What is sewage fungus?

It is slimy, murky brown, smells bad, and fills river enthusiasts and anglers with gloom.

But it is probably useful to know that sewage fungus, so called because of its fungal-shaped mass of filaments, is not fungus at all, but is primarily made up of several species of bacteria, most commonly Sphaerotilus natans, Beggiatoa alba, Carchesium polypinum, and Flexibacter species. By the time you see its characteristic slimy, floating fronds, it will contain some fungi and algae, too, and it always spells bad news for the river in which it is spotted and for any unfortunate wildlife living there.