The 720-acre East Harbor system, which includes Moon Meadow, Salt Meadow and the East Harbor lagoon (sometimes called "Pilgrim Lake"), was artificially isolated from the Cape Cod Bay marine environment since the filling of the original 1000-ft wide inlet at the northwest end of the system in 1868 (large image). The exclusion of tides 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 has caused numerous ecological problems. The lack of tidal flushing deprived the system of salty, nutrient-poor, oxygen-rich Cape Cod Bay water and it became a stagnant freshwater pond. These conditions resulted in midge outbreaks, fish kills, and a proliferation of exotic species.