That Green Slime Blooming in the Schuylkill Is Just Duckweed, Y’all

Duckweed on the Schuylkill | Photo by Jared Brey

We saw some pictures floating around the social media airwaves this morning showing a long strand of green stuff floating on top of the Schuylkill River. Was it an algae bloom, some wondered? Evidence that the Schuylkill is increasingly toxic?

No, says the Philadelphia Water Department. It’s just duckweed. 

“Duckweed is naturally occurring, typical to low-flow and/or stagnant water conditions, and not harmful to the health of the river,” said Laura Copeland, public information officer for PWD, in an email. 

Fine, fine, but why is it popping up now?

“Schuylkill River flows during most of this summer have been lower than normal,” Copeland said. “Over the past week, we have seen heavy rainfall saturate the watershed, increasing river flows. The heavy rain fall has flushed the duckweed from upstream tributaries and areas along the Schuylkill River down river.  The large biomass of Duckweed observed in the downstream portions of the Schuylkill River is not a common occurrence and is related to the atypical low flow conditions that the Schuylkill drainage has experienced this summer.”

Copeland said duckweed shouldn’t cause anyone concern about water quality, either.

Algae blooms, on the other hand, can sometimes cause harm to wildlife. So, we’ll take duckweed any day.

“The Schuylkill River has not experienced a harmful algae bloom in recent memory,” Copeland said. “During every season, different types of algae can be observed in our rivers. PWD scientists monitor the algae along the rivers to ensure that our rivers are healthy and our drinking water is safe.”

OK, and so just one more question: Is the whole green mass going to flow over the falls and into the Delaware River in one piece?

“The biomass is likely to separate as it flows over the dam and into the Delaware River,” Copeland said. “The Delaware River is a much larger river system than the Schuylkill with considerably more flow and tidal impacts. It is highly likely that the biomass will become more dispersed as the duckweed moves down into the Delaware River and eventually out to the bay.”

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