Concentrations of the algae Microcystis aeruginosa are so great that even breathing vapors caused by water skiing could cause illness, officials said, and swallowing even a few ounces of lake water could bring effects as severe as organ failure.

The algae is not a new problem on the Klamath. But the highly toxic concentrations now present are yet another bruise for a troubled river that also triggered a commercial salmon fishing disaster this year. 

The bright green algae is so thick that parts of Irongate and Copco reservoirs on the Klamath look like radioactive putting greens.

"It's no longer just a matter of fish. It's now a human health issue," said Zeke Grader, director of the Pacific Coast Federation of Fishermen's Associations.

The algae itself is usually not toxic and is common, at low levels, in many water bodies. But in warm, stagnant water it often blooms in huge floating mats, releasing a toxin, called microcystin, that can cause illness.

"There isn't any way to dance around this. It's a serious situation," said Peggy Lehman, an environmental scientist at the state Department of Water Resources and an expert on the algae. Lehman is studying another large Microcystis bloom in the Sacramento-San Joaquin Delta suspected of contributing to the decline of several fish species.

A consultant hired by the Karuk Tribe recently tested water samples from the reservoirs and found algae concentrations 3,900 times greater than a standard set by the World Health Organization, the operating standard for California water agencies.

In a joint news release Monday, the federal Environmental Protection Agency and California Water Quality Control Board warned that an adult who ingests just 4 ounces of water from the reservoirs could be exposed to microcystin at 100 times the standard. A child's exposure would be nearly 400 times the standard.

"They're pulling up values that are very high and quite dangerous to all wildlife and humans," Lehman said.

Officials warn people to avoid contacting the algal blooms, to avoid swimming near them and not to eat fish caught during the bloom. Children, pets and livestock should be kept away from the affected waters.

Skin contact with the affected water can cause rashes, eye irritation and mouth ulcers.

"The presence of a blue-green algal scum in a swimming area means there is a high probability of adverse health effects for those who come in contact," said Catherine Kuhlman, executive officer of the North Coast Regional Water Quality Control Board.

The toxin itself is invisible and highly stable in the water, persisting for weeks. As a result, the Karuk Tribe fears water running out of the reservoirs poses a hazard to people downstream.

The reservoirs have not been closed to the public. State officials left that decision to Siskiyou County, which has kept the reservoirs open and posted warnings.

The hydroelectric dams that impound the reservoirs are owned by PacifiCorp., an electric utility based in Portland, Ore. Spokesman Deston Nokes blamed the algae on pollution from upstream farms and towns, which provides nutrients for the algae to feed on.

Craig Tucker, spokesman for the Karuk Tribe, agreed pollution is a factor. But he said stagnant water in the reservoirs gives the algae a chance to bloom, citing this as another reason the dams should be removed.

The dams could be subject to new operating rules, and even removal, as part of a federal relicensing process now under way.

"There's a real water quality problem up there and it's going to have to be addressed, but everyone's in denial about it," said Grader.