological, hydrological, or agricultural), one 

measure used to quantify drought is the Palmer Drought 

Severity Index (PDSI), a measure of long-term drought that 

uses precipitation, temperature, soil moisture, and other 

factors. The index accounts for long-term trends to define 

wet and dry periods, thus limiting its use in the most recent 

record. The PDSI uses zero as normal, negative numbers to 

represent drought, and positive numbers to represent abovenormal precipitation. The National Climatic Data Center 

calculates the PDSI (and other drought indices) for States 

by climate division; most of the Wood River Valley is within 

Idaho climate zone 4 (or Central Mountains division; U.S. 

Department of Commerce, 2006). The PDSI for Idaho climate 

zone 4 from January 1900 through October 2006 is shown 

in figure 2. The 241 months between October 1986 and 

October 2006, are characterized by two long periods of drier 

than normal conditions interrupted by four periods of wetter 

than normal conditions. Of these 241 months, 14 had PDSI 

values less than -0.5 and greater or equal to -1, indicating 

drier than normal conditions, 158 experienced mild to extreme 

drought conditions (less than -1), and 53 months experienced 

slightly to very wet conditions (greater than 0.5) with the 

remaining months in the normal range. The range of PDSI 

values is 3.69 (January 1997) to -4.89 (February 1988). The 

average PDSI value for these 241 months is -1.58.
Description of Study Area 9

-8

-6

-4

-2

0

2

4

6

8

10

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

PALMER DROUGHT SEVERITY INDEX: IDAHO CLIMATE ZONE 4

Current

drought

Early 1990s

drought

1930s

drought

YEAR

PALMER DROUGHT SEVERITY INDEX (PDSI) VALUE

EXPLANATION

-3 to -2: Moderate drought

-2 to -1: Mild drought

-4 or less: Extreme drought -1 to -0.5: Incipient dry spell 

-4 to -3: Severe drought -0.5 to 0.5: Near normal

0.5 to 1: Incipient wet spell

1 to 2: Slightly wet

2 to 3: Moderately wet

3 to 4: Very wet

4 or more: Extremely wet

Figure 2. Palmer drought severity index for Idaho climate zone 4.

Table 4. Summary of data from selected weather stations in and near the Wood River Valley, south-central Idaho.

[Western Regional Climate Center (2006). Summary values are through December 31, 2005, for active stations. Abbreviations: °F, degree Fahrenheit; in., 

inches; max., maximum, min., minimum; RS, Ranger Station; *, inactive station]

Station name

Mean July 

maximum

air

temperature

(°F)

Mean January 

air temperature (°F)

Mean 

annual air 

temperature

(°F)

Mean 

annual 

precipitation 

(in.)

Mean 

annual 

snowfall 

(in.)

Mean 

February 

snow 

depth 

(in.)

Mean freeze date

Minimum Maximum Last First

Fairfield 3SE * 82.3 1.9 27.7 39.9 12.6 50.8 14 05-11-06 09-19-06

Fairfield RS 85.5 5.1 28.9 41.5 15.0 69.8 19 05-05-06 09-22-06

Galena * 76.3 4.4 31.6 35.0 24.7 183 45 07-01-06 08-21-06

Hailey 3NNW * 84.8 7.8 30.2 43.0 15.9 81.1 23 05-03-06 10-13-06

Ketchum RS 80.2 5.5 31.2 39.5 18.5 114 27 05-22-06 09-20-06

Picabo 86.3 7.5 30.9 42.6 12.9 42.9 8 05-05-06 10-03-06

Richfield 87.1 13.2 30.8 45.6 11.0 32.4 7 04-27-06 10-15-06

Sun Valley * 82.5 .3 30.0 38.0 17.3 121 29 05-31-06 09-03-06
10 Water-Resource Trends and Comparisons, Wood River Valley, South-Central Idaho

When the 92 months from March 1999 through October 

2006, the current drought, are compared to the PDSI values 

for the 20th century, they are matched in duration and severity 

by only the well-known drought of the 1930s (fig. 2). The 

1930s drought consisted of three periods of drier than normal 

conditions and two periods of wetter than normal conditions. 

For the 14-year period between 1928 and 1941 (168 months), 

the PDSI was less than -0.5 and greater or equal to -1 for 

9 months and 109 months were less than -1. Twenty-three 

months were greater than 0.5 with the remaining 27 months 

were in the normal range.

Population

The population in the study area was about 15,000 in 

2000 (census tracts 9601, 9602, and 9603), which is about 

78 percent of the population of Blaine County (U.S. Census 

Bureau, 2006). The population of Blaine County increased 

about 11.5 percent between April 1, 2000 and July 1, 2005 

4,900

8,387

4,473

3,768

5,295 5,384

4,598

5,749

9,841

13,552

18,991

21,166

0

5,000

10,000

15,000

20,000

25,000

POPULATION

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2005

YEAR

Figure 3. Blaine County population by decennial census, 1900–2000, and projected 2005 population, south-central Idaho

as compared to a growth rate of 10.4 percent for the State 

of Idaho as a whole (U.S. Census Bureau, 2007). Campbell 

(1996) forecast the total population of Idaho to increase 

between 20 and 40 percent between 2005 and 2025.

Because tract level census data for Blaine County does 

not exist for much of the 20th century, total population in 

Blaine County was used to show population growth by 

decennial census for 1900–2005 in figure 3 (Forstall, 1995; 

U.S. Census Bureau, 2007). These data show a relatively 

stable Blaine County population through 1970 and hence a 

relatively stable period of water withdrawal and consumption 

although water use for mining, power generation, and 

irrigation may have varied. This period is considered to 

represent predevelopment conditions. Population then rapidly 

increased after 1970, with a 171-percent increase between 

1970 and 1980. This figure accounts for 27 percent of the 

total population growth between 1970 and 2005. The largest 

increase in population occurred between 1990 and 2000. This 

report considers the time period up to the mid-1980s to be 

partial-development before the continuing development of the 

Wood River Valley through 2005. 
Methods and Data Sources 11

Methods and Data Sources

Ground-Water Level Trend Analysis

Water levels were measured semi-annually in three 

wells in the Wood River Valley for more than 50 years. 

Mean annual water levels in these wells were evaluated 

with the nonparametric Mann-Kendall test to determine any 

statistically significant trends in water levels. If the Kendall’s 

S statistic is significantly different from zero, it may be 

concluded that there is a monotonic trend in water levels over 

time. Temporal trends in ground-water levels were considered 

to be statistically significant if the confidence level was at least 

99.5 percent and the p-value (or level of significance) was less 

than 0.005. A detailed discussion of this technique is given in 

Helsel and Hirsch (1992).

Mean annual depth to water and total annual precipitation 

at the Ketchum Ranger Station were normalized using 

equations 1 and 2 to allow direct comparison between the two 

datasets in a plot.

d y y i i = − , (1)

N

d

D

d i

y

i

i

i

= 

where equals the sample deviation of

is the

,

,

sample in the series of samples,

is the mean of sa

i n

y n mples,

is the normalized value of sample ,

is the sampl

N i

di e deviation of sample , and

 is the maximum absolute va

i

D lue of d in the series.

(2)

Streamflow Trend Analysis

Long-term trend analysis was performed on three gaging 

stations in the Wood River Valley that have at least 20 years 

of record. Mean annual and monthly streamflow trends were 

analyzed, as well as 7-day and 30-day low-flow values for the 

period of record. The Base Flow Index (BFI), derived by a 

systematic method of calculating the volume of base flow for 

annual data, also was analyzed for any statistically significant 

trends. Long-term trends in precipitation and temperature 

were determined and were compared with streamflow data to 

account for anomalies.

Kendall’s tau was used to test for a significant trend for 

all of the flow statistics with all of the p-values provided in 

table 5. In addition, the magnitude of the trend was estimated 

using Sen’s Estimate of Slope, also provided in table 5. 

The Kendall’s tau nonparametric hypothesis test measures 

monotonic correlation strength between two datasets (Helsel 

and Hirsch, 1992). In these cases, the two datasets are the flow 

statistic (mean annual and monthly discharge, 7- and 30-day 

low-flow values, and the BFI) and time. A strong relation 

between data sets indicates an increase or decrease in flow 

over time. The calculated p-value is the probability that the 

null hypothesis (no trend in data) is in fact true; thus statistical 

significance increases as p-values decrease. Surface-water 

trends were judged statistically significant at the 90-percent 

confidence level.

The 7- and 30-day low-flow statistics were calculated 

using a process that combines the U.S. Environmental 

Protection Agency (USEPA) BASINS program (Aquaterra 

Consultants, 2006; U.S. Environmental Protection Agency, 

2006), S-Plus statistical software (Insightful Corp., 2005), and 

the USGS statistical library for S-Plus. This process calculates 

the annual low flow from the minimum value of an n-day 

moving average of daily streamflow. The annual minima are 

then plotted over the period of record to identify long-term 

trends for that site.

