Boston Harbor Islands National Recreation Area - Natural Features

Water is perhaps the key visible landscape element for the Boston Harbor Islands. The 34 islands of the park are surrounded by the marine waters of Massachusetts Bay and the Boston Harbor estuary. They are part of a "drowned drumlin field," the only collection of glacial drumlins that intersects a coast in the United States. The 500-square miles of Harbor waters profoundly influence the islands, from twice-daily tidal fluctuations and currents, to storm waves from New England's Noreasters . Except on those islands composed largely of bedrock, these coastal forces continually shape and reshape the islands. Rates of erosion on the islands can be dramatic.

In general, the geographic history of the islands can be summarized as, first, glacial shaping of landforms with a new northwest-southeast orientation; second, sea level rise after glacial retreat, which partly immersed the glacial hills to form islands, with erosion and deposition reorienting the islands to a northeast-to-southwest bearing; and third, human intervention, leading to greater local erosion control. Boston Harbor, with a mean tidal range of 10.33 feet, has one of the largest tidal ranges in the United States (although tides vary from location to location in the harbor due to local circumstances; differences greater than 18 feet have been recorded). Most of the harbor varies in depth from about 3 to 30 feet, but the north and south ship channels, between Logan International Airport and Spectacle Island, and between Georges Island and Hull, can reach depths of 60 feet. Tidal currents within Boston Harbor vary greatly in speed because of the irregular bottom topography and the large number of islands; the highest speeds are found at the deepest levels, and the slowest speeds are found in shallow areas. Boston Harbor has a relatively slow "flushing" rate in which about one-third of the water is "replaced" during each tide cycle.