"It scares me," said Lawton Weber, a fly fishing guide, who first spotted it on the Connecticut River in northern Vermont in June. "It's an aesthetic eyesore when it's in full bloom mode and its impact on the trout population is going to be significant."
It looks like a clump of soiled sheep's wool, a cottony green or white mass that's turning up on rocks and river bottoms, snarling waterways.
Over the past 10 years, the algae with a scientific name of Didymosphenia geminata, or didymo, has turned up in California, Washington, Alaska, Idaho, Montana, Nevada, Utah, Wyoming, Colorado, the Dakotas, Missouri, Arkansas and Tennessee.
"We're starting to realize it's all over the place," said Karl Hermann, a regional waste monitoring and assessment coordinator for the Environmental Protection Agency in Denver.
What started out in Vancouver Island in British Columbia "has suddenly just skyrocketed," he said.
The algae has the potential to bloom into thick masses with long stalks, blanketing the bottoms of some streams, threatening aquatic insect and fish populations by smothering food sources.
In New England, it has turned up in the White River, Connecticut River and the Batten Kill, a trout fishing mecca in southern Vermont that's famed for its hard-to-catch fish. Quebec is grappling with it in Matapedia River in the lower St. Lawrence.
There's no easy way to get rid of it. Experts say the only hope is to keep it from spreading. But that's a lofty challenge, since a single cell carried on absorbent fishing gear or clothing can be transferred — unknowingly — into other waters.
Vermont and New Hampshire have launched a radio campaign urging river users to scour their boats and clean their gear.
"Please don't take chances, disinfect your fishing gear," said Scott Decker, program supervisor with the New Hampshire Fish and Game Department.
It's unknown yet what effect, if any, the algae will have on fish populations, according to Sarah Spaulding, an ecologist with the U.S. Geological Survey and the EPA. But many are concerned.
"Once you remove (insects), young fish don't have anything to eat," said David Deen, a Vermont lawmaker who is a fishing guide and river steward for the Connecticut River Watershed Council. "Growth is slowed at best, and at worst they could starve to death."
In South Dakota, the algae is suspected of decimating brown trout populations in some spots.
Not only does rock snot threaten fish, but it's an unsightly nuisance. Jeff Williams, Arkansas' trout program supervisor, said out-of-staters were disappointed to find that the White River "wasn't as clean of a river as it used to be. The gravel is covered with didymo."
Once rare, the algae is perplexing scientists with the frequency of nuisance blooms.
"It seems to be something different — this expansion and production of big masses in the U.S.," Spaulding said.
Where it once preferred high-altitude, low-nutrient rivers, rock snot has shown up in rivers in Missouri, Arkansas and Tennessee, raising questions about what triggers its growth. Dammed rivers provide a constant water flow for it to bloom, and drought and changes in sunlight may also play a role.
"We think there are other factors that we don't yet know about," said Spaulding.
In New Zealand, the algae has infested as many as 55 waterways on South Island, growing up to 7 inches thick. Scientists are trying a copper treatment to manage it, and anyone who knowingly spreads the algae is treated as a criminal. The penalty: up to five years in prison and/or a fine of up to $100,000.
For now, the algae is in its early stages in New England, forming nubby brown growths on rocks.
"I think all of the Northeast is tuned in to see what the effects will be, so they can start taking preventive measures," said Mary Russ, executive director of the White River Partnership.