Officials have discovered large swaths of "rock snot" algae covering the bottom of a 40-mile stretch of the upper Delaware River.

Aquatic biologist Erik Silldorff was near the New York/ Pennsylvania border for a meeting last week when he strapped on his wading shoes and ventured into the Delaware to discover the blooms.

"In the shallow waters, where the flow was pretty low, it's everywhere," Silldorff said. "There's a couple-of-inches-thick layer of algae oozing in and around my toes as I walk into the river."

Rock snot -- scientific name Didymosphenia geminata, also known as Didymo -- has been spotted in small amounts all over the U.S., but large blooms started appearing out West during the past decade.

Researchers don't know why, but think it might have to do with genetic mutations in algae.

Blooms in the Delaware River could disrupt plant and animal life cycles in the spring.

But Silldorff, who works for the Delaware River Basin Commission, said a bigger concern is that it may spread to areas where it could choke out other algae -- an important food source -- year round.

"Any cold water stream, any stream for instance that would have trout living in it year-round," Silldorff said, "has a risk that Didymo could dominate the ecosystem year round."

Plus, it's ugly.

"When Didymosphenia blooms in the river, it looks like the river is filled with old decaying toilet paper, so it's really unpleasant," said Academy of Natural Sciences researcher Marina Potapova.

Potapova said "rock snot" could take hold in the Delaware River near Trenton, but the water near Philadelphia is too salty to sustain it.

Silldorff urges anglers and boaters to decontaminate their gear with soap, bleach or hot water to prevent the algae's spread.