Scientific Investigations Report 2007–5258
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5258
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The map of the partial-development water table in the unconfined aquifer is shown on plate 1. Although water-level measurements from 206 wells were used to construct this map (see section, “Ground-Water-Level Maps”), water-level 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, partial-development conditions are not shown for these areas. Water-table 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 ground-water 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.
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 ground-water 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.
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 water-table maps, and positive values indicate a rise in ground-water levels. Dashed lines indicate uncertainty in either the partial-development 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.
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.
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.
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.
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 ground-water 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.
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 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 ground-water 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.
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