Coal-fired plant's owners expected to vote on its future

The bureau uses power from plant to pump Colorado River water through the Central Arizona Project aqueduct system to farms, cities and tribes in central and southern Arizona, and a bureau official said it recognizes that the plant is "an economic driver" for much of the state.

NV Energy has said it plans to cut its ties with the plant in order to meet a 2013 Nevada mandate for reduced reliance on coal-fired power generation, but Harelson said the owners' current re-assessment is being driven by economics, "not environmental or regulatory issues."

The federal Environmental Protection Agency announced in 2014 that the plant could either shut down one of its three 750-megawatt units or reduce power generation by an equal amount by 2020 to cut haze-causing pollution at places like the Grand Canyon.