BOISE — The benefits of raising Arrowrock Dam — decreased flood risk and increased water supply on the Boise River system — do not equal the costs, Idaho Water Resource Board members were told by federal officials May 18.

A U.S. Army Corps of Engineers study that determined raising the dam is the best option for solving water supply and flood risk problems on the Boise River system is now on hold.

For Congress to approve the project, which would cost about $1.2 billion, the benefits must at least match the total cost, said Lt. Col Timothy Vail, commander of the Corps Walla Walla District.

But the study determined the benefits would equal only 70 percent of the cost.

“The alternatives we identified are ultimately too expensive,” Vail said. “The project is currently in hiatus at this milestone.”

It’s up to the board, which paid half the cost of the study, to decide whether to terminate it or ask Corps headquarters to reformulate and continue it.

In a separate presentation May 18, U.S. Bureau of Reclamation officials asked the board to help fund a proposed feasibility study that would look at increasing the height of Anderson Ranch Dam by 6 feet. That would add 28,954 acre-feet of storage space in that reservoir, which is also on the Boise system.

Arrowrock and Anderson Ranch supply most of the irrigation water for farmers in the valley.

The Arrowrock study found that raising the dam by 30-70 feet would greatly decrease flood risks in the Treasure Valley and provide about 100,000 acre-feet of additional reservoir storage space.

The Corps’ study found that adding 100,000 acre-feet of water supply on the Boise River system was worth about $11.6 million annually. That number represents what it would cost the state to get that water from another source.

But IWRB members told Corps officials that total greatly underestimates what it would cost the state to obtain that much additional water on the Boise system.

People have tried to identify additional water supplies in the Boise system for years without success, said IWRB member Albert Barker. The benefit to the local economy of having an additional 100,000 acre-feet of water would be tremendous and much more than $11.6 million, he said.

“It seems like you are significantly under-valuing the benefit you would get from that (much) water,” he said. “If we could take that into account, that might change the overall cost-benefit ratio of this project.”

Vail said the Corps ideally needs an answer on whether the board wants to reformulate the study by July.

The cost of an Anderson Ranch study would also be about $3 million, and federal regulations require that non-federal partners must split the cost.

Officials representing the Corps, Bureau and IWRB will meet in the next few weeks to discuss their options and the idea of coordinating the two studies.

“I think we need further discussion on the whole issue before we do anything,” said IWRB member Chuck Cuddy.