The BFI is calculated as the ratio of the volume of base 

flow to the volume of total flow for a given time period. It 

was developed by the Institute of Hydrology of the United 

Kingdom (1980a, 1980b) in an attempt to standardize baseflow calculations for reproducibility. This method was further 

improved and automated by Wahl and Wahl (1988) for use 

on unregulated streams in the United States. Traditional 

calculations originally developed for this application are based 

on a 5-day interval local minima slope estimate. For this study, 

a longer duration interval was used to account for large spring 

runoff events in the snowmelt-dominated streams of the Wood 

River Valley that typically occur over a several week period.

BFI calculations are made by an algorithm that identifies 

local hydrograph minima at a user-defined interval. As 

initially developed for rainfall-dominated basins in the United 

Kingdom and Midwestern United States, the interval was 

taken at 5 days. Turning points are calculated by a threepoint linear regression when a user-defined threshold slope is 

exceeded, commonly taken as 10-percent difference. The baseflow hydrograph is then created by connecting the turning 

points and integrating the area below the line. 

The BFI calculations for the Big Wood River at Hailey 

gaging station data (13139510) were based on a 30-day 

interval over a year to identify base flow of a snowmeltdriven system as opposed to one dominated by rainfall. The 

Big Wood River near Bellevue (13141000) gage data used a 

40-day interval for the BFI calculation and the Silver Creek at 

Sportsman Access (13150430) gage data used a 5-day interval. 

A shorter interval period, such as the default 5-day period, 

will introduce “false peaks” during springtime flows from 

melting snowpack for the Big Wood River gages. Although 

these longer intervals may lead to slight underestimation of 

base flow for the increased springtime runoff, it provides a 

consistent and reproducible approach for the analysis of longterm datasets. 
12 Water-Resource Trends and Comparisons, Wood River Valley, South-Central Idaho

Low-flow trends

Gaging station Period p-value Slope

Big Wood River at Hailey Total 7-day 0.117 0.001

30-day .097 .001

Big Wood River near Bellevue 7-day .084 -.263

30-day .076 -.319

Silver Creek at Sportsman Access 7-day .074 -.875

30-day .101 -.897

BFI annual trends 

Gaging station p-value Slope

Big Wood River at Hailey Total 0.200 0.001

Big Wood River near Bellevue .905 .000

Silver Creek at Sportsman Access .184 -.170

Annual trends

Gaging station p-value Slope

Big Wood River at Hailey Total 0.601 0.339

Big Wood River near Bellevue .611 -.936

Silver Creek at Sportsman Access .018 -1.627

Table 5. Surface-water trend analyses for three streamflow-gaging stations, Wood River Valley, south-central Idaho.

[All p-values are derived from Kendall’s tau with the Sen slope estimator. Numbers in bold are greater than 90 percent statistically significant. 

Slopes with negative values represent declining trends and positive values show increasing trends]

Monthly trends

Gaging station Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Big Wood River at 

Hailey Total

p-value 0.123 0.183 0.006 0.845 0.864 0.626 0.626 0.237 0.194 0.268 0.219 0.663

Slope .216 .175 .538 .180 .444 2.000 .704 .543 .431 .311 .253 .079

Big Wood River near 

Bellevue

p-value .006 .095 .908 .773 .941 .337 .494 .458 .615 .221 .223 .014

Slope -.485 -.364 -.057 .333 .230 2.843 .479 .167 .083 -.293 -.490 -.700

Silver Creek at 

Sportsman Access

p-value .002 .051 .308 .010 .773 .197 .010 .014 .007 .001 .003 .007

Slope -1.167 -1.332 -.700 -1.792 -.200 -.895 -2.043 -2.407 -3.200 -3.987 -2.523 -1.867
Methods and Data Sources 13

Ground-Water Levels

To estimate water-level changes in the Wood River Valley 

aquifer system between historic and current conditions (2006), 

one map representing each set of conditions was needed. 

Historic or predevelopment conditions refer to the period 

before 1970 with a relatively stable population (fig. 3), water 

withdrawals, and consumption. However, this study uses 

ground-water level data from 1970 to1986 to fill spatial data 

gaps in the study area and to coincide with previous studies for 

comparison. For purposes of this report, partial development 

refers to the period including predevelopment conditions to 

1986. Current (2006) conditions were based on water levels 

measured in 98 wells in the study area during October 23–27, 

2006. Maps were constructed of partial development and 

October 2006 conditions for the unconfined and confined 

aquifers. These maps were then used to develop the maps 

showing water-level changes between the two time periods. 

In addition, streamflow was measured at 13 sites concurrently 

with the October 2006 water-level measurements to identify 

gaining and losing stream reaches. Finally, stream discharge 

and ground-water levels were analyzed for trends.

Partial Development Ground-Water Levels

Historic ground-water levels used for the partialdevelopment ground-water level maps were obtained from 

the USGS National Water Information System (NWIS) and 

the Idaho Department of Water Resources (IDWR) wellinformation and ground-water level databases. Water-level 

measurements stored in these databases were made at different 

dates throughout the year and over a range of years. However, 

there were insufficient ground-water level measurements 

during October of a single year to represent the entire Wood 

River Valley aquifer system. Therefore, it was necessary to use 

water levels from multiple years and months.

The decennial census data shown in figure 3 shows a 

relatively stable Blaine County population through 1970; 

ideally, water levels measured prior to 1970 would be used 

to construct a predevelopment map representing stable 

ground-water withdrawal and consumption rates. However, 

there are insufficient pre-1970 water-level measurements to 

adequately represent the entire Wood River Valley aquifer 

system. Consequently, water-level data collected as recently 

as 1986 was selectively used to expand map coverage into 

recently developed areas. Because these later water-level 

data were collected after significant development began, 

the conditions represented are more properly described as 

partial development. Despite this broad definition of partial 

development, historic data still were unavailable for some 

tributary canyons, and thus are not included on the partial 

development ground-water level maps. In addition, despite the 

desirability of comparing water levels collected in October 

of partial-development years to October 2006 levels, groundwater levels measured during August through December 

were used to construct the partial-development maps because 

an insufficient number of historic October measurements 

were available. The final set of historic ground-water level 

measurements contained historic (1952-1986) water levels for 

242 wells: 206 wells in the unconfined aquifer and 36 wells in 

the confined aquifer (appendix A).

The compromises described here are often necessary in 

the analysis of past ground-water conditions. They are made 

assuming that monthly and annual water-level fluctuations 

were less than 10 ft prior to 1986, an assumption discussed 

in more detail in sections, “Estimating Water-Level Change” 

and “Annual and Long-Term Fluctuation of Water Levels.” 

Because streamflow in the area is intimately related to groundwater levels, it integrates upgradient ground-water conditions 

and is thus another indicator of water-table variation when 

used in conjunction with the maps. It should also be cautioned 

that, except where historic and October 2006 water-level 

measurements coincide, the maps of water-table change 

indicate broad conditions and may not be suitable for precise 

site-specific interpretation of change. 

The 242 wells used for the partial-development maps 

were recorded in the NWIS database with elevation accuracies 

ranging from 0.01 to 20 ft, depending on whether the elevation 

was determined from topographic maps, level surveys from 

benchmarks, or Global Positioning System surveys. The 

IDWR well-information database does not contain elevation 

accuracies for the wells it contains. Elevations for the 

wells used to construct the partial-development maps were 

determined from 10-meter digital elevation models (DEMs) 

for all wells with an elevation accuracy greater than 1 ft. 

There were 26 coincident wells between the 2006 and partialdevelopment ground-water levels. For these wells, the Global 

Positioning System (GPS) surveyed elevations were used. The 

2006 surveying methods and accuracy are discussed in section, 

“Survey of Well Locations.”

The GPS-surveyed elevations, the NWIS-listed well 

elevations, and well elevations derived from 10-meter 

DEMs were compared for the 98 wells measured in October 

2006 in order to qualitatively evaluate the accuracy of the 

water-level changes determined by the GIS methods used to 

generate the change maps (appendix A). Overall, the average 

difference between the DEM-derived elevation and the 

GPS-surveyed elevation of the 98 wells was smaller than the 

average difference between the NWIS-listed well elevation 

and GPS-surveyed elevation. The mean, median, and range 

for the GPS/DEM elevation comparison were 5.1, 2.8, and 

26.8 ft, respectively. For the GPS/NWIS elevation comparison, 

the mean, median, and range were 15.3, 3.1, and 1,000 ft, 

respectively. The NWIS elevations had one obvious outlier. 

However, if the outlier is removed, then the mean, median, 

and range for the GPS/NWIS comparison for the remaining 97 

wells are still greater than that of the GPS/DEM comparison. 
14 Water-Resource Trends and Comparisons, Wood River Valley, South-Central Idaho

Of the 98 GPS-surveyed wells, 18 were listed in NWIS with 

elevation accuracies of 1 ft or less. The comparisons of just 

these higher accuracy elevations in NWIS with the DEM 

and GPS derived elevations resulted in the DEM comparison 

having a mean, median, and range of 1.7, 1.2, and 7.4 ft, 

respectively, which are larger than the NWIS comparison. 

October 2006 Ground-Water-Level Measurement

Water levels in 98 wells in the Wood River Valley study 

area were measured by personnel from the USGS during 

October 23-27, 2006. In non-flowing wells, ground-water 

levels were measured using either steel tapes or calibrated 

electric tapes. In flowing wells, water levels were measured 

with a calibrated digital pressure gage. Of the 98 wells, 

10 wells were completed in the confined aquifer, and the 

remaining wells were completed in the unconfined aquifer.

