The Gowanus Canal is 1.8 miles long, stretching through the neighborhoods of Red Hook, Carroll Gardens, Park Slope and, of course, Gowanus. It was an important entryway into those neighborhoods for cargo-laden ships, with five bridges placed at ground level to allow automobiles to travel back and forth between each side. Three of these bridges are bascule bridges – what you may think of as the “stereotypical” drawbridge, with sides that flip up to open the passage. A fourth span, the Ninth Street Bridge, is lifted into the air to let ships pass underneath. The fifth bridge, however, is a bit of a misfit. The wooden span that crosses Carroll Street was built in 1889 to replace an earlier bridge, and is one of only four remaining retractile bridges in the United States – that’s a style of bridge where the span actually slides or swings out of the way.…