Once upon a time in New York (in 1677, to be precise), people got water from a well. It was dug down at the southern tip of Manhattan, where most non-native residents lived back then. For about 100 years, it supplied plenty of water for everyone living in the city. Then, with a growing population of 22,000, in 1776 NYC was forced to build its first reservoir to meet the water needs of its people.
Today there are over 8 million people living in the city. They use 1 billion gallons of water a day. Obviously, one well and one reservoir couldn't possibly supply that much water. So what does?
It starts with a watershed. This is basically just land—in New York City's case, 1,972 square feet of land north and west of the city. Rain and snow fall on the watershed. And the watershed drains that moisture into 19 reservoirs. These reservoirs are all connected by water-moving passageways called aqueducts. (Pictured below, the inside of the Catskill Aqueduct as it was being built in 1923.) They, along with 3 lakes, supply all the water for NYC.
From the reservoirs and lakes, water runs through the aqueducts into a big reservoir called Kensico. It sits about 30 miles north of the city and it holds 30 billion gallons of water. From there, the water travels again to another, smaller reservoir called Hillview. It's right outside the city and it holds 30 million gallons of water. The water is treated with a chemical called chlorine, which kills any bacteria that might be floating around in it. And also with ultra violet light, which kills any viruses.
Visit here to learn more about the science and history of water in New York City!