Study of Wood River aquifer shows poor recharge retention

Water shortages in the Wood River Valley of south-central Idaho got some members of the Idaho Water Resource Board asking whether recharge in the area made sense.

The answer is there’s not much potential if the goal is to hold water over from wet years to dry years, said Brian Patton, the board’s planning bureau chief.

At the board’s request, Mike McVay, a water resource engineer with the Idaho Department of Water Resources, looked into the potential of recharge flows in the area and found water-retention rates were low.

Retention times are short because the Wood River Aquifer is so small. Unlike the much larger Eastern Snake Plain Aquifer, there isn’t a lot of distance from any recharge location to a discharge point, so water moves in a short amount of time, McVay said.

Looking at four different hypothetical recharge locations, he found recharge water ran back into the Big Wood River in a matter of months.

Using the groundwater model for the Wood River Valley, he looked at a recharge site near Ketchum, one between the Big Wood River and the bypass canal near Walker Drive, one near Silver Creek and one south of Bellevue.

The recharge water from the first three sites flowed back to the Big Wood River in less than a year. Even at the best site — south of Bellevue — 85 percent of the recharge water flowed back to the river within a year, and 10 percent more returned to the river in the second year.

In addition, there’s not a lot of surplus water available for recharge, and the aquifer returns to healthy levels quickly in wet years, he said.

“It’s kind of a conundrum in the Wood River Valley,” he said.

From the board’s perspective, the results of the modeling are not very promising. The board’s goals are stabilization and recovery of the aquifer, and it would be difficult in the Wood River Valley to use recharge to accomplish that, he said.

McVay’s analysis shows recharge can only be done in above-average water years when the Magic Reservoir fills and spills. It also shows it’s not a good way to store water year to year to make up for the dry years because the recharge water is already out of the aquifer at that point, Patton said.

“There are no other options in that valley at this point,” he said.

Several pending water calls between surface water users and more junior groundwater users in the Wood River Valley led to the analysis.

Water shortages vary from year to year, but senior water right holders downstream of junior pumpers could run out of water in July if it’s an extremely dry year, Patton said.

New water for recharge would only be available if water users reduced their consumption, McVay said.

There is some discussion between parties along those lines, but the board is not directly involved, Patton said.

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