Those giant, bright green patches swamping the shores of the Schuylkill River in Philadelphia's Fairmount Park aren't toxic waste or algae, rather aquatic vegetation.

The tracts are concentrations of Duckweed, a naturally occurring plant that flourishes in slow-moving water, according to Philadelphia Water Department spokeswoman Laura Copeland.

A few days ago, the plants started showing up ago along the riverbanks above the Schuylkill River dam at the foot of Boathouse Row. Copeland said recent heavy rains flushed the plants downstream.

"Despite being churned up by the recent heavy rainfall, the Schuylkill River is healthy," she said in a statement.

The plants can produce an odor, but there have been no complaints about the smell so far, Copeland said.

Water department officials expect the blooms to break up as they go over the dam and flow into the Delaware River.