Well locations were selected to represent groundwater levels throughout the Wood River Valley, but parts of 

the study area lacking wells are inadequately represented. 

Although an emphasis was made to find wells located in the 

tributary canyons in the northern part of the study area and the 

transitions of the canyons with the main Wood River Valley, 

these canyons often contained few available wells.

Survey of Well Locations

Elevations of the 98 wells were measured using a 

combination of real-time kinematic (RTK) and fast-static 

differential GPS-surveying techniques. Trimble 5700 and 5800 

GPS systems were used for the surveying. In areas with poor 

satellite reception, a nearby location with good reception was 

selected, and a Topcon GTS-312 electronic total station was 

used to survey the well location relative to the GPS-surveyed 

location. GPS surveying data were processed with Trimble 

Geomatics Office software (Trimble Navigation Ltd., 2007), 

and any well that was surveyed using the total station was 

corrected to its proper GPS position. 

Errors in the GPS fast-static surveying ranged from 0.02 

to 0.26 ft horizontally and 0.05 to 0.52 ft vertically, with a 

mean horizontal error of 0.07 ft and a mean vertical error of 

0.16 ft. The RTK survey method produced smaller positional 

precision errors. However, because each surveyed point 

originated from a fast-static surveyed position, these errors 

translated into the larger fast-static original position errors. 

Fast-static errors are a result of short measurement times, poor 

satellite reception due to areal obstructions (such as tree cover 

or narrow canyons), or lack of satellites available at the time 

of measurement. The fast-static technique was used in areas 

where RTK was not possible.

Ground-Water-Level Maps

The October 2006 and partial-development ground-water 

level maps were produced using a geographic information 

system (GIS). The study area boundary was defined by using 

a DEM converted to a slope model to determine the transition 

from the adjacent bedrock hills (high slope values) to the flat 

portions of the valley filled with unconsolidated sediment 

(low slope values). This transition represents the approximate 

location of the boundary between the unconfined aquifer 

and the adjacent bedrock in the tributary canyons and the 

main valley. This location matches the approximate aquifer 

boundary defined by Moreland (1977) and Frenzel (1989), 

and the Quaternary alluvium boundary defined in Worl and 

Johnson (1995) and in Breckenridge and Othberg (2006).

The ground-water-level surfaces were created using a 

spline interpolator technique that includes barriers in order to 

incorporate the effects of aquifer boundaries along the narrow 

northern valley and tributary canyons (Environmental Systems 

Research Institute, Inc., 2007). The spline interpolator is 

a deterministic interpolator that creates a smooth surface 

that must pass through each measured value. It is ideal for 

smoothly varying surfaces such as a typical potentiometric 

surface. The output of the spline interpolator is a grid with an 

interpolated value at each cell except for those cells containing 

a ground-water-level measurement that is assigned the 

measured ground-water level. The cell size for the resultant 

grids is 10×10 m.

The spline interpolator was applied to the entire study 

area except in the vicinities of Warm Springs Creek on the 

October 2006 map and Trail Creek on the partial-development 

map. The spline with barriers interpolator did not provide 

hydrologically realistic results due to a lack of data in these 

two areas, so a tension spline interpolator was used instead. 

The resulting grids for each map were then combined to 

create one ground-water surface map. Contour lines were 

then created from the ground-water surface map for display 

purposes (pls. 1 and 2).

The tension spline technique includes a weight option 

that controls the amount of curvature in the interpolated 

surface between known ground-water level points and an 

option to determine the number of data points used in the 

calculation of each interpolated cell. The two areas in which 

the tension spline technique was utilized used high weights 

(value of 10), allowing for more variation in the output surface 

at a greater distance from the data points. The few data 

points available in these areas resulted in less control of the 

interpolated surface for the tributary canyons. 
Methods and Data Sources 15

The confined aquifer was evaluated separately for 

the October 2006 and partial-development potentiometricsurface maps with ground-water level measurement data from 

wells completed in the confined aquifer. The tension spline 

interpolator was used to create these maps because the data 

is contiguous and no irregular barriers or boundaries isolate 

data points. The confined aquifer interpolations also used as 

many available data points as possible for the October 2006 

potentiometric-surface map (due to the limited amount of 

data available) and used a weight option of 10. The partialdevelopment potentiometric surface map was created using 

this same interpolator and settings.

Cells of the interpolated surfaces or grids are better 

controlled when the interpolated cell is surrounded by data 

points. The interpolator will evaluate cells if there are data 

points on only one side of the evaluated cell. This can have 

unpredictable results due to the lack of control over the 

interpolated cell. All sections of contour lines derived from 

interpolated cells that are surrounded by actual data points are 

displayed as a solid line, whereas the remaining contours are 

dashed to indicate this lack of control over the interpolated 

value of the grid cell. The potentiometric surfaces derived 

from wells completed in the confined aquifer are only defined 

for the areas between or surrounded by ground-water level 

data points because the full extent of the confined aquifer is 

not well known.

Estimating Water-Level Change

Water-level changes between partial-development and 

October 2006 ground-water maps were estimated by using 

a GIS to subtract the October 2006 water-level surfaces 

of the unconfined and confined aquifers from the partialdevelopment water-level surfaces. The results are groundwater level change maps for the two aquifers showing 

changes in the water-table and potentiometric surfaces. Lines 

representing water-level change were then created from the 

resulting ground-water change surfaces—ground-water level 

declines are shown as negative values and ground-water-level 

rises are shown as positive values.

For the water-level change maps, any line that was within 

both the October 2006 and partial-development maps’ solidline contour was displayed as a solid line. If the line occurs in 

an area where either the October 2006 or partial-development 

maps’ lines were dashed, then the water-level change map was 

dashed to indicate an interpolated value with less confidence.

As previously described in section, “Partial Development 

Ground-Water Levels,” historic data were not always available 

to match the exact timing of the October 2006 groundwater level measurement. Therefore, it was necessary to use 

measurements made August through December from 1952 

through 1986 to augment the dataset. To evaluate the effect 

of natural annual and multi-year variation in ground-water 

levels on the water-level change maps, wells with multiple 

water-level measurements were analyzed to constrain probable 

variation. The maximum water-level range was calculated for 

a well with multiple water-level measurements for a single 

year during the 5-month period from August to December as 

well as the entire year. A well could have several water-level 

range calculations if it was measured multiple times during 

multiple years. The range of water-level measurements in 

wells vary from those measured only twice during the period 

of record to those measured multiple times in a single week. 

Surface-Water Discharge Measurements

Streamflow was measured at selected sites in order to 

evaluate the interaction between ground and surface water. 

Changes in streamflow between measurement points on a 

stream indicate whether the stream is gaining flow from 

ground-water discharge (an increase in flow between the 

stations) or if the stream is losing flow to ground-water 

and recharging the aquifer (a decrease in flow between the 

stations). Late October was selected for the mass-measurement 

of ground-water levels and streamflow because the irrigation 

season and related surface-water diversion typically ends by 

mid-October, and July–October usually are the driest months 

of the year. Consequently, streamflow from small tributary 

basins and the return flow from drains have ceased by late 

October. 

During October 23-27, 2006, streamflow was measured 

at 13 sites: 7 on the Big Wood River, 3 on its tributaries, 1 on 

a diversion, and 2 on Silver Creek (fig. 4). However, during 

the week of October 23-27, 2006, a large number of tributary 

streams were still flowing into the Big Wood River, and many 

active diversions were still removing water for irrigation. 

Because of the large number of streams and diversions, not 

all inflows and outflows to the Big Wood River could be 

measured for discharge. Without measuring the discharge of 

each individual inflow and outflow, the comparison of flow 

between each measurement site is somewhat limited and 

accurate calculation of stream gains and losses is not possible 

for the entire river reach. 

Measurements of discharge were made with a current 

meter using the general methods adopted by the USGS. These 

methods are described in USGS Water-Supply Paper 2175 

(Rantz, 1982; Rantz and others, 1982) and the Techniques 

of Water-Resources Investigations of the United States 

Geological Survey (TWRIs), Book 3, Chapters A1 through 

A19 and Book 8, Chapters A2 and B2, which may be accessed 

at http://water.usgs.gov/pubs/twri/.
16 Water-Resource Trends and Comparisons, Wood River Valley, South-Central Idaho

Figure 4. Location of surface-water discharge measurements, Wood River Valley, central Idaho, October 23–26, 

2006.

