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Plan to Raise Arrowrock Dam Fails Feasibility Study

“We need water bad,” said Idaho Water Resource Board chairman Roger Chase. “We’ve got to find water.

Fri September 02, 2016 - West Edition

Lt. Col. Timothy Vail called the end result of the cost-benefit study an “unpleasant surprise” that he found disappointing.
Lt. Col. Timothy Vail called the end result of the cost-benefit study an “unpleasant surprise” that he found disappointing.

BOISE, Idaho (AP) Hopes of Idaho officials to increase the height of a Boise River dam to double water storage and also create enough space to handle a 500-year flood ran aground after the project failed a federal feasibility study.

The U.S. Army Corps of Engineers told the Idaho Water Resource Board back in May that the $1.26 billion cost of raising century-old Arrowrock Dam 70 ft. (21.3 m) outweighed the benefits, killing the project.

The board absorbed a second blow shortly after when U.S. Bureau of Reclamation officials said the federal agency will need $1.75 million in cost sharing to study a much less ambitious plan costing an estimated $31 million to raise Anderson Ranch Dam 6 ft. (1.8 m).

Board chairman Roger Chase called it a tough day, noting growth in Boise and the surrounding area means future water demand will exceed supply.

“We need water bad,” he said. “We've got to find water. And we need flood control.”

Initial planning that started in 2009 identified raising Arrowrock Dam as the top-ranked storage option for both flood risk and water supply.

Lt. Col. Timothy Vail said early on it looked like raising Arrowrock Dam would meet the cost-benefit requirements. But he said work in the region in the intervening years to improve water supply reduced the benefit, while the costs increased with the unknowns of raising an older dam such as Arrowrock, which started operating in 1915.

Vail called the end result of the cost-benefit study an “unpleasant surprise” that he found disappointing.

“Boise is what keeps me up at night in terms of flood risk,”' he said.

The Army Corps of Engineers does feasibility studies because there are potential dam projects throughout the country competing for limited federal dollars. Ultimate approval for projects must come from Congress.

Officials said raising Arrowrock Dam just over 70 ft. would add 270,000 acre ft. of storage to the reservoir that can already hold 272,000 acre ft. An acre ft. is the amount of water it takes to cover an acre with a foot of water.

Besides supplying additional water for the region, the study found the extra storage would mostly absorb a 500-year flood event that would otherwise cause billions of dollars in flood damage downstream, including inundating Boise State University.

Board member Albert Barker said he thought the cost-benefit analysis undervalued the benefit of additional water to the region.

“What is the benefit to the local economy if you had another 100,000 acre feet of water?” he asked Army Corps officials. “If you could take that into account that might change the overall cost-benefit of the project.”

Board members also contended that the cost-benefit analysis was flawed because it calculated the benefit of water that board members said didn't exist, suggesting the benefit of a scarce resource was undervalued.

The smaller project upstream of Arrowrock Dam at Anderson Ranch Dam would add about 30,000 acre ft., increasing storage capacity by about 7 percent at that reservoir. But that project is stalled until local governments and other entities can pool $1.75 million to pay for half of a federal feasibility study.

Federal officials said the board had various options, including asking for another study on a smaller height increase at Arrowrock Dam that might meet the cost-benefit requirement.

The board, openly dismayed with the day's events, asked for another meeting with federal officials to discuss options.

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