New York has a lot of famous landmarks, but the humble, ubiquitous water tower is, perhaps, the true symbol of the city and the overriding memory that remains in most visitors minds forever after leaving NYC. These workhorses of water supply are found all over New York City, ensuring an adequate supply of drinking water to the city's population.
There is an extraordinarily large concentration of small water towers dotting the city's rooftops with recent estimates suggesting that there are over 10,000 water towers in the city, with over 100 built or replaced each year. There are only two large companies that build them, both of which have been competing since the 19th century.
Each tank is custom-built for the building it serves and is designed to hold an adequate capacity of water for the day. At night, more water is pumped into the tower for use the next day. Some buildings hide their towers within enclosures, but most expose them for all the world to see, often on platforms above the roof surface. The rooftops of southern Manhattan are dotted with many visible examples of this form.
The towers are needed because of the city's weak gravity-driven water supply. Water to NYC is provided through a 19th century system of aqueducts fed by various reservoirs. This system relies on gravity to deliver water and since it doesn't use any moving parts, this design has aged extremely well. However, the designers didn't envisage there would be many tall buildings in New York and to overcome the weak gravity-drive, any building over six storeys needs a water tower to ensure adequate pressure to every floor. By placing the water supply on the roof, buildings are able to use gravity to increase their water pressure.
Water towers have been appreciated as a whimsical aspect of the NYC skyline for well over 60 years now and there is just something mildly comical about seeing a modern stone and glass edifice with a pokey little water tower perched on the roof.