ID19_0147_WoodRiver_Fig_05

Adams Gulch

Quigley Creek

Willow Creek

District Canal No. 45

Camas Creek

East Fork Big Wood River

Oregon Gulch

Warm Springs Cr

Greenhorn Creek

BIG

WOOD

RIVER

Croy

Creek

Seamus Creek

Slaughterhouse Creek

Indian Creek

Elkhorn

Gulch

Magic

Reservoir

Trail Creek

Deer Creek

Silver Creek

Eagle

Creek

Lake Creek

Cove Creek

Little Wood River

Wood R

N Fk Big

Big Wood River

Bradford Rd

Gannett Rd

East Fork Rd

Muldoon Canyon Rd

Baseline Rd

Warm

Springs

Rd

Hailey

Bellevue

Deer Creek Rd

Greenhorn

Gulch Rd

Croy

Creek Rd

CAMAS

CUSTER

BLAINE

20

26

93

93

75

75

20

Glendale Rd

Trail Creek

at Ketchum

Big Wood

River

at Hailey

Big Wood

River at

Gimlet

Big Wood

River at

Ketchum

East Fork

Big Wood

at Gimlet

Big Wood River

at Glendale

Bridge

Big Wood River at

Stanton Crossing

Silver Creek

at Sportsman

Access

Warm Springs

Creek at Guyer

Hot Springs

District Canal

below Elm Street

at Bellevue

Big Wood River

near Ketchum

Silver Creek

at Hwy 20

near Picabo

Big Wood River

near Bellevue

Ketchum

Sun

Valley

EXPLANATION

Canal/ditch

Boundary of study area

Surface-water discharge

 measurement sites

Basin boundary

Warm Springs

Creek at Guyer

Hot Springs

0 2 4 6 8 MILES

0 2 4 6 8 KILOMETERS

114°25' 114°20' 114°15' 114°10' 114°05' 114°00'

43°

45'

43°

50'

43°

40'

43°

35'

43°

30'

43°

25'

43°

20'
Trend Analysis 17

Trend Analysis

Ground-Water Level Trends

Temporal trends in ground-water levels for three 

wells in the Wood River Valley with more than 50 years 

of measurements varying in frequency from weekly to 

biannually were evaluated using techniques described 

in section, “Ground-Water Level Trend Analysis.” All 

three wells, 432042114163801, 432143114114301, and 

432657114144801, showed statistically significant (the level 

of significance, or p-value, was less than 0.005) declining 

trends in ground-water levels. Well 432042114163801 is 

completed in the confined aquifer near the intersection of 

U.S. Highway 20 and State Highway 75. The remaining two 

wells are completed in the upper unconfined aquifer: well 

432143114114301 near Gannett and well 432657114144801 

south of Bellevue.

To examine the response of water levels to precipitation, 

normalized plots of mean annual depth to water for the three 

wells and total annual precipitation at the Ketchum Ranger 

Station were constructed as described in section, “GroundWater Level Trend Analysis” (fig. 5). Although there is broad 

agreement of trends and many individual years between the 

water level and precipitation plots, a large number of years do 

not match or show water-level declines in response to periods 

of increased precipitation. A simple linear regression between 

water levels and precipitation confirms a poor correlation 

between the two: all R 

2 (coefficient of determination) values 

are less than 0.5. This lack of correlation is further confirmed 

in that water levels in these three wells show statistically 

significant downward trends, while, as previously mentioned, 

total annual precipitation at the Ketchum Ranger Station 

shows a statistically weak (p-value of 0.258) increasing trend.

Water levels in the three wells unquestionably respond 

to recharge from precipitation. However, other factors such 

as withdrawals, infiltration from surface-water irrigation 

(including canal leakage), and land-use changes probably have 

an effect. Water levels in these wells also have fluctuated over 

their period of record. Table 6 shows the extremes and annual 

variation of ground-water levels in these three wells.

The statistically significant downward trend in groundwater levels helps to explain the statistically significant 

downward trends in low-flow values for the Big Wood River 

near Bellevue and the Silver Creek at Sportsman Access 

gaging stations. Because both streams are fed largely by 

ground water in these reaches, any lowering of ground-water 

levels will reduce streamflow.

Streamflow Trends

During analysis of streamflow at the Big Wood River at 

Hailey gaging station, it was observed that flows prior to 1945 

had periodically decreased to zero due to upstream diversions 

at the Justus Ditch for hydroelectric production and seasonal 

irrigation. An unknown quantity of water was then returned 

to the river downstream of the gaging station. Use of the ditch 

was discontinued sometime in the late 1970s to early 1980s. 

The combined records of the Big Wood Slough at Hailey and 

the Big Wood River at Hailey were used during this period for 

trend calculations.

No statistically significant trends were found in the 

mean annual streamflow for the Big Wood River at Hailey 

gaging stations (table 5). A statistically significant trend 

exists for the mean monthly flow for March, for which there 

was a 25-percent increase in mean monthly base flow from 

1951-2005 (p-value of 0.005). A weak statistical increasing 

trend also exists for January (p-value of 0.123). The March 

increase in streamflow may be a function of earlier snowpack 

runoff, seen in basins throughout the Western United States 

(Stewart and others, 2005). 

Analysis of the annual 7- and 30-day low-flow 

calculations at the Big Wood River at Hailey gaging stations 

indicate an increase in base flow (water years 1915–2005). 

Weak statistical trends for the 7-day (p-value of 0.117) and 

30-day (p-value of 0.097) low-flow calculations suggest an 

increase of 20 ft3

/s since 1915 (fig. 6A). As mentioned in the 

section, “Streamflow Trend Analysis,” the confidence interval 

for surface-water analysis in this report was restricted to 

90 percent; while the trends for 7-day and 30-day low-flow 

calculations have confidence intervals of 88 and 90 percent, 

respectively, they may suggest a developing trend. If these 

trends exist, a possible explanation may be a statistically weak 

increasing trend (p-value of 0.258) of total annual precipitation 

at the Ketchum Ranger Station.

The Big Wood River near Bellevue gaging station 

(13141000) did not have a statistically significant annual trend 

in discharge or a BFI trend (table 6). However, mean monthly 

discharge values show statistically significant decreasing 

trends for December, January, and February (p-values of 

0.014, 0.006, and 0.095, respectively, fig. 7). Statistically 

significant decreasing trends also exist for both the 7-day 

and 30-day low-flow analyses (p-values of 0.084 and 0.076, 

respectively), showing a mean decrease of about 15 ft3

/s since 

the 1940s (fig. 6B). 

To examine whether or not ground-water inflow has 

changed, data from the Silver Creek at Sportsman Access near 

Picabo gaging station (13150430) was examined for trends. 
18 Water-Resource Trends and Comparisons, Wood River Valley, South-Central Idaho

Figure 5. Normalized residual plots of mean annual depth to water for three wells and total annual 

precipitation at the Ketchum Ranger Station, Wood River Valley, south-central Idaho.

ID19_0147_WoodRiver_Fig_05

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0 -1.00

-0.80

-0.60

-0.40

-0.20

0.00

0.20

0.40

0.60

0.80

1.00

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

NORMALIZED MEAN ANNUAL DEPTH TO WATER, IN FEET

-1.00

-0.80

-0.60

-0.40

-0.20

0.00

0.20

0.40

0.60

0.80

1.00

NORMALIZED ANNUAL PRECIPITATION, IN INCHES, AT KETCHUM RANGER STATION

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1950 1960 1970 1980 1990 2000 2010

-1.00

-0.80

-0.60

-0.40

-0.20

0.00

0.20

0.40

0.60

0.80

1.00

Annual mean water levels, normalized

Annual mean precipitation, normalized

A. Well 432042114163801

B. Well 432143114114301

C. Well 432657114144801
Trend Analysis 19

Figure 6. Trend analysis of annual 7- and 30-day low streamflows for two gaging stations on the Big Wood River, southcentral Idaho.

ID19_0147_WoodRiver_Fig_06

0

50

100

150

200

250

300

STREAMFLOW, IN CUBIC FEET PER SECOND

7-day

7-day low flow trend

30-day

30-day low flow trend

P-value = 0.117 

P-value = 0.097 

B. Big Wood River near Bellevue gaging station

 1915–96

A. Big Wood River at Hailey gaging station

 water years 1915–2005 

0

50

100

150

200

250

300

1910 1920 1930 1940 1950 1960

YEAR

1970 1980 1990 2000 2010

7-day

7-day low flow trend

30-day

30-day low flow trend

P-value = 0.084

P-value = 0.076 
20 Water-Resource Trends and Comparisons, Wood River Valley, South-Central Idaho

Table 6. Extremes of ground-water levels and annual variation for selected wells in the Wood River Valley, south-central Idaho, 

1954–2006.

