Riverkeeper's water quality data show that combined sewer overflows remain one of the most important sources of pollution affecting the Hudson River and its largest tributary, the Mohawk River. Combined sewers carry sewage from homes and businesses to treatment plants in dry weather. During rain, however, the same pipes carry street runoff, leading to overflows of polluted street water and raw sewage. Each point where there's an overflow is known as a combined sewer overflow, or CSO. In addition to hundreds of CSOs in New York City, there are 92 in the Capital District, 58 in the Hudson River Estuary, 52 in the Mohawk River, 26 in New Jersey and 12 in the Upper Hudson.

Cities have been investing to reduce the frequency and volume of sewage overflows, in order to comply with the Clean Water Act. State investments, including the landmark Clean Water Infrastructure Act, have provided critical grants to help with this work. The video below was produced by the City of Poughkeepsie to describe the challenge posed by its combined sewer system, and the work being done to reduce overflows.