If you've taken a stroll along the Central Canal in the past couple of days, you've seen the muck that's suddenly bubbling up on the water's surface. This can be a little disconcerting, since the Central Canal, which is fed by the White River, carries a majority of the city's drinking water. The Indiana Department of Environmental Management says not to worry: What we're seeing is a harmless form of single-celled green algae. IDEM recommends not touching the stuff. But if you do come in contact with it, clean the affected areas as soon as possible.