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Scientific Investigations Report 2007–5258

Scientific Investigations Report 2007–5258

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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 ground-water 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 water-resource concerns, were evaluated.

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.

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