[Annual variation is calculated for calendar year. Water levels are depth below land surface, negative numbers indicate height above land surface. Abbreviations: 

USGS, U.S. Geological Survey; ft, foot]

USGS well No. Well name

Water levels (ft) Mean 

depth to 

water, 

period of 

record (ft)

Annual variation in water levels (ft)

Lowest Date Highest Date Maximum Year Minimum Year Mean, period 

of record

432042114163801 01S 18E 14AAB1 -11.33 06-27-58 -46.47 09-22-94 -27.25 20.04 1958 1.54 2005 8.82

432143114114301 01S 19E 03CCB2 26.05 11-03-94 2.55 08-12-65 14.95 15.46 1962 1.53 1996 9.41

432657114144801 01N 18E 01DAA1 51.87 03-23-93 31.51 06-19-65 43.72 15.82 1967 2.65 1996 8.63

BIG WOOD RIVER NEAR BELLEVUE 

MEAN MONTHLY DISCHARGE 

WATER YEARS 1915–96 

0

50

100

150

200

250

300

350

1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

STREAMFLOW, IN CUBIC FEET PER SECOND

December

December Monthly trend (p-value 0.014)

January

January Monthly Trend (p-value 0.006)

February

February Monthly Trend (p-value 0.095)

Figure 7. Mean monthly discharge at Big Wood River near Bellevue, south-central Idaho, water 

years 1915–96. 
Partial Development and October 2006 Hydrologic Conditions 21

Because Silver Creek and its tributaries are fed primarily by 

ground water discharging through seeps and springs, and 

because irrigation diversions and returns to and from Silver 

Creek are minimal, seasonal fluctuations in streamflow are 

directly related to water levels in the Wood River Valley 

aquifer system (Moreland, 1977). Therefore, streamflow in 

Silver Creek can be used as a proxy for trends in ground-water 

discharge to the Big Wood River on the Bellevue fan. Silver 

Creek at Sportsman Access has a statistically significant 

annual discharge decreasing trend (p-value of 0.018) for the 

period of 1975—2005 and mean monthly discharge values 

at this gaging station also show statistically significant 

decreasing trends for July through February and April 

(p-values all less than 0.052), table 5. Statistically significant 

decreasing trends also exist for both the 7-day and 30-day 

low-flow analyses (p-values of 0.074 and 0.101, respectively). 

The Silver Creek at Sportsman Access gage data did not have 

a statistically significant trend for the BFI analysis. 

The decreasing trends in the Big Wood River near 

Bellevue since the 1940s, and in Silver Creek since 1975, 

indicate declining ground-water levels. There have been 

wet and dry cycles during this time. However, population 

has increased since the 1960s, and therefore the declines in 

ground-water levels could be due to increased consumptive 

use of water in the valley from increased population. Increased 

consumptive use may result in lowered ground-water levels, 

which reduces spring flows to the Big Wood River and Silver 

Creek. These reduced flows are most evident during the winter 

months for the Big Wood River near Bellevue gage since flow 

is primarily derived from ground-water inputs to the river 

during this time. Because Silver Creek is spring fed year round 

the effect of declining ground-water levels is evident for most 

of the year. More information is needed on water-use patterns 

to test this hypothesis.

Partial Development and October 2006 

Hydrologic Conditions

Unconfined Aquifer

Partial-Development Water Table

The map of the partial-development water table in the 

unconfined aquifer is shown on plate 1. Although waterlevel measurements from 206 wells were used to construct 

this map (see section, “Ground-Water-Level Maps”), waterlevel measurements were not available for some areas of 

the unconfined Wood River Valley aquifer system for the 

partial-development period. This is true for three of the eight 

tributary canyons shown on the October 2006 water-table map 

(Croy Creek, Indian Creek, and Deer Creek). Thus, partialdevelopment conditions are not shown for these areas. Watertable contours for the northern part of the study area and all 

mapped tributary canyons were approximated due to a lack of 

data in those areas.

The partial-development map generally shows a uniform 

southerly water-table gradient until the Bellevue fan, at which 

point the gradient decreases and a poorly defined groundwater divide forms and gradients slope to either the southwest 

toward the discharge area of the Big Wood River and Willow 

Creek or southeast toward the discharge area of Silver Creek. 

October 2006 Water Table

The October 2006 water-table map of the unconfined 

aquifer in the Wood River Valley was constructed from 88 

wells and is shown on plate 2. The map includes the main 

valley and eight tributary canyons and shows a uniform 

gradient southward until the Bellevue fan where the valley 

widens and the ground-water gradient declines. A groundwater divide forms near Baseline Road; it trends nearly 

due south, thus splitting the gradient to the southwest and 

southeast.

Previous studies show a seasonal migration of the 

ground-water divide with a westward shift during the irrigation 

season (May to October) and a return eastward during autumn 

and winter. This is due to increased recharge to ground water 

in the middle to eastern part of the Bellevue fan and decreased 

infiltration from the Big Wood River along the western edge 

of the Bellevue fan as a result of decreased or no streamflow 

during this period (Castelin and Chapman, 1972). Castelin 

and Chapman’s (1972) map of May ground-water conditions 

shows an eastward position of the divide. Smith’s (1959) map 

shows ground-water conditions during August and a central 

position of the divide. Moreland’s (1977) map, along with this 

study, shows October ground-water conditions and a westward 

position of the divide. Although the west-east shift apparently 

occurs annually, its actual position may vary from year to 

year due to changes in irrigation, pumping, land use, and 

precipitation. The ground-water divide is not as well-defined 

on the partial-development water-table map as it is for the 

October 2006 water-table map. This is most likely due to the 

range of dates used in the creation of the partial-development 

map: 5 months over a 35-year period as opposed to a single 

week in October 2006. 
22 Water-Resource Trends and Comparisons, Wood River Valley, South-Central Idaho

Changes in the Water Table, Partial Development 

to October 2006

The water-table change map of the unconfined aquifer 

(pl. 3) shows changes in the water table at a 10-ft interval; 

negative values indicate a decline in ground-water level 

between the partial-development and October 2006 watertable maps, and positive values indicate a rise in ground-water 

levels. Dashed lines indicate uncertainty in either the partialdevelopment or October 2006 water-table maps. Some of 

the ±10-ft lines are dashed to represent water-level changes 

in this range falling within the average range of water-level 

fluctuation (5 ft) plus the average elevation uncertainty of the 

partial-development map (5 ft). These dashed lines occur in 

areas influenced by partial-development wells with elevation 

accuracies greater than 1 ft.

Changes in the water table in the study area north of 

Ketchum primarily indicate no change or an increase in 

ground-water levels. However, ground-water level changes in 

this area are approximated due to the lack of data coverage. 

Ground-water level declines are small in the upper and lower 

parts of the Warm Springs tributary canyon, with a large rise 

in ground-water levels in the middle section. Ground-water 

levels rose in the upper part of the Trail Creek tributary and 

declined in the lower part of the tributary. Ground-water 

levels in this tributary canyon, as well as all others, were 

approximated due to the lack of data coverage. Ground-water 

levels primarily declined or did not change in the area from 

Ketchum south to Indian Creek, with ground-water level 

rises in three areas. Ground-water levels declined or did not 

change in the remainder of the study area south of the Indian 

Creek tributary. Ground-water levels declined more than 

10 ft in the area southwest of Bellevue near Glendale Road. 

Moreland (1977) found this area to have natural ground-water 

level fluctuations as much as 40 ft, but water-level fluctuation 

calculations performed for this study show fluctuations of less 

than 40 ft. Ground-water levels declined greater than 10 ft in 

the southeastern part of the study area near Picabo; Moreland 

(1977) noted that ground-water levels in this area fluctuate less 

than in other parts of the study area.

Confined Aquifer

Partial-Development Potentiometric Surface

The partial-development potentiometric-surface map 

of the confined aquifer was constructed using ground-water 

levels measured in 36 wells. The partial-development map 

(pl. 4A) shows a broad area of high water levels (fairly well 

defined because of the number of measurements used), from 

which gradients slope to the southwest and east.

October 2006 Potentiometric Surface

The October 2006 potentiometric-surface map of the 

confined aquifer was constructed using ground-water levels 

measured in 10 wells, and is shown on plate 4B with 10-ft 

contours. Even with the limited number of data points, the 

October 2006 potentiometric-surface map shows a broad 

area of high water levels from which gradients slope to the 

southwest and east.

Changes in the Potentiometric Surface, Partial 

Development to October 2006

The potentiometric-surface change map (pl. 4C) 

predominately shows declines in ground-water levels greater 

than 10 ft. Moreland (1977) found natural water-level 

fluctuations of 10 ft or less in these areas, although Castelin 

and Chapman (1972) indicated ground-water level fluctuations 

of 5 ft or less in these same areas.

Annual and Long-Term Fluctuation of Water 

Levels

Previous authors have noted large annual fluctuations 

of water levels in the Wood River Valley aquifer system. 

Therefore, annual and August–December fluctuations were 

evaluated for different years in 376 wells completed in the 

unconfined aquifer and 121 wells completed in the confined 

aquifer. In the unconfined aquifer, the mean water-level 

range for the August to December period is approximately 

5 ft with a maximum of 70 ft. In the confined aquifer, the 

mean water-level range for the August to December period 

is approximately 3 ft with a maximum of 20 ft. Castelin and 

Chapman (1972) documented larger water-level fluctuations; 

however, their analysis used water levels measured during 

May to August and May to October. Moreland (1977) found 

annual ground-water level fluctuations ranging from 5 to 

40 ft depending on the well location in the study area. The 

smallest fluctuations were in the southeastern part of the 

study area because of spring discharge into Silver Creek and 

its tributaries. Ground-water level fluctuations are somewhat 

damped by seeps and springs because increases in groundwater levels increase seep and spring discharge thus tempering 

the ground-water level rise. As ground-water levels decline, 

seep and spring discharge decreases. The largest fluctuation 

rates were near the intermittent reach of the Big Wood River 

below Glendale Road because of surface-water diversions. 

