Samples taken by the tribe at Iron Gate and Copco reservoirs found extremely high concentrations of algae. The tribe said that blue-green scum on the water suggests a toxic compound that Microcystis aeruginosa algae can produce poses a health risk to swimmers. The algae cell counts are nearly 4,000 times that considered a moderate health risk by the World Health Organization.

The tribe called on Siskiyou County and dam owner Pacificorp to close the reservoirs to swimmers and water skiers.

”It's worse than last year,” said the tribe's water quality coordinator Susan Corum. “The responsible thing to do is to close these reservoirs immediately.”

The Siskiyou County Public Health Department has put out an alert saying that people should avoid wading and swimming in areas with algae mats, and be careful that children don't swallow the water. Fish should be eaten only after their guts and liver are removed and the fillets are rinsed with clean water.

”Sampling from previous years indicates that these algae are capable of releasing toxins that are potentially harmful to human health,” the statement reads.

While it said that there have been no documented cases of human illness from algae in California, studies from around the world have shown the toxic algae can cause eye irritation, rashes, mouth ulcers and vomiting and diarrhea.

The algae blooms in slow-moving water in the summer and fall. Several years ago, dogs died after exposure to much lower levels of toxic algae in the South Fork of the Eel River and Big Lagoon.

In September last year, Humboldt County health officials warned people on the lower Klamath River about algae blooms in the reservoirs, recommending that children and pets be kept out of the river.

There is a statewide effort to try to determine how best to sample algae-rich water, how to deal with big blooms and exactly what health risks are possible from exposure.

Pacificorp must also have a water quality certification from the state to satisfy the requirements of an ongoing relicensing process through the Federal Energy Regulatory Commission. Catherine Kuhlman with the North Coast Regional Water Quality Control Board said that blue-green algae is a part of the certification, and another state water quality improvement process called Total Maximum Daily Load.

”FERC has to have a certification that water quality will be met as Pacificorp operates the dams,” Kuhlman said.