system, New York is one of only four major cities in the United States the majority of whose drinking water is pure enough not to require purification by water treatment plants. The Croton Watershed north of the city is undergoing construction of a US$3.2 billion water purification plant to augment New York City's water supply by an estimated 290 million gallons daily, representing a greater than 20% addition to the city's current availability of water. Manhattan, surrounded by two brackish rivers, had a limited supply of fresh water. To satisfy its growing population, the City of New York acquired land in adjacent Westchester County and constructed the old Croton Aqueduct system there, which went into service in 1842 and was superseded by the new Croton Aqueduct, which opened in 1890. This, however, was interrupted in 2008 for the ongoing construction of a US$3.2 billion water purification plant that can supply an estimated 290 million gallons daily when completed, representing an almost 20% addition to the city's availability of water, with this addition going to Manhattan and the Bronx. Water comes to Manhattan through the tunnels 1 and 2, completed in 1917 and 1935, and in future through Tunnel No. 3, begun in 1970. Address algorithm The address algorithm of Manhattan refers to the formulas used to estimate the closest east–west cross street for building numbers on north–south avenues. It is commonly noted in telephone directories, New York City travel guides, and MTA Manhattan bus maps. See also History of New York City List of Manhattan neighborhoods List of people from Manhattan Manhattanhenge Manhattanization Manhattoe National Register of Historic Places listings in Manhattan Sawing-off of Manhattan Island Timeline of New York City Notes References Citations Sources Burke, Katie. ed. Manhattan Memories: A Book of Postcards of Old New York (2000); Postcards lacking the (c) symbol are not copyright and are in the public domain. Jackson, Kenneth T. and David S. Dunbar, eds. Empire City: New York Through the Centuries (2005), 1015 pages of excerpts Still, Bayrd, ed. Mirror for Gotham: New York as Seen by Contemporaries from Dutch Days to the Present (New York University Press, 1956) Virga, Vincent, ed. Historic Maps and Views of New York (2008) Stokes, I.N. Phelps. The Iconography of Manhattan Island, 1498–1909 compiled from original sources and illustrated by photo-intaglio reproductions of important maps plans views and documents in public and private collections (6 vols., 1915–28).