Because wells used in this analysis have varying measurement 

dates and measurement frequencies, no coherent spatial 

distribution similar to that found by Castelin and Chapman 

(1972) is apparent.
Partial Development and October 2006 Hydrologic Conditions 23

Surface-Water Discharge Measurements and 

Streamflow Gain/Loss

During October 23–27, 2006, stream discharge was 

measured at 13 sites: 7 on the Big Wood River, 3 on its 

tributaries, 1 on a diversion, and 2 on Silver Creek (fig. 4). 

Measured discharge at each site and change in discharge 

between sites is shown in table 7, listed in downstream order. 

The three measured tributaries of the Big Wood River enter 

the river between the former sites of the Big Wood River 

at Ketchum (13136000) and the Big Wood River at Gimlet 

(13138500) gaging stations.

Changes in the discharge of the Big Wood River in the 

Wood River Valley primarily was due to flow from tributaries 

in the northern part, ground-water outflow, inflow and 

surface-water diversions in the southern part, and a mixture 

of these factors in the central part. As measured in October 

2006, discharge in the Big Wood River increased by 46 ft3

/s 

from the Big Wood River near Ketchum (13135500) site 

to the Big Wood River at Ketchum (13136000) site, which 

was attributed to the inflow of small streams such as the 

North Fork Big Wood River, Eagle Creek, and Lake Creek. 

Measurement error at these sites could account for up to 

5 ft3/s of the measured flow. Discharge increased by 101 ft3/s 

between the Big Wood River at Ketchum (13136000) site 

and the Big Wood River at Gimlet (13138500) site; most of 

this gain was from the combined 82 ft3

/s inflow measured at 

the Warm Springs Creek at Guyer Hot Springs (13136500), 

Trail Creek at Ketchum (13137500), and the East Fork Big 

Wood River near Gimlet (13138000) sites. The remaining 

19 ft3/s was attributed to ungaged inflow from Elkhorn Gulch, 

inflow from the wastewater-treatment facility, ground-water 

seepage into the river, and measurement error. Measurement 

error could account for most of this inflow to the Big Wood 

River. Discharge in the Big Wood River increased about 

32 ft3/s from the Big Wood River at Gimlet (13138500) to the 

Big Wood River at Hailey (13139500) sites. Of the ungaged 

tributary basins in this reach, Greenhorn Creek and Deer 

Creek are the most likely to contribute significant flow to the 

river. The wastewater-treatment facilities input a minimal 

amount of flow in this reach (less than 2 ft3/s) (M.A. Maupin, 

written commun., 2007) and measurement error could account 

for up to half of the additional flow. Due to diversions and 

seepage to ground water, the channel of the Big Wood River 

is typically dry for most of the reach between the District 

Canal below Elm Street, at Bellevue diversion (13140350) 

and the Big Wood River at Glendale Bridge (13140500) sites 

except during snowmelt and intense precipitation events. 

During October, 2006, the entire flow of the Big Wood River 

Table 7. Streamflow measurements, Wood River Valley, south-central Idaho, October 23–26, 2006.

[Abbreviations: ft3/s, cubic foot per second; mi2, square miles; –, no flow or not applicable]

Site name

Site 

identification 

No.

Measurement 

date and time

Measured 

discharge 

(ft3

/s)

Discharge 

change from 

upstream site 

(ft3

/s)

Basin area 

upstream of 

measurement 

point (mi2

)

Percentage 

of basin 

upstream of 

measurement 

point

Major tributaries to the Big Wood River

Warm Springs Creek at Guyer Hot Springs 13136500 10-25-06 14:02 39 – 94 100

Trail Creek at Ketchum 13137500 10-25-06 16:10 23 – 64 100

East Fork Big Wood River near Gimlet 13138000 10-26-06 09:42 20 – 86 100

Big Wood River

Big Wood River near Ketchum 13135500 10-25-06 09:20 79 – 178 22

Big Wood River at Ketchum 13136000 10-25-06 11:50 125 46 240 30

Big Wood River at Gimlet 13138500 10-26-06 12:39 226 101 517 65

Big Wood River at Hailey 13139500 10-23-06 17:32 258 32 628 79

Big Wood River at Glendale Bridge, near 

Bellevue

13140500 10-25-06 – – -258 745 84

Big Wood River at Stanton Crossing 13140800 10-23-06 11:54 122 122 774 97

Big Wood River near Bellevue 13141000 10-23-06 14:36 169 47 795 100

Diversion

District Canal below Elm Street, at Bellevue 13140350 10-24-06 16:33 35 – – –

Silver Creek

Silver Creek at Sportsman Access 13150430 10-24-06 09:58 176 – 72 78

Silver Creek at Highway 20, near Picabo 13150500 10-24-06 13:05 154 -22 93 100
24 Water-Resource Trends and Comparisons, Wood River Valley, South-Central Idaho

was still diverted into the District Canal below Elm Street, 

at Bellevue diversion (13140350), which had a discharge of 

35 ft3/s. The river remained dry for many miles until groundwater seepage into the river reaches 122 ft3

/s at the Big Wood 

River at Stanton Crossing (13140800). The river gained an 

additional 47 ft3/s before it reached the Big Wood River near 

Bellevue (13141000) site, a quarter of which could be due to 

measurement error.

Surface-water discharge measurements and streamflow 

gain/loss analyses on the Big Wood River have been made by 

Castelin and Chapman (1972) for February 1971; Moreland 

(1977) for the week of September 29–October 1, 1975; 

Luttrell and Brockway (1984) for September 12–14, 1983, 

and March 12–14, 1984; and Frenzel (1989) for August 1986. 

Because measurement sites and times, and more importantly, 

discharge varied among the studies, direct comparison among 

them is difficult. However, allowing for different measurement 

points, the general identification of gaining and losing stream 

reaches on the Big Wood River and Silver Creek for this study 

generally agree with previous studies.

Two discharge measurements were made on Silver 

Creek on October 24, 2006, for this study. Discharge was 176 

ft3

/s at the Silver Creek at Sportsman Access gaging station 

(13150430) and 154 ft3/s at the Silver Creek at Highway 20 

near Picabo gaging station (13150500), thus showing a loss 

of 22 ft3/s between the gaging stations. Moreland (1977) 

measured discharge and analyzed streamflow gain/loss on 

Silver Creek and its tributaries for three time periods in 

1975: the weeks of May 19–22, June 23–27, and September 

29–October 3. These measurements indicated a loss of 4, 10, 

and 15 ft3/s, respectively. 

Summary and Conclusions

The Wood River Valley of south-central Idaho extends 

from Galena Summit southward to the Timmerman Hills. 

The study area of this report is within the Wood River Valley 

extending from the boundary with the Sawtooth National 

Forest southward to the Timmerman Hills. Because the entire 

population of the area (about 15,000 in 2000) depends on 

ground-water for domestic supply, either from domestic or 

municipal-supply wells, rapid population growth since the 

1970s has raised concerns about the continued availability 

of ground and surface water to support existing uses and 

streamflow. 

The Wood River Valley aquifer system includes the 

Quaternary sediments of the Wood River Valley and its 

tributaries; and locally, underlying igneous, sedimentary, or 

metamorphic rocks where they are hydraulically connected 

and used for water supply. The aquifer system is comprised of 

a single unconfined aquifer and an underlying confined aquifer 

present south of Baseline Road. Streams are well-connected to 

the unconfined aquifer throughout the valley. Depth to water in 

the upper valley is commonly less than 10 feet, increasing to 

about 90 feet southward. Water levels in the lower valley range 

from less than 10 to about 150 feet below land surface in the 

unconfined aquifer, whereas most wells in the confined aquifer 

are under artesian pressure and flow.

Most of the Wood River Valley is drained by the Big 

Wood River or its tributaries, except for the southeastern part 

of the Bellevue fan, which is drained by the ground-water-fed 

Silver Creek. The Big Wood River gains flow from a number 

of perennial and ephemeral tributaries and meanders across 

the narrow upper valley until Bellevue, where it flows along 

the western side of the Bellevue fan, finally exiting the valley 

at Stanton Crossing. A network of irrigation canals and drains 

exists throughout the study area.

Mean annual water levels in three wells (two completed 

in the unconfined aquifer and one in the confined aquifer, all 

in the southern part of the study area) with more than 50 years 

of semi-annual measurements were evaluated for statistically 

significant trends. All three show statistically significant 

declining trends.

Long-term trend analysis was performed on three 

gaging stations in the Wood River Valley that had at least 

20 years of record. The Big Wood River at Hailey gaging 

station (13139500) showed a statistically significant trend of 

a 25-percent increase in mean monthly base flow for March 

over the 90-year period of record. This increase in streamflow 

may be a function of earlier snowpack runoff, as seen in 

basins throughout the Western United States. No statistically 

significant long-term trends were found in mean annual or 

calculated BFI values. Both the 7-day and 30-day low-flow 

analyses for the Big Wood River near Bellevue gaging station 

(13141000) show a mean decrease of about 15 cubic feet per 

second since the 1940s, and mean monthly discharge values 

show statistically significant decreasing trends for December, 

January, and February. To examine whether or not groundwater inflow has decreased, streamflow in Silver Creek was 

used as a proxy for trends in ground-water discharge to the 

Big Wood River on the Bellevue fan. The Silver Creek at 

Sportsman Access near Picabo gaging station (13150430) 

showed statistically significant decreasing trends in annual and 

mean monthly discharge for July through February and April. 

