The Croton is New York City's oldest system. It came on line on July 4, 1842, and was subsequently expanded and then completed in 1911. It covers about 380 square miles, East of Hudson (EOH) in northern Westchester, Putnam and Dutchess Counties, and a small part of Connecticut. It supplies about 10% of the City's needs under normal circumstances, and up to 30% in times of drought.. Presently, it includes ten reservoirs asnd three controlled lakes. The New Croton Reservoir is the terminal reservoir for the system. It feeds into the New Croton Aqueduct which goes to Jerome Park Reservoir, in the Bronx and further south.

The Catskill System, mostly West of Hudson (WOH), was started in 1907; its two connecting reservoirs are linked via a 92-mile aqueduct to the Kensico Reservoir, situated EOH. This component of the Catskill System was completed in 1927, but further additions within the City limits have been ongoing. It covers 570 square miles and supplies 40% of the City's needs.

As the City's needs grew with its population and per capita use (flush toilets and showers became widespread), the City, starting in the 1940s, tapped into the headwaters of the Delaware River. Four reservoirs were built, WOH, and their waters conveyed via a 105-mile aqueduct to the West Branch and Kensico Reservoirs. The Delaware system was completed in 1964. It covers 1010 square miles and supplies 50% of the City's water.

As the terminal reservoir for the Catskill/Delaware system, the Kensico serves 8 million people. In addition, over 80% of Westchester is supplied by the New York City system, whether the Croton, Catskill or Delaware.

Together, the three systems deliver 1.3 billion gallons per day to New York City and the metropolitan area. They supply over half of New York State with safe, unfiltered and still affordable water.