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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5258

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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.

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 Wood-Silver 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 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 year-round. 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).

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).

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.

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 above-normal 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.

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 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.

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