This is Didymo algae (Didymosphenia geminata), also appropriately known as "rock snot". It continues to spread a deadly path in trout habitat. This algae forms a thick layer on stream beds, in time choking out most aquatic insects and greatly reducing food supplies for trout. New Zealand has had a significant problem with Didymo for some time. Since the mid-Eighties, Didymo has become a growing nuisance in North America. It is present in rivers throughout the West, Arkansas, and Canada. This summer the algae was discovered for the first time in New England waters (Connecticut, White and Battenkill Rivers). The rapidly growing territory of this invasive single celled organism has led many scientists to believe humans play a significant role in it's spread by inadvertently transporting Didymosphenia geminata on fishing boots, waders and boats. Please check out the links below to learn more about Didymo and what you can do to help prevent the spread of this freshwater diatom.