Exploring options for Minnidoka Dam

By Cindy Snyder, Ag Weekly correspondent
Friday, September 26, 2008 6:06 PM CDT

CINDY SNYDER/Ag Weekly Raising Minidoka Dam to store more water is one option, but the Bureau of Reclamation and irrigation districts that divert water at the dam have been planning to rennovate the structure for years.

RUPERT, Idaho - Water users in the Mini Cassia area have long known Minidoka Dam needed work, but with the state taking a closer look at projects that could increase water storage capacity, a proposed rehabilitation project is getting more attention.

Adding 5 feet to the height of the Minidoka Dam, which could provide an additional 50,000 acre-feet of storage, is one option being looked at during the comprehensive aquifer management process for the Eastern Snake Plain Aquifer.

The 2,200-foot-long structure was built in 1905-06 at a cost of about $505,000. It was a fairly modest structure rising approximately 60-feet above the original river bed.

While the structure has served irrigators well for 100 years, it’s time for replacement, said Larry Heib, operations and maintenance manager for the Bureau of Reclamation in Burley. One disadvantage of a rock-filled dam is that it leaks like a sieve, and the Minidoka Dam is no exception. So much water has leaked over the years that a wetland and put-and-take fishery have been created near the face of the dam.

Failing concrete and maintenance costs are two of the primary issues being considered in a study of how best to rehabilitate the structure, Heib said. Several years ago the Bureau knocked down one pier to see what was really inside and found very little reinforcement.

“There’s enough reinforcement for support against the water but not for lateral stability,” Heib said. That means the structure is susceptible to an earthquake.

It took about one swing with a wrecking crane to bring down the pier, he said. Workers found just five pieces of rebar inside the pier.

In addition to replacing the spillway, new diversion structures will be built for both the Minidoka and Burley irrigation districts. The anticipated $61 million cost of rehabilitating the structure would be shared by users who benefit from it. Bonneville Power Administration would pay half the costs with Minidoka Irrigation District paying 27.5 percent, Burley Irrigation District paying 16.3 percent and wildlife and recreation interests paying the remaining 8 percent.

Randy Bingham, manager of BID, estimates the rehabilitation project will cost his district over $10 million or about $210 per acre.

Chris Ketchum, deputy area manager for the BOR in Burley; Larry Hieb, operations and maintenance manager for BOR, and John Tiedeman, a manager for BOR in Boise, discussed options for Minidoka Dam with members of the Idaho Water Resources Board during a tour in mid-September. Photo by Cindy Snyder.

Adding 5 feet to the dam could push the cost between $100 and $150 million. If the dam is raised, the Bureau would determine who benefits from the additional storage, and those beneficiaries would be billed for their share of the cost.

From an engineering and cost standpoint, adding 1 foot or 5 feet to the structure’s height doesn’t change much. But a taller structure could have significant impacts on fish and wildlife species and even hydropower generation.

At the current height, there is 47 feet of head at the turbines in the hydropower plant. Adding 5 feet is a 10 percent increase.

“That’s significant,” Heib said. “Does that put us out of our efficiency range? We may have to replace the turbines. We don’t know, but it has to be considered now.”

Impacts on fish and wildlife from a higher structure must not just be considered in the summer, but year-round, said Chris Ketchum, deputy area manager for the Bureau of Reclamation in Burley. Water levels behind the dam are lowered 5 feet in the winter because of icing concerns. Even if the dam is rehabilitated and not raised, the Bureau hopes to leave water levels stable through the winter.

Endangered species concerns regarding two snails may also become an issue.

“Those are big unknowns,” Ketchum told members of the Idaho Water Resources Board. “I don’t know that they’re deal killers.”

Rehabilitation of the structure was scheduled for 2013. Bureau of Reclamation personnel said they are moving down two parallel paths regarding the project: replacement and raising the structure.

Rehabilitation remains the baseline, and work continues on design and engineering. Engineers began looking at the impacts of adding 5-feet in September, thanks to a special study paid for by the Idaho Department of Water Resources. In November, operations and how to manage the additional 50,000 acre-feet of storage will be looked at. The Bureau expects to begin analyzing the environmental and socioeconomic impacts. The final study could be completed by the winter of 2010.

“We can’t stop replacement and wait for the raising to catch up,” said John Tiedeman, who is in the Boise office. “That would throw the schedule off two years or more.”

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