East Harbor is a tidal estuary in Truro, Massachusetts that was originally a harbor until it was cut off from Cape Cod Bay to form a salt marsh lagoon, later renamed Pilgrim Lake. It is now within the Cape Cod National Seashore. Original Harbor Until the latter half of the nineteenth century, the East Harbor was a natural embayment deep enough to shelter Provincetown's fishing fleet during the winter, and was connected to Cape Cod Bay through a inlet. This effectively isolated neighboring Provincetown from Truro and other towns on Cape Cod. Until late in the 19th century, there was not a single road leading in or out of Provincetown – the only way to travel by land to the rest of Cape Cod was to first head north, traversing a series of tall, rolling sand dunes, and to then follow the thin strip of beach along the northern shore line, known as the "backshore". A wooden bridge was erected over the East Harbor in 1854, only to be destroyed by a winter storm and ice two years later. Although the bridge was replaced the following year, a traveler using it still had to traverse several miles over a sand route, which, together with the backshore route, would occasionally be washed out by storms. Prior to the railroad's arrival in 1873, Provincetown was like an island, in that it relied almost entirely upon the sea for its communication, travel, and commerce needs. Back Barrier Lagoon – Pilgrim Lake The harbor was artificially separated from Cape Cod Bay and cut off from tidal flow in 1868, when a dike was constructed across the inlet to facilitate the laying of track for the arrival of the railroad. The wooden bridge and sand road were finally replaced by a formal roadway in 1877. The exclusion of tides changed the harbor into a back barrier salt marsh lagoon. In the early 1900s, a real-estate developer named this lagoon "Pilgrim Lake", and the U.S. Geological Survey officially adopted that name in February 1910. But it also caused salinity to decline from natural levels of ~25-30 parts per thousand (ppt) to nearly freshwater by the time of the first documented fish survey in 1911. By this time the native estuarine fauna were largely extinguished. Over time, the blockage of tides had caused further ecological problems. The lack of tidal flushing deprived the system of salty, nutrient-poor,