The city has shut down the 'flushing tunnel' for an overhaul. Built in 1911, this tunnel pumped clean(er) water into the canal. To combat the stagnation and stank caused by the lack of new water, the city has installed an oxygenation system to aerate the canal while the tunnel is worked on. I don't think it works.

(click photo for larger version)

Summertime, and the Living is Easy
2010-09-06

Gowanus Canal
2010-09-06

Gowanus Canal
2010-09-06

Gowanus Canal
2010-09-06