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Scientific Investigations Report 2007–5258

Scientific Investigations Report 2007–5258

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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 ground-water 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 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 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.

Using a GIS, current and partial-development ground-water 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 water-table 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.

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