Sometimes, during heavy rain and snow storms, combined sewers receive higher than normal flows. Treatment plants are unable to handle flows that are more than twice design capacity and when this occurs, a mix of excess stormwater and untreated wastewater discharges directly into the City’s waterways at certain outfalls. This is called a combined sewer overflow (CSO).
---NYC Department of Environmental Protection

There are 434 Combined Sewer Outfalls in New York’s sewer system. These 434 sites pass approximately 27 billion gallons of raw sewage into New York’s waterways each year. 28 Outfalls consists of video and audio recorded at the 28 largest Combined Sewer Outflows in New York City. Together these 28 sites contribute 70% of the total flow of untreated sewage into New York’s waterways.

For more information these sites are a good starting point

NY Transported
Article on CSOs