Federal officials plan to create an environmental study that will look at raising one of three Boise River system dams to capture more water for fast-growing southwestern Idaho.
The U.S. Bureau of Reclamation is taking comments through Sept. 9 and holding three meetings involving the proposed project to raise Anderson Ranch Dam by 6 feet (2 meters).
That would increase storage by 29,000 acre-feet. An acre-foot is about 326,000 gallons (1.2 million liters).
“There’s a consensus that the Treasure Valley will see an increase in demand, and the state wants to be prepared for that,” said Cynthia Bridge Clark, water projects section manager at Idaho Department of Water Resources.
The Idaho Water Resource Board, whose eight members are appointed by the governor, is paying $3 million for the environmental study and a feasibility study to raise the dam. Reclamation is also paying $3 million for the $6 million cost for the studies.
To do the project, Reclamation must first complete an environmental impact statement to study potential effects. The comments will be used to shape that environmental study. Reclamation is holding the public meetings on Aug. 27 in Pine, Aug. 28 in Boise and Aug. 29 in Mountain Home.
Idaho officials have long sought to increase water storage in the Boise River system to supply water to Boise and nearby fast-growing cities as well as create enough space to handle a 500-year flood.
A 2015 study done by a water engineering company for the Idaho Water Resource Board and Idaho Department of Water Resources found that the population in the Treasure Valley will more than double by 2065, greatly increasing water demands.
Lucky Peak Reservoir, Arrowrock Reservoir and Anderson Ranch Reservoir combined hold about 1.1 million acre-feet. A plan to raise Arrowrock Dam 70 feet (21 meters) at a cost of $1.26 billion fell apart in 2016 when it failed a U.S. Army Corps of Engineers feasibility study.
The estimated cost to raise Anderson Ranch Dam 6 feet is $31 million.
Bridge Clark said one of the concerns about water in the Treasure Valley is climate change and the possibility of warmer winters. Currently, a big part of the water strategy for Boise and surrounding cities relies on water stored in snowpack in the mountains.
But if precipitation falls as rain rather than snow, that could lead to a reduced snowpack and a greater reliance on the three reservoirs.
“If the snowpack isn’t as robust — if we don’t manage to store that water in the snowpack — than there are thoughts that we need a bigger bucket in the reservoirs,” Bridge Clark said.
In related action, Republican Idaho Gov. Brad Little earlier this year signed legislation to end years of litigation involving water that refills the three reservoirs following flood-control releases. Ambiguity over who owned the rights to that refill made some water rights holders uncomfortable, leading to litigation.