The official name for Rock Snot is Didymo.

It’s a single-cell organism, a type of algae, in the form of a tiny stalk, but with a weird ability to split, multiply and weave itself into a slimy mat. It’s that multiplying effect that creates tufts of brown, white, or yellow at the bottom of freshwater streams.

The gooey-looking substance then covers rocks and is hard to pull off. It looks nasty and untouchable, but it feels like wet wool.

Didymo is mainly found in freshwater streams and is spreading through tributaries that flow into the Chesapeake Bay.

North East River from North East Community Park (Maryland)
North East Community Park at the entrance to the North East River into the Chesapeake Bay

It traveled to us from Scotland, but scientists don’t know exactly how. It wasn’t mentioned in scientific reports in the 1800s and fishermen, who are the ones who report it the most these days, didn’t write about it in the early 1900s.

Officials first confirmed Rock Snot in Maryland in 2008.  They found it in Baltimore County’s Gunpowder River below Prettyboy Reservoir, a major drinking water supply.

The state’s response was to issue warnings and “strongly” encourage fishermen to wear rubber-soled waders and boots. Standard felt-soled waders were algae sponges. The hope was to keep the invasive species isolated because once found, it’s nearly impossible to eradicate. The state then constructed wash stations for anglers to clean their gear before leaving the river area.

It didn’t work. The state banned felt-soled waders in 2011. That just annoyed the fishermen; many didn’t believe it was a serious problem. Not enough washed their gear clean of the algae.

Rock snot spread to Savage River in 2009, the north branch of the Potomac River in 2011, and Big Hunting Creek in 2012.  By then, about four percent of wild trout stream miles were infested as well as four of the six tailwater trout area.

Anglers come across it mainly in the Spring when they start boating again.

rock snot (didymo algae) on a rock
Rock Snot, an alga called didymo which is an invasive alga found in freshwater tributaries flowing into the Chesapeake Bay

It’s not dangerous to people but could be to fish. Scientists are still trying to figure out long-term impacts on the Chesapeake Bay region. Researchers think climate change could make it worse, but they’re not sure yet.

The best solution? If you come across it, wash the snot off your boots and equipment to keep it from spreading.

Courtesy Maryland Department of Natural Resources