In a park in West Baltimore, a spectacular arched stone bridge rises over a stream called the Gwynns Falls, which flows into Baltimore’s Inner Harbor and the Chesapeake Bay.Although the bridge is beautiful and trees beside the stream are blossoming, when you look at the stream, you see that the Gwynns Falls is troubled. A whirpool of sludge twists under the bridge, with a gyre of Styrofoam cups and plastic bottles.