But the agency's preference for cooling the Snake could raise Idaho Power customers' rates.
The utility proposes that it spend $3 million a year to restore trees to shade the Snake's upstream tributaries, and to raise flows along with some other measures.
But EPA administrators doubt the plan will be enough to keep the water cool throughout the 100-mile journey through the Snake and its reservoirs. The agency is eyeing an alternate that could cost the utility up to $250 million.
The Endangered Species Act requires the region to do something about the spawning problem.
"Right now it's very unclear what these projects are going to be and that they would represent any temperature benefits downstream," said John Palmer, an EPA senior policy adviser who wrote a letter to Idaho Power expressing the agency's doubts. "We think to get the level of temperature reduction it would require a significant amount of restoration."
Idaho Power argues that it exceeds federal water quality standards only during a two-week period in October. The company opposes spending so much for a temperature control structure needed for such a short time.
"It's really addressing a symptom instead of the cause," said Idaho Power attorney Jim Tucker.
The dispute is the last major issue holding up Idaho Power's nearly 15-year effort to obtain a new license to operate Brownlee, Oxbow and Hells Canyon dams. It has cost $107 million so far.
In 2004, the company estimated the process and its requirements would cost more than $325 million over 30 years.
Other groups and officials have said they prefer the company's watershed restoration approach.
The decision ultimately rests with the Federal Energy Regulatory Commission, which is waiting for responses from Idaho and Oregon officials. A decision from the FERC isn't expected until at least 2011.