From the New York Department of Environmental Conservation
-- A threat to angling in fresh water streams and rivers across New York and other parts of the world is the invasive algae known as Didymo or "rock snot." (http://www.dec.ny.gov/animals/54244.html)
Creating a green thick mat across the stream bottom, didymo inhibits light and kills many organisms on the water bed causing a limited food chain for trout and other fish species. A high risk vector of didymo is believed to be felt-soled waders, since its absorbent material easily retains moisture and the microscopic cells of the algae.
The department encourages anglers to consider alternatives to felt-soled waders such as rubber studded boots, and asks that anglers remember waders are not the only human vector associated with the spread of aquatic invasives. Therefore, other fishing and boating equipment should be dried or disinfected before use.
Read more about how to help stop the spread of didymo and other aquatic invasives at http://www.dec.ny.gov/animals/48221.html and http://www.dec.ny.gov/animals/48221.html.