The cost of raising Arrowrock Dam to reduce flood risk and increase water supply for the Treasure Valley exceeds the benefits, the Army Corps of Engineers told a stunned Idaho Water Resources Board on Wednesday.
The Corps said the risk of flood remains, but it can’t justify being involved in an Arrowrock Dam construction project. The U.S. Bureau of Reclamation presented a separate proposal to raise the larger Anderson Ranch Dam upstream on the South Fork of the Boise River, after the Corps said it would not move forward with raising Arrowrock.
The water board is in charge of planning future water uses and supplies for the state, and provides bonding and other financing for water projects. The Army Corps and the Bureau of Reclamation jointly operate the Boise River dams and major diversions; the Corps has Lucky Peak while the bureau has Arrowrock, Anderson Ranch and Lake Lowell.
The board, along with Idaho Gov. Butch Otter and the Idaho Legislature, was hoping the Corps would support rebuilding the aging Arrowrock Dam. The board on Wednesday asked the two agencies to consider a new proposal to jointly study flood control and water supply in the valley and at all three of the dams upstream from Boise. The state had paid $1.5 million of the cost to study raising the 348-foot Arrowrock Dam by more than 70 feet.
The Corps study found that the cost-benefit ratio of flood control came to just .5, said Corps’ Arrowrock Project Manager Karen Zelch. (A 1.0 is the break-even point). The cost-benefit for the entire $1.3 billion construction project came to .7.
“It’s still not enough to get us a project we can construct,” Zelch said.
The Bureau of Reclamation wants to study raising Anderson Ranch Dam by 6 feet — adding about 29,000 acre-feet of capacity to the 946,700 acre-feet in the Boise River reservoirs — at a cost of about $31 million. But it will need $1.75 million in cost-sharing from a state agency, local government or businesses to conduct the $3.5 million study.
At the state’s request, the Corps limited the scope of the Arrowrock study by not considering alternative flood-risk reduction measures such as expanding flood plains and wetlands or redesigning neighborhoods.
Zelch said that the existing Boise River dams reduce potential damages for most floods, but they do not have the capacity to prevent significant damage during a very large but less frequently expected flood. The study estimated one of these rare floods could cost millions or even billions of dollars, and is likely to occur at some point.