Didymo (Rock Snot)

Didymo is an invasive form of aquatic algae has been spreading in clean cold rivers and streams along the Eastern Seaboard. This highly invasive algae was detected in Maryland in 2008 in Gunpowder Falls below Prettyboy Reservoir and later in the Savage River below Savage River Reservoir. In 2012, didymo was detected in Big Hunting Creek (Maryland) and parts of the Delaware River (New York, Pennsylvania).

Under certain conditions, didymo can form dense mats along stream bottoms, engulfing and suffocating fish and other aquatic life. Didymo, sometimes called "rock snot", can also make fishing or other stream recreation difficult or impossible. Didymo has a slimy appearance, but feels like wet wool or cotton. It can be white, brown or yellow in color.

Biologists theorize that didymo is spread from stream to stream by human contact. Several state wildlife agencies recommend that outdoor enthusiasts thoroughly clean and dry all equipment that has been in streams, particularly fly-fishermen's waders or boots. In response to didymo outbreaks, several Mid Atlantic states have moved to ban felt-sole boots which are are thought to act as a carrier of the algae.