Heavy metals from this type of spill can contaminate the bottom of the food chain, environmentalists warn, and can cause harm to other species.

"Fish eat plankton, fish eat shad, and each time they do that, they obtain just a little bit of the heavy metal into their system,” said Wayne Gustaveson, a fisheries biologist who studies Lake Powell. “And it builds up in their system time after time after they eat many meals."

Fragile fish eggs are especially vulnerable to the heavy metals, and chemical damage to DNA can result in harmful mutations. In some parts of the Lake Powell region, which is pockmarked with abandoned mines, runoff from mines has already rendered some streams fishless.

The Utah Department of Health recommended Wednesday that people be cautious around Lake Powell, recommending "that recreationists on the San Juan River avoid drinking the water and wash after contact with the river as soon as possible," a National Parks Service release said.

The extent of the pollution from the spill remains unclear. The EPA has said it will know more about the concentrations of metals in the water by Monday, but a full assessment of the consequences of the spill could take years.

Anderson said a curve in the San Juan River could provide the metals a place to accumulate and eventually spill over into Lake Powell, which straddles the Arizona-Utah border.

"This flows down the river and will land in the sediment delta in the San Juan arm” of Lake Powell, Anderson said.

The metals will “kind of get locked in there, and as the river goes up and down, the river can cut into the sediment and release that stuff back into the lake,” said Anderson. “We need some long-term technology and monitoring plan to understand the implications of this over time.”

Lake Powell is man-made body of water that rose after the damming of the Colorado River, into which the San Juan flows. The spot attracts millions of vacationers, boaters and fishers each year. So far, tourism hasn’t taken a hit, USA Today reports, but officials have warned visitors not to swim in the San Juan River arm of the lake.

But worries persist.

“We’ve got concerned people in the community,” said Danny Woods, a fishing guide, told the paper. “This is our livelihood. This is our drinking water. This is our business.”