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A spill that sent 1 million gallons of wastewater from an abandoned mine into Colorado's Animas River, turning the river orange, set off warnings Thursday that contaminants threaten water quality for those downstream and into Utah on the San Juan River.

The Bureau of Land Management's Utah spokeswoman, Megan Crandall, urged the boating public to exercise caution before putting in on the San Juan at Bluff and Mexican Hat.

The contamination is expected to reach Utah by Friday on its journey to Lake Powell.

Utah environmental regulators recommended staying clear of the river until the extent of the pollution is better understood.

"It's going to be highly acidic and laden with heavy metals. You should stay away from that water," said Walt Baker, director of the Utah Division of Water Quality. "We don't know how evident it will be when it reaches Utah. We made contact with tribal leaders and health authorities."

The Environmental Protection Agency confirmed it triggered the release while using heavy machinery to investigate pollutants at the Gold King Mine north of Silverton.

Health and environmental officials are evaluating the river in Colorado's San Juan County. They said the spill contained zinc, iron, copper and other heavy metals, prompting the EPA to warn agricultural users to shut off water intakes along the river and tell recreational users to avoid the river.

"There's nothing that can be done to stop the flow of the river," said Joe Lewandowski, a spokesman for Colorado Parks and Wildlife. "We can only wait until the flows slow down. We had a big heavy spring [of rain] here."

Lewandowski said the EPA is testing to determine the river's metal levels and that results should be returned by mid-Friday.

Downstream in Durango, where the wastewater is expected to reach by late afternoon, city officials are asking residents to cut back on their water use and irrigation of city land at Fort Lewis College has been stopped.

The La Plata County Sheriff's Office closed the river from San Juan County line — including Durango — to New Mexico. Authorities say they will re-evaluate the closure once the EPA tests are confirmed.

Photos from the area showed the river's waters turned a thick, radiant orange and yellow.

The water release was triggered at 10:30 a.m. Wednesday at the abandoned mine on the upper portions of Cement Creek, about 55 miles north of Durango. The fluid was being held behind unconsolidated debris near an abandoned mine portal, the EPA says.

The agency called the release "unexpected."

Deputy Stephen Lowrance of the San Juan County Sheriff's Office said authorities are keeping people away from the Animas. In Silverton, signs have been posted warning of the danger.

"The river looks pretty nasty," Lowrance said. "It doesn't look like water; it just looks like sludge."

He said it was too early to tell, for him, if anything was being left behind on the banks.

"Nobody would want to get close to that," he said.

In Farmington, New Mexico, city officials shut down water-supply intake pumps to avoid contamination and advised citizens to stay out of the river until the discoloration has passed. Don Cooper, emergency manager in San Juan County, said people should not panic because the EPA had told the county the spill would not harm people and that the primary pollutants were iron and zinc.

"It's not going to look pretty, but it's not a killer," Cooper told The (Farmington) Daily Times.

The mine is owned by Golden-based San Juan Corp., Durango attorney Nancy Agro said Thursday afternoon. She said the EPA had been operating at the site for years under an access agreement.

"Upon suspending work last year, the EPA backfilled the portal to the mine," Argo said in a statement. "On [Wednesday], while the EPA was removing the backfill from the portal to the Gold King Mine to continue its investigation this year, the plug blew out, releasing contaminated water behind the backfill."

At the time of the spill, EPA responders were at the scene evaluating the toxic materials already leaking into Cement Creek.

"There were several workers at the site at the time of the breach. All were unharmed," the San Juan Basin Health Department said in a news release Thursday. "The EPA recommends that recreational users of the Animas River avoid contact with or use of the river until the pulse of mine water passes."

Pet owners have been told to keep their dogs and livestock out of the Animas River until further testing is done.

Steve Salka, utilities manager for Durango, said he pulls water from the Animas in the summer to help replenish the Terminal Reservoir. He said while the city's main water source is another river, the Florida River, the contamination could cause serious problems.

"I want to know what's in it," he said Thursday. "The most important thing is what's in it. I need to know. Back in the 1800s, things were used in mining that aren't allowed anymore."

The Animas is a 126-mile river that flows into the San Juan River in Farmington, N.M. The San Juan eventually joins the Colorado River in Utah.

Todd Hartman, spokesman for the Colorado Department of Natural Resources, says the mine operated around the turn of the century. It was permitted again in 1986 but never produced in the modern era.

"Its permit was revoked in 2005," Hartman said.

Bill Simon, one of the coordinators for the Animas River Stakeholders Group, a water-quality collaborative, said the Gold King Mine was one of several abandoned sites in the area being monitored for decades.

"The EPA may have misinterpreted what was going on," Simon said.

Simon said iron oxide in the spill is his biggest concern, especially since it can clog the gills of fish and macro invertebrates.

"This river system is somewhat used to pretty poor water quality anyway," he said, "so it remains to be seen what effect its going to be on aquatic life."

The EPA agrees and says due to current and longstanding water-quality impairment associated with heavy metals in Cement Creek, there are no fish populations. Further, federal officials say, the Animas River has historically been impaired for several miles downstream of Silverton.

It's unknown if the spill could have any human health impacts, officials say.

"We are monitoring the situation very closely and working with the EPA to get testing results to make sure we minimize any health impacts," said Flannery O'Neil, spokeswoman for the area's health department.

Colorado Parks and Wildlife says it is monitoring wildlife health in the area.

The agency has placed a cage with fish in the river to see how they react to the waters. Officials say they should know by Friday morning if there were any effects.

"This is a significant spill," said Elizabeth Holley, an assistant professor of mining engineering at the Colorado School of Mines.

Salt Lake Tribune reporter Brian Maffly contributed to this story.