A million-gallon mine waste spill that sent a plume of orange-ish muck down a river in southwest Colorado on Thursday was caused by a federal mine cleanup crew.

The U.S. Environmental Protection Agency said that a cleanup team was working with heavy equipment Wednesday to secure an entrance to the mine. Workers instead released an estimated 1 million gallons of mine waste into Cement Creek.

"The project was intended to pump and treat the water and reduce metals pollution flowing out of the mine," agency spokesman Rich Mylott said in a statement.

The creek runs into the Animas River, which then flows into the San Juan River in New Mexico and joins the Colorado River in Utah.

Officials emphasized that there was no threat to drinking water from the spill. Downstream water agencies were warned to avoid Animas water until the plume passes, said David Ostrander, director of EPA's emergency response program in Denver.

Officials weren't sure how long it would take the plume to dissipate, Ostrander said. The acidic sludge is made of heavy metal and soil, which could irritate the skin, he said.

The EPA said in an earlier statement that that the polluted water "was held behind unconsolidated debris near an abandoned mine portal."

The agency did not elaborate what was in the water, saying it contained "sediment and minerals flowing as an orange-colored discharge."

"The water associated with the release is obvious and highly discolored," the agency said.

No one was injured in the accident.

The plume made its way to Durango on Thursday afternoon, prompting La Plata County health officials to warn rafters and others to avoid the water. Pet owners were advised to keep dogs and livestock out of the Animas.

"It's really, really ugly," Butch Knowlton, the county's director of Emergency Preparedness, told The Durango Herald. "Any kind of recreational activity on the river needs to be suspended."

It was not clear when the waste would reach New Mexico or Utah.

The immediate impact on wildlife wasn't clear. There are no fish in the Cement Creek watershed because of longstanding problems with water quality, the Colorado Department of Public Health and the Environment said.

The city of Durango stopped pumping water out of the Animas River on Wednesday to make sure none of the waste could be sucked up into the city reservoir. It also suspended the transfers of raw water to a local golf course and Fort Lewis College.

City officials said they would not water any city-owned parks for the next three days to help conserve. Durango will rely on another source of water, the Florida River, from which is can pump 5.3 million gallons a day.

Colorado Parks and Wildlife planned to place four cages containing fish in the Animas River to monitor what happens to them, spokesman Joe Lewandowski said.

"We'll see if those fish survive," Lewandowski said. "We're also monitoring to make sure we don't get infiltration into the hatchery, because that could be a problem."

Durango resident Lisa Shaefer said she was near the mine Wednesday when a mine bulwark broke and sent a torrent of water downstream that raised the water level 2 to 3 feet in Cement Creek. The initial wall of water carried rocks and debris and made a roar as it pushed through a culvert, she said.

"What came down was the filthiest yellow mustard water you've ever seen," she told the newspaper.

In Farmington, New Mexico, city officials shut down all water-supply intake pumps to avoid contamination and advised citizens to stay out of the river until the discoloration has passed.