On a sunny stretch of the Bear River near Colfax, the cool water carries a nasty surprise for swimmers and fishermen.

Look closely at the water flowing by. It carries clots of a feathery substance that looks like shredded toilet paper. Step into the gravelly shallows. Your feet will scream at you to get out of the sewage spill.

But this isn't sewage. About 10 miles of the Bear River below Rollins Reservoir is infested with a strange algae called "didymo," short for its scientific name, Didymosphenia geminata.

The algae's slang name describes the species better: "rock snot." Though it looks slippery, it feels more like a wet shag carpet.

"No doubt, it is pretty yucky, and that's the complaint from a lot of recreationists," said Leah Elwell, program director at the Center for Aquatic Nuisance Species in Livingston, Mont., which monitors the problem nationwide. "If you're kayaking, you don't want to get a mouthful of that. It does kind of foul up your day."

Didymo is considered native to North America. It has been reported in a handful of other California locations, including portions of the American River's south fork and the Feather River.

But scientists know very little about the algae, and they've grown alarmed by a mysterious change in its behavior in recent years.

So-called "nuisance blooms" of didymo, like that in the Bear River, are being reported with increasing frequency around the world. Experts don't know why, but suspect everything from climate change to a genetic mutation in the algae itself.

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