[ROCKLOBSTER]

then moves through the massive sewer system. That snow is filthy dirty...

Which then runs into the river.

Don't they realize the snow is no dirtier than rain runoff, which also runs unrestricted into the river

[SamAdams76]

Actually the snow gets dirtier. The rain quickly runs off into the sewers but the snow is pushed to the side of the street by the plows, picking up all the oil and other residue in the street. Then the exhaust fumes from the cars and taxis slowly blacken the piles of snow with soot. Not to mention people trudging through it, spitting on it, tossing chewing gum and other such things into the snow pile.