 Although still an island by name, Deer Island has been connected to the mainland since the former Shirley Gut channel, which once separated the island from the town of Winthrop, was filled in by the 1938 New England hurricane.Today, Deer Island is the location of the Deer Island Waste Water Treatment Plant, whose egg-like sludge digesters are major harbor landmarks.The island has a permanent size of , plus an intertidal zone of a further .