A team of US regulators investigating contamination at a gold mine have accidentally released a million gallons of orange-coloured waste water into a local river system.
The waste contained sediment and metals and had been held behind a barrier near the abandoned Gold King Mine in Colorado.
It spilled into Cement Creek, which flows into the Animas River in San Juan county, said Environmental Protection Agency spokesman Rich Mylott.
Several workers were using heavy equipment to pump and treat the waste when the breach happened.
No one was injured, Mr Mylott said.
He added: “The primary environmental concern is the pulse of contaminated water containing sediment and metals flowing as an orange-colored discharge downstream.”
Pictures showed a trio of kayakers floating down a orange-hued stretch of the Animas River, near Durango.
Federal and Colorado health officials have warned water users downstream to turn off intakes and avoid activities in the water until the contaminated water passes.
Officials in Durango said tap water was safe for its water utility customers, saying they have stopped pumping water from the Animas River and are instead drawing water from the unaffected Florida River.
The EPA said it will be taking samples downstream to confirm the waste has passed and poses no risk to aquatic life or water users over the coming days.
The first set of results is expected on Friday.
The EPA also said it will assess the damage near the mine and any residual releases of mine water.