The 7-day low flow analysis also had a statistically significant 

decreasing trend.

The decreasing trends at the Big Wood River near 

Bellevue and Silver Creek gages indicating declining groundwater levels could be due to increased consumptive use of 

water in the valley from increased population. Increased 

consumptive use may result in lowered ground-water levels 

which reduces spring flows to the Big Wood River and Silver 

Creek. These reduced flows would be most evident during 

the winter months for the Big Wood River near Bellevue gage 

since flow are primarily derived from ground-water inputs to 

the river during this time. Since Silver Creek is spring fed year 

round the effect of declining ground-water levels is evident for 

most of the year.
References Cited 25

Using a GIS, current and partial-development groundwater level maps were made of the unconfined and confined 

aquifers, along with ground-water level change maps between 

the two development periods. Development conditions in 

the valley were based on decennial census data that shows 

a relatively stable Blaine County population through 1970, 

after which it increased from 5,749 in 1970 to 21,166 in 2005. 

Ideally, ground-water levels measured prior to 1970 would 

be used for a pre-development map, but pre-1970 water-level 

measurements are insufficient to construct a map of the entire 

valley. Therefore, ground-water levels measured up to 1986 

were used to represent the partial-development period.

In general, the current and partial-development watertable maps of the unconfined aquifer show a uniform southerly 

gradient until the Bellevue fan, at which point the gradient 

decreases and a poorly defined ground-water divide forms and 

causes gradients to slope either southwest or southeast. The 

potentiometric-surface maps of the confined aquifer show a 

broad area of high water levels; from this broad area, gradients 

slope to the southwest and east.

The ground-water level change map of the unconfined 

aquifer shows a mixture of increases and declines in the 

northern part of the study area. The changes for this area, 

however, are estimated due to data limitations. More data were 

available for the southern part of the study area, and that area 

shows either no ground-water level changes or declines in 

ground-water levels. Ground-water changes less than 10 feet 

can be attributed to average natural variation in ground-water 

levels plus average elevation inaccuracies of some wells. 

Mapped contours in this range are dashed to indicate these 

areas and to represent estimated change conditions.

The potentiometric-surface change map of the confined 

aquifer in the southern part of the study area shows declines 

in ground-water levels beyond the expected range of natural 

annual variation plus elevation inaccuracies.

During October 23–27, 2006, stream discharge was 

measured at 13 sites—7 on the Big Wood River, 3 on its 

tributaries, 1 on a diversion, and 2 on Silver Creek. Typically, 

the irrigation season and related surface-water diversion ends 

by mid-October, and July–October usually are the driest 

months of the year. However, during the week of October 

23–27, 2006, a large number of tributary streams were still 

flowing into the Big Wood River and many active diversions 

were still removing water. Because of the large number of 

streams and diversions, not all inflows and outflows to the 

Big Wood River could be measured for discharge. Without 

measuring the discharge of each individual inflow and 

outflow, the comparison of flow between each measurement 

site is somewhat limited. However, allowing for different 

measurement points, the general identification of gaining and 

losing stream reaches on the Big Wood River and Silver Creek 

for this study generally agree with those identified in previous 

studies.

Acknowledgments

First and foremost, the authors thank the well owners 

who allowed access to their property and wells. Special thanks 

are due to A.L. Brown, B.W. Lium, and W.J. Pabich for their 

early support for this study, background information, and 

technical discussions that contributed to a better understanding 

of the conditions in the study area. Field measurements 

were performed by A.M. Campbell, D.G. Cole, R.G. 

Dickinson, R.W. Erickson, and R.E. Reaves, all of the U.S. 

Geological Survey. Both the elected and appointed officials 

for the cooperating governments and the cooperating nongovernmental organizations provided funding in response 

to their constituents’ and members’ concerns. Technical 

discussions with G.M. Clark and S.W. Lipscomb of the U.S. 

Geological Survey were particularly helpful in the evaluation 

of hydrologic trends. Technical reviews of the manuscript 

were provided by D.L. Berger, J.E. Hortness, K.D. Peter, and 

G.W. Rattray of the U.S. Geological Survey; W.J. Pabich; and 

T. Klahr of the Nature Conservancy.

References Cited

Aquaterra Consultants, 2006, BASINS 4.0 (beta version 

December 1, 2006), accessed Dec. 15, 2006, at URL: http://

www.aquaterra.com/basins4/

Berenbrock, C.E., Bassick, M.D., Rogers, T.L., and Garcia, 

S.P., 1995, Depth to water, 1991, in the Rathdrum 

Prairie, Idaho; Spokane River valley, Washington; 

Moscow-Lewiston-Grangeville area, Idaho; and selected 

intermontane valleys, east-central Idaho: U.S. Geological 

Survey Water-Resources Investigation Report 94-4087, 

scales 1:250,000, 1:317,000, 1:500,000, 3 maps on 2 sheets.

Breckenridge, R.M., and Othberg, K.L., 2006, Surficial 

geologic map of the Hailey Quadrangle, Blaine County, 

Idaho: Idaho Geological Survey Digital Web Map 55, 

scale 1:24,000. Available online at URL: http://www.

idahogeology.org/PDF/Digital_Data_(D)/Digital_Web_

Maps_(DWM)/Hailey_Surf_DWM-55-M.pdf

Brennan, T.S., Lehmann, A.K., and O’Dell, I., 2005, Water 

resources data, Idaho, 2005; Volume 1. Surface water 

records: U.S. Geological Survey Water-Data Report 

ID-05-1, 510 p. Available online at URL: http://pubs.usgs.

gov/wdr/2005/wdr-id-05-1/

Briar, D.W., Lawlor, S.M., Stone, M.A., Parliman, D.J., 

Schaefer, J.L., and Kendy, Eloise, 1996, Ground-water 

levels in intermontane basins of the northern Rocky 

Mountains, Montana and Idaho: Hydrologic Atlas 738-B, 

scale 1:750,000, 1 sheet.
26 Water-Resource Trends and Comparisons, Wood River Valley, South-Central Idaho

Brockway, C.E., and Grover, K.P., 1978, Evaluation of 

urbanization and changes in land use on the water resources 

of mountain valleys: Moscow, University of Idaho Water 

Resources Research Institute, 104 p. plus appendix.

Brockway, C.E., and Kahlown, M.A., 1994, Hydrologic 

evaluation of the Big Wood River and Silver Creek 

watersheds Phase I: Kimberly, University of Idaho Water 

Resources Research Institute, Kimberly Research Center, 

53 p. plus 5 appendices. Available online at URL: http://

conserveonline.org/docs/2004/06/hydrology_phase1_1994.

pdf

Brown, A.L., Jr., 2000, Summary Report: Hydrologic 

evaluation of the Big Wood River and Silver Creek 

Watersheds: Ketchum, Hydronetics, 84 p. 

Bureau of Reclamation, 2006, AgriMet-The Pacific Northwest 

Cooperative Agricultural Weather Network-AgriMet 

network map: accessed Nov. 14, 2006, at URL: http://www.

usbr.gov/pn/agrimet/agrimetmap/agrimap.html

Campbell, P.R., 1996, Population projections for states by age, 

sex, race, and hispanic origin: 1995 to 2025: U.S. Bureau of 

the Census, Population Division, PPL-47, 105 p. Available 

online at URL: http://www.census.gov/population/www/

projections/ppl47.html#tr-size-state and table 1 at URL: 

http://www.census.gov/population/projections/state/stpjpop.

txt

Castelin, P.M., and Chapman, S.L., 1972, Water resources 

of the Big Wood River-Silver Creek area, Blaine County, 

Idaho: Boise, Idaho Department of Water Administration, 

Water Information Bulletin 28, 44 p. Available online at 

URL: http://www.idwr.idaho.gov/hydrologic/info/pubs/wib/

wib28-big_wood_river-silver_creek_area.pdf

Castelin, P.M., and Winner, J.E., 1975, Effects of urbanization 

on the water resources of the Sun Valley-Ketchum area, 

Blaine County, Idaho: Boise, Idaho Department of Water 

Resources, Water Information Bulletin 40, 86 p. Available 

online at URL: http://www.idwr.idaho.gov/hydrologic/info/

pubs/wib/wib40-sun_valley-ketchum_area.pdf

Critchfield, H.J., 1983, General climatology, 4th ed.: New 

York, Prentice Hall, 453 p.

Environmental Systems Research Institute, Inc., 2007, 

ArcGIS 9.2 desktop help—How spline with barriers works: 

accessed May 10, 2007, at URL: http://webhelp.esri.

com/arcgisdesktop/9.2/index.cfm?TopicName=How%20

Spline%20with%20Barriers%20works

Fenneman, N.M., 1931, Physiography of Western United 

States: New York, McGraw-Hill, 534 p., 1 plate in pocket.

