The scientific name is Didymosphenia geminata. Some people call it didymo for short, while others refer to it as rock snot.
Since it first appeared in Tennessee seven years ago, this single-cell algae has invaded the tailwaters below Norris, Cherokee, Wilbur, South Holston and Fort Patrick Henry dams, growing in thick mats that stick to the river bottom.
And while rock snot isn’t a human health risk, wildlife biologists say the fast-growing algae could be a threat to fish because it can smother their eggs and crowd out the organisms the fish feed on.
A native of cold rivers in the northern hemisphere, rock snot started spreading south about 10 years ago. In 2003, the algae was reported in the tailwater below Wilbur Dam in northeastern Tennessee — its first finding in the U.S. east of the Mississippi River.
Rock snot has spread aggressively around the world, and is growing at an accelerated rate even in its native range. It needs cold, well-oxygenated water and plenty of sunlight. Scientists say once it invades a river, there is no known way of eradicating it.
Tyler Baker, an environmental scientist with the Tennessee Valley Authority, said if a tailwater is cold enough to support trout, it’s a candidate for rock snot.
“It definitely prefers cold water and doesn’t tolerate shade,” Baker said. “It doesn’t seem to grow well beneath trees or in deep pools. We see it below Cherokee Dam, but it’s not as thick there as in the colder tailwaters.”
So far, rock snot has not been documented in any of Tennessee’s free-flowing streams. The algae, which tends to be beige or brown in color, also has not been found in the Hiwassee River, a popular trout fishery below TVA’s Appalachia powerhouse in Polk County.
Baker said the severity of the infestation diff ers with each tailwater, with the Wilber Dam tailwater being the worst, followed by the South Holston.
“Below Wilbur Dam, it looks like a brown, shag carpet covering 95 percent of the bottom,” he said.
The algae does not appear to be harming fish or other aquatic life, biologists say.
“We do aquatic insect surveys, and so far we’re not seeing any major shifts we can attribute to it,” Baker said.
“Everyone is pleased that what we feared would happen so far hasn’t.”
Mike Bone, owner of Clinch River Outfitters, said his biggest beef with rock snot is that it fouls his clients’ trout flies and fishing line. Bone said it’s a bigger problem for waders than boat fishermen because walking on the river bottom tends to stir the algae up.
Bone said rock snot also becomes a problem when water is released below the dams for hydropower generation.
“When the water pulses, it picks up all that algae and moves it down the river,” Bone said. “It’s aggravating stuff . It would be nice to get rid of it.”
Bart Carter, stream and river biologist for the Tennessee Wildlife Resources Agency, said rock snot colonizes the length of the tailwater, from below the dam downstream to the headwaters of the next reservoir.
Biologists say the potential for people to transport rock snot is substantial since the microscopic algae can survive in a drop of water, or on the felt sole of a wading boot.
In an effort to educate the public and prevent further spread, TWRA has posted signs at boat launches urging visiting anglers to clean and dry their gear.
So far, rock snot has not been confirmed in the Great Smoky Mountains National Park, or in any trout stream in the Cherokee National Forest.
Steve Moore, head fisheries biologist for the Great Smoky Mountains National Park, said he hopes the thick vegetation along the park’s streams will shade out rock snot.