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Map courtesy of Jim Bartolino, U.S. Geological Survey
Based on a continuous water-level sampling record dating back at least 50 years at three different local well sites, shown here as wells “A,” “B” and “C,” hydrologists with the U.S. Geological Survey have determined that groundwater levels in the Wood River Valley are in decline. The lighter-colored portion of the map indicates the extent of the valley’s unconfined aquifer, which is responsible for spring-fed flows on the lower Big Wood River as well as Silver Creek.
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Based on the results of a recently completed water study, hydrologists with the U.S. Geological Survey can conclusively point to declines in both groundwater and surface-water resources in the Wood River Valley. Pinpointing just how extensive those declines are is another matter.
The same goes for why the water levels are dropping.
What the hydrologists can say based on the results of their sampling is that groundwater levels and streamflows in some parts of the valley are declining beyond what might be expected from typical, short-term fluctuations. The study was initiated due to local concerns related to growth and the long-term sustainability of the area's water supplies.
It was funded through a cooperative agreement between the USGS, Blaine County and the cities of Bellevue, Hailey, Ketchum and Sun Valley. Also funding the study were the Idaho chapter of The Nature Conservancy, the Sun Valley Water and Sewer District, the Blaine Soil Conservation District and local nonprofit Citizens for Smart Growth.
Explaining the results of the first phase of the four-phase water study to a crowd gathered at the Ketchum City Hall on Wednesday, the hydrologists said the results indicate statistically significant downward trends in water levels based on samples taken from three different wells. The wells—one near Bellevue and the other two farther south in the Bellevue Triangle—have a continual sampling record dating back at least 50 years.
The lengthy record provided the hydrologists—Kenneth Skinner, Jim Bartolino and Andrew Tranmer—with a long-term trend to analyze.
In addition to the wells, the scientists also considered long-term surface-water trends based on three local streamflow gauges with at least 20 years of continuous record. The gauges are found on the Big Wood River in Hailey and near Stanton Crossing on U.S. Highway 20 and on Silver Creek at the sportsmen's access point near Picabo.
However, unlike the data from the three wells, the information provided by the streamflow gauges varies somewhat, Bartolino said.
He said that at the Hailey gauge, the sampling actually shows a 25 percent increase in mean-March streamflows between the years of 1951-2005. He said this is partially explained by the gauge's location near the river's many tributaries.
"It's at the lower end of a gaining-rich region," he said.
Farther south, sampling from the Stanton Crossing and Silver Creek streamflow gauges indicates substantial declines in stream flows, he said.
Bartolino thinks the discrepancy between the Hailey gauge and the Stanton Crossing and Silver Creek gauges is likely attributable to changing climate patterns that have created an earlier spring snowmelt cycle in recent years. Moreover, he said the decreasing low-flow trends they observed on the lower two streamflow gauges correspond to declining trends they've observed in water levels at the sampled wells.
Flows on both Silver Creek and the lower Big Wood are fed by springs originating from the valley's underground aquifer.
The results of the first phase of the study completed by Skinner, Bartolino and Tranmer are published in a lengthy 40-page report that can be viewed by logging onto the Blaine County Web site at www.blainecounty.org.
Back in October 2006, the USGS hydrologists measured water levels in 98 wells and streamflows at 13 different sites spread throughout the valley. They used these measurements to construct a groundwater contour map of the valley's aquifers. Using available data for the period 1952 to 1986, the scientists constructed a second groundwater contour map.
By analyzing the two different groundwater contour maps, they were able to determine changes in water levels. Those changes are represented on groundwater change maps that are also included in the USGS report.
In general, the scientists said they are less confident with changes shown by the map indicating anything less than a 10-foot decline in groundwater levels. Speaking to the crowd during the presentation, Skinner said large portions of the valley fall within this 10-foot margin of error. He said that uncertainty is due to a lack of long-term monitoring data.
Skinner said they are confident of the declining water levels that exceed the 10-foot margin of error. He said one of the most pronounced declines they observed is located in an area south of Bellevue in the upper Bellevue Triangle.
Skinner said that in the years to come, the groundwork established by the recent study at the 98 wells and 13 streamflow sites will allow for a more detailed analysis of changing groundwater levels.
"We'll be able to detect smaller groundwater level changes," he said.
What the hydrologists' report doesn't answer is whether the declines are due to drought conditions, increased water use or a combination of both.
In the next phase of the water study, the hydrologists will create a detailed water budget for the basin. Bartolino said the water budget will show both the amount of water that flows into the valley's aquifer, as well as what flows out of it.
He said that locally, aquifer recharge comes from tributaries, the Big Wood River, leakage from irrigation canals, from septic systems, the application of irrigation water and precipitation.
On the discharge side, Bartolino said, they will consider direct evapotranspiration, the process by which moisture is transferred from vegetation, ponds and open water into the atmosphere, as well as discharge from the lower Big Wood River, Silver Creek, subsurface flow out of the basin and pumping for human needs.
Bartolino cautioned that a water budget's usefulness only extends so far. He said that while they can be useful for local planners, water budgets do not provide a detailed recommendation on how much water use the local groundwater and surface-water system can take.
"Water budgets can get you towards that number, but they can't get you to that number," he said.
To arrive at an estimate for how much water use is sustainable, local values like the preservation of streamflows, agricultural practices and the need for things like green lawns must be considered, Bartolino said.
Of course, he said, not having a detailed knowledge of how a local water system works would make that task very difficult.
"To make informed decisions you have to have a scientific basis," he said.
Responding to a statement from someone in the crowd who noted the data only show a 4 percent to 10 percent decline in local aquifer levels, Bartolino expressed caution against viewing the declines in such terms. He said that just a small decline beyond current aquifer levels could endanger flows on the lower Big Wood River and Silver Creek.
He said these spring-fed waterways must have the level of the aquifer reach the streambed. If the aquifer drops just below the streambed, all spring-fed flows will be lost, he said. If it drops even farther below the streambed, he said, riparian vegetation like cottonwood trees, whose roots can only drop so far, will become endangered.
Bartolino said that certain places along the Big Wood River in the Bellevue Triangle have already seen large cottonwoods die due to declining aquifer levels.
"It's that upper portion (of the aquifer) that's important," he said.
Bartolino said people also need to be careful thinking that simply reducing water use due to a transition from more water-intensive agricultural use to less-intensive residential use will solve the problem. He said diverting water into irrigation canals in the triangle actually helps recharge the aquifer because the canals lose much of their flows to underground seepage.
"That's probably where most of the water in Silver Creek is coming from," he said.