Forstall, Richard, 1995, Idaho population of counties by 

decennial census: 1900 to 1990: Accessed February 13, 

2007 at URL: http://www.census.gov/population/cencounts/

id190090.txt

Frenzel, S.A., 1989, Water resources of the upper Big 

Wood River basin, Idaho: U.S. Geological Survey WaterResources Investigation Report 89-4018, 47 p. Available 

online at URL: http://pubs.er.usgs.gov/pubs/wri/wri894018

Godfrey, Bruce, 2000, Köppen climate classification for the 

conterminous United States: accessed Nov. 14, 2006, at 

URL: http://snow.ag.uidaho.edu/Clim_Map/koppen.htm

Helsel, D.R., and Hirsch, R.M., 1992, Statistical methods in 

water resources: Amsterdam, Elsevier Science Publishers, 

529 p.

Insightful Corp., 2005, S-Plus 7—Statistical application 

software: accessed Dec. 15, 2006, at URL: http://www.

insightful.com/products/splus/default.asp

Institute of Hydrology, 1980a, Low flow studies: Wallingford, 

Oxon, United Kingdom, Institute of Hydrology Report 

No. 1, 41 p.

Institute of Hydrology, 1980b, Low flow studies: Wallingford, 

Oxon, United Kingdom, Institute of Hydrology Report 

No. 3, p. 12-19.

Jones, R.P., 1952, Evaluation of streamflow records in the Big 

Wood River basin, Blaine County, Idaho: U.S. Geological 

Survey Circular 192, 59 p., 1 plate in pocket. Available 

online at URL: http://pubs.er.usgs.gov/usgspubs/cir/cir192

Luttrell, S.P., and Brockway, C.E., 1984, Impacts of individual 

on-site sewage disposal facilities on mountain valleys – 

Phase II – Water-quality considerations: Moscow, Idaho 

Water & Energy Resources Research Institute, University 

of Idaho, Research Technical Completion Report 

WRIP/371403, 74 p.

Maupin, M.A., 1992, Depth to water in the eastern Snake 

River Plain and surrounding tributary valleys, southwestern 

Idaho and eastern Oregon, calculated using water levels 

from 1980 to 1988: U.S. Geological Survey WaterResources Investigation Report 90-4193, scales 1:1,000,000 

and 1:750,000, 2 maps on 1 sheet. 

Moreland, J.A., 1977, Ground water-surface water relations in 

the Silver Creek area, Blaine County, Idaho: Boise, Idaho 

Department of Water Resources, Water Information Bulletin 

44, 42 p., 5 plates in pocket. Available online at URL: http://

www.idwr.idaho.gov/hydrologic/info/pubs/wib/wib45-

silver_creek_area.pdf. Also published as U.S. Geological 

Survey Open-File report 77-456, 66 p., plates in pocket. 

Available online at URL: http://pubs.er.usgs.gov/pubs/ofr/

ofr77456
References Cited 27

Mundorff, M.J., Crosthwaite, E.G., and Kilburn, Chabot, 

1964, Ground water for irrigation in the Snake River Basin 

in Idaho: U.S. Geological Water-Supply Paper 1654, 224 

p., 6 plates in pocket. Available online at URL: http://pubs.

er.usgs.gov/pubs/wsp/wsp1654

Natural Resources Conservation Service, 2006, Idaho snow 

survey program: accessed Nov. 14, 2006, at URL: http://

www.id.nrcs.usda.gov/snow/

Rantz, S.E., 1982, Measurement and computation of 

streamflow: Volume 2, Computation of discharge: U.S. 

Geological Water-Supply Paper 2175, 347 p.

Rantz, S.E., and others, 1982, Measurement and computation 

of streamflow: Volume 1, Measurement of stage and 

discharge: U.S. Geological Water-Supply Paper 2175, 

284 p.

Schmidt, D.L., 1962, Quaternary geology of the Bellevue 

area in Blaine and Camas Counties, Idaho: U.S. Geological 

Survey Open File Report 62-120, 127 p. Available online at 

URL: http://pubs.er.usgs.gov/pubs/ofr/ofr62120

Smith, R.O., 1959, Ground-water resources of the middle Big 

Wood River-Silver Creek area, Blaine County, Idaho: U.S. 

Geological Survey Water-Supply Paper 1478, 61 p., 2 plates 

in pocket. Available online at URL: http://pubs.er.usgs.gov/

usgspubs/wsp/wsp1478

Smith, R.O., 1960, Geohydrologic evaluation of streamflow 

records in the Big Wood River basin, Idaho: U.S. 

Geological Survey Water-Supply Paper 1479, 64 p., 4 plates 

in pocket. Available online at URL: http://pubs.er.usgs.gov/

pubs/wsp/wsp1479

Stewart, I.T., Cayan, D.R., and Dettinger, M.D., 2005, 

Changes toward earlier streamflow timing across 

western North America: Journal of Climate, v. 18, no. 4, 

p. 1136-1155. Available online at URL: http://tenaya.ucsd.

edu/~dettinge/stewart_timing.pdf

Street, L.V., 1990, Geothermal investigations in Idaho, part 17, 

Geothermal resource analysis in the Big Wood River valley, 

Blaine County, Idaho: Boise, Idaho Department of Water 

Resources, Water Information Bulletin 30, part 17, 26 p., 1 

plate in pocket. Available online at URL: http://www.idwr.

idaho.gov/hydrologic/info/pubs/wib/wib30p17-geothermal_

big_wood_river_valley.pdf

Trimble Navigation Ltd., 2007, Trimble Geomatics Office 

software: accessed Feb. 12, 2007, at URL: http://www.

trimble.com/geomaticsoffice.shtml

U.S. Census Bureau, 2006, Census 2000 Summary File 1 (SF 

1) 100-Percent Data: Accessed November 18, 2006 at URL: 

http://factfinder.census.gov/servlet/DCGeoSelectServlet?ds_

name=DEC_2000_SF1_U

U.S. Census Bureau, 2007, State & County QuickFacts, 

Blaine County: Accessed February 13, 2007 at URL: http://

quickfacts.census.gov/qfd/states/16/16013.html

U.S. Department of Commerce, 2006, National Climatic Data 

Center: accessed Nov. 14, 2006, at URL: http://www.ncdc.

noaa.gov

U.S. Environmental Protection Agency, 2006, BASINSBetter Assessment Science Integrating point and Nonpoint 

Sources, A powerful tool for managing watersheds: 

accessed Dec. 15, 2006, at URL: http://www.epa.gov/

waterscience/basins/index.html

Wahl, K.L., and Wahl, T.L., 1988, Effects of regional groundwater declines on streamflows in the Oklahoma panhandle: 

Symposium on water-use data for water resources 

management, American Water Resources Association, 

Tucson, Arizona, p. 239-249. Available online at URL: 

http://www.usbr.gov/pmts/hydraulics_lab/twahl/bfi/bfi_

beaver_river.pdf

Western Regional Climate Center, 2006, Idaho climate 

summaries: accessed Nov. 16, 2006, at URL: http://www.

wrcc.dri.edu/summary/climsmid.html

Wetzstein, A.B., Robinson, C.W., and Brockway, C.E., 

1999, Hydrologic evaluation of the Big Wood River and 

Silver Creek watersheds, phase II: Kimberly, University 

of Idaho Water Resources Research Institute, Kimberly 

Research Center, 136 p. Available online at URL: http://

conserveonline.org/docs/2004/06/hydrology_phase2_1999.

pdf

Worl, R.G., and Johnson, K.M., 1995, Geology and mineral 

resources of the Hailey 1°x2° quadrangle and the western 

part of the Idaho Falls 1°x2° quadrangle, south-central 

Idaho—-An overview, chap. A of Worl, R.G., Link, P.K., 

Winkler, G.R., Johnson, K.M., eds., Geology and mineral 

resources of the Hailey 1°x2° quadrangle and the western 

part of the Idaho Falls 1°x2° quadrangle, Idaho: U.S. 

Geological Survey Bulletin 2064, pl. 1.
28 Water-Resource Trends and Comparisons, Wood River Valley, South-Central Idaho

This page intentionally left blank.
Appendix A. 29

Appendix A. —Water-Level and Well Information Used to Construct PartialDevelopment and October 2006 Water-Table and Potentiometric-Surface Maps, 

Wood River Valley, South-Central Idaho.

Appendix A data is distributed with this report as a Microsoft Excel® spreadsheet and is available at URL: http://pubs.usgs.

gov/sir/2007/5258.
30 Water-Resource Trends and Comparisons, Wood River Valley, South-Central Idaho

This page intentionally left blank.
Manuscript approved for publication, October 31, 2007

Prepared by the USGS Publishing Network, 

Publishing Service Center, Tacoma, Washington

Bill Gibbs 

Bob Crist

Bobbie Jo Richey

Jackie Olson

Linda Rogers

Sharon Wahlstrom

For more information concerning the research in this report, contact the 

Director, Idaho Water Science Center 

230 Collins Road 

Boise, Idaho 83702-4520

http://id.water.usgs.gov
SIR 2007–5258 Skinner and others Water-Resource Trends and Comparisons Between Partial Development and 

 October 2006 Hydrologic Conditions, Wood River Valley, South-Central Idaho