About a million gallons of mine waste spilled into Cement Creek, which flows into the Animas River.
The Environmental Protection Agency was investigating toxic wastewater at an abandoned Gold King mine 55 miles north of Durango Wednesday morning when a dirt barrier that covered the mine tunnel collapsed and released the wastewater into the river, according to The Denver Post.
EPA officials took responsibility for what happened, saying they were sorry for the "huge tragedy."
"It's hard being on the other side of this. Typically we respond to emergencies, we don't cause them. It's something we sincerely regret," said Dave Ostrander, EPA's Region 8 director for emergency preparedness. "We are responsible for this and we are not running anywhere," he added, according to another report from The Denver Post.
On Thursday, Durango residents gathered on bridges and river banks to witness the blue waters of the Animas River turn into a thick, yellow-orange sludge.
The river was closed from the San Juan County line all the way to New Mexico, where the contamination was headed. Affected residents had been advised to stay away from the Animas, and even irrigation of land at Fort Lewis College was halted.
Stephen Lowrance, San Juan County Sheriff deputy, said a public health advisory to avoid Cement Creek and the Animas River had been issued, and warning signs had been posted near the water routes. The river will be closed until further notice as the EPA conducts tests on water samples.
"The river looks pretty nasty," Lowrance said. "It doesn't look like water; it just looks like sludge," The Denver Post reports.
The mine waste first contaminated Cement Creek and flowed into Animas River, which spills into the San Juan River in New Mexico. The San Juan River then flows into Utah's Colorado River. Authorities expect the contamination to finally settle in Lake Powell, a reservoir on the Colorado River.
Ostrander said the wastewater contained lead, cadmium, arsenic, copper, calcium and aluminium in "varying levels."
Meanwhile, New Mexico officials blasted the EPA for not warning them sooner about the contamination headed their way.
Chris Sanchez, spokesman for New Mexico Gov. Susana Martinez, said state officials were informed of the spill almost a day after the accident happened. He also said the EPA did not warn the state's environmental agencies, according to ABC News.
Authorities in Farmington, Nex Mexico announced all activities in the river should be suspended until the yellow-orange sludge had passed. Water supply intake pumps were also stopped.
The Colorado Parks and Wildlife tested the contamination's effect on animals by placing cages with fish in the Animas River, according to spokesman Joe Lewandowski.
Right now on the #AnimasRiver in #Durango. Currently monitoring fish in the river - some results may be known today. pic.twitter.com/tHk876ldLg
— CO Parks & Wildlife (@COParksWildlife) August 7, 2015
"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," he added, according to ABC News.
EPA issued a statement Friday saying: "Potentially impacted water systems have been notified and precautions are in place to ensure drinking water in homes is protected. EPA and New Mexico Environment Department (NMED) are providing assistance to community water systems and closely monitoring the situation."
