Stormwater is generated from rain and melting snow, which is conveyed over impervious surfaces such as rooftops, streets, and sidewalks. Rather than being absorbed into the ground, much of the stormwater in New York City flows over impervious surfaces into roof drains or catch basins in the streets, and from there into the sewers. These impervious surfaces cover approximately 72% of New York City’s 305 square miles in land area and generate a significant amount of stormwater. Stormwater can pose challenges to the City by triggering combined sewer overflows, washing pollutants into our waters through the separate storm sewer system, and causing flooding. This section describes how stormwater is conveyed through the City, the challenges it creates, and what the City is doing to overcome them.