As the Colorado River and its once massive reservoirs shrink from overuse and climate change, officials are faced with a decision that pits conservation against ratepayer costs for electricity.

To fight off predators of the humpback chub, a threatened fish native to the river, Glen Canyon Dam in northern Arizona would need to do what is known as a “cool mix flow,” where cold water is released from deep in its reservoir to cool the river below. But there are no hydropower turbines in the cool, deep section, so significant power generation would be lost.

The proposal comes after the worst snowpack on record for the Colorado River Basin, relied upon by farmers, industries, wildlife and more than 40 million people in seven U.S. states, tribal nations and Mexico. It also comes as those states fail to reach a long-term agreement on how to share the river’s dwindling resources beyond this year, when the guidelines expire.

“There is a limited water supply. It’s getting even lower. And with that, a lot of hard decisions need to be made,” said John Berggren, regional policy manager for the environmental nonprofit Western Resource Advocates.

Utilities that buy this hydropower say the cool water releases would be costly because they would have to spend millions to buy alternative energy and would increase financial hardship for customers. But supporters say that without cool releases, the warm waters projected downstream this summer would allow non-native predatory fish to spawn, further threatening the humpback chub, and would destroy a world-famous trout fishery nearby.

The Bureau of Reclamation, which is expected to announce a decision in the next couple of weeks, said in a statement that it is weighing several factors including the ecological health of the river and the hydropower production of the dam. The Interior Department, which oversees the bureau, declined to comment. If the cool water release is approved, it would likely happen from June to October through jet tubes, bypassing the turbines near the warmer surface.

How mixing cool water protects fish

Lake Powell, one of two massive reservoirs on the Colorado River, is just 23% full after decades of overuse and evaporation of water as average temperatures rise because of climate change. A record low inflow is expected this summer. With such a low reservoir, warm water near the surface gets sucked through the generators and sent downstream.