and out of the harbor, river systems, and tidal reservoirs. This interpretation came to be known as the theory of Tidal scour. This understanding of the harbor as a dynamic landscape assuaged concerns some had over the negative impacts of land fill operations of land and real estate developers. As the 19th century progressed, the acceleration of urban growth dramatically increased the need for more land. The Ordinance of 1641 extended the property rights of riparian owners from the line of low tide to a maximum distance of from the line of high tide. Generally, other states drew the line of private property at high tide. However, extending shore lines into bordering bodies of water was not unique to Boston. Chicago built into Lake Michigan, New York extended itself into the Hudson and East rivers, and San Francisco reclaimed sections of its bay. The Boston Harbor's unique geography inspired the law that made land reclamation such a widespread activity in Boston. By the end of the nineteenth century, the city had created more land in two generations than it had in the previous two centuries. Harbor Islands Boston Harbor contains a considerable number of islands, 34 of which have been part of the Boston Harbor Islands National Recreation Area since its establishment in 1996. The following islands exist within the harbor, or just outside it in Massachusetts Bay: Bumpkin Island Button Island Calf Island Castle Island Deer Island Gallops Island Georges Island Grape Island Great Brewster Island Green Island Hangman Island Langlee Island Little Brewster Island Little Calf Island Long Island Lovells Island Middle Brewster Island Moon Island Nixes Mate Outer Brewster Island Peddocks Island Raccoon Island Ragged Island Rainsford Island Sarah Island Shag Rocks Sheep Island Slate Island Snake Island Spectacle Island Spinnaker Island The Graves Thompson Island Two former islands, Castle Island and Deer Island, still exist in a recognizable form. Castle Island was joined to the mainland by land reclamation, while Deer Island ceased to be an island when the channel which formerly separated it from the mainland was filled in by the New England Hurricane of 1938. Nut Island is a small former island in Boston Harbor that was joined by landfill to the Houghs Neck peninsula in northeastern Quincy by the 1940s so it could be used as the site of a sewage treatment facility. Two other former islands, Apple Island and Governors Island, have