An invasive algae called rock snot is rapidly spreading through the Delaware River.
There are big concerns that it will spread to nearby streams, causing serious damage.
Those who protect the Delaware are putting out a warning.
It's the time of year when people flock to the Delaware River in the Poconos for fishing and all kinds of water activities.
Now those enjoying the water above need to know what's below.
It is called rock snot, technically didymo. It is an invasive algae that looks almost like wet toilet paper clinging to rocks.
It chokes out insects and plants fish and birds rely on.
"The way you tell you pick it up and roll it and oh yeah, that has substance it doesn't just fall apart in your hands," said biologist Erik Silldorf.
He discovered the widespread outbreak of the rock snot. It has appeared in small patches in the past. This year it spreads over 150 miles of the Delaware, from far north of Bushkill, all the way past the Lehigh Valley.
One reason for it could be the mild, dry winter.
"It might be and maybe was a one off event we saw it in 2012 and maybe as we get toward more normal flows and normal winters and springs that we won't see these infestations in the future, but we're not sure," Silldorf said.
While there are concerns about the Delaware itself, the biggest concern is that this rock snot will get transferred to nearby creeks and streams. That's why people who use the river need to take precautions.
"Get it down get it wet, get it covered by your soapy water," said Ranger Kathleen Sandt.
Rangers at the Delaware Water Gap National Recreation Area are asking anyone who uses to river to wash anything that gets wet with dish detergent or two percent bleach solution.
"If you come out fishing in the Delaware River and your gear gets wet and you don't clean it thoroughly and properly and allow it to dry thoroughly, you can easy bring the cells of the didymo algae into another body of water when you go fishing or recreating somewhere else," Sandt added. "If they don't, they're killing their fish. They're killing their sport."
Fisherman Dennis Csekitz has heard the dangers. He goes from the Delaware to other streams and lakes all the time. He will be washing his boat and gear. He knows the rock snot can dangerously clog a small stream.
"I get home tonight I'm going to put a detergent on it. I'm going to wash off my tires if it is on this ramp it could spread elsewhere," Csekitz said.
Signs will be going up all along the Delaware warning people to clean their gear every time they use the river.
Park rangers stress the Delaware River is safe to use, as long as you take precautions.