Oil sheens were visible on parts of the Clark Fork River on Friday, a day after a pipe broke at the Noxon Rapids Dam, spilling about 1,250 gallons of transformer oil.

Emergency response officials were working with the dam’s operator, Avista Corp., to evaluate the situation and craft a cleanup plan. A helicopter flew over a 20-mile stretch of the lower Clark Fork River. Pockets of oil were spotted from the air, but no dead fish turned up.

The oil is “not the goopy, heavy stuff,” said Bruce Howard, Avista’s director of environmental affairs, but rather a highly refined mineral oil used to cool electrical transformers.

The oil tends to float on top of the water, Howard said. He said the spill shouldn’t put the river’s fish population or other wildlife at risk.

Smallmouth and largemouth bass live in the reservoir downstream from the Noxon Rapids Dam, said Loren Albright, a member Trout Unlimited’s Panhandle Chapter. Endangered bulltrout also spend part of their life cycle in the reservoir, and the Clark Fork is a major tributary of Lake Pend Oreille.

Dam operators discovered the spill shortly after noon Thursday. A chunk of ice apparently fell from the face of the dam, smashing the pressure gauge on the valve of a quarter-inch pipe. The spilled oil collected in storm-water drains that empty into a stretch of the Clark Fork River between the Noxon Rapids and Cabinet Gorge dams. Employees shut off the value manually and alerted authorities.

Officials from the U.S. Environmental Protection Agency, the states of Idaho and Montana, and two counties were on the scene Friday.

Avista is operating the two dams to keep the spilled oil in the reservoir, said Anna Scarlett, a company spokeswoman. At Noxon Rapids Dam, only one generating unit is running at a time. The reduced activity will keep the water calm, assisting cleanup efforts, she said.

Avista has hired a contractor with expertise in oil spills, Howard said. The initial cleanup will focus on using absorbent pads to soak up the oil, with additional cleanup and evaluation to follow.

“We don’t think there will be a threat to the long-term health of the river,” Scarlett said.