Complex sentence: The original reservoir was called "Croton Lake", and was New York City's first source of water not located in New York City, being completed in 1842 by damming the Croton River, a tributary of the Hudson River.
Split into simpler sentences: The original reservoir was called Croton Lake, and was New York City's first source of water not located in the city itself. The reservoir was created in 1842 by damming the Croton River, a tributary of the Hudson River.