Combined Sewer Overflow

The 1.8-mile-long Gowanus Canal was constructed in the 1860s on the site of a former salt marsh and creek. It has a long history of environmental issues, including industrial pollution and combined sewer overflow (CSO). Combined sewer systems carry sewage from homes and businesses, as well as stormwater that flows over streets and other paved surfaces. A typical rainstorm in NYC generates more water than our sewer system can handle, which causes raw sewage and untreated stormwater to overflow into our waterways. CSOs are the greatest source of ongoing pollution to the Gowanus Canal. The overloaded sewer system currently discharges about 363 million gallons of raw sewage and polluted run-off, or combined sewer overflow (CSO), into the Gowanus Canal each year, spread over about 40 rainfall events.

Reducing CSO

The NYC Department of Environmental Protection (DEP) is designing and constructing grey and green infrastructure across the Watershed to reduce CSO and decrease the amount of raw sewage flowing into the Canal. DEP installations currently underway include a growing number of curbside rain gardens, or bioswales; a high level storm sewer system; and two large underground sewage detention tanks, as part of the Superfund remedy. Learn more about DEP’s work in Gowanus here. Visit the DEP’s Green Infrastructure page to learn more about green infrastructure across the city.

While planned grey and green infrastructure investments are part of the solution for improving water quality in the Gowanus Canal, they do not fully address CSO. Once these projects are complete, there will still be about 115 million gallons of annual CSO discharge left unmanaged. While we recognize that 100% CSO elimination in the Gowanus Canal may not be feasible without the complete separation of more than 7,000 miles of sewer pipes that make up NYC’s combined sewer system, GCC’s long-term vision for a clean and resilient Gowanus Canal and Watershed is to achieve as close to zero CSO as possible. Through advocacy and action towards comprehensive watershed planning, we should work to strategically address existing and future sources of pollution to reach this goal.

Net Zero CSO and Unified Stormwater Rule

During the Gowanus Rezoning process, we advocated for a net zero CSO rezoning in Gowanus where new development would not increase pollution by contributing additional sewage and stormwater to the Gowanus Canal. The City met this community priority through the Unified Stormwater Rule (USWR), which updated the City’s requirements for how stormwater is managed on all new and redeveloped sites that discharge to the City sewer in both combined and separately sewered areas. In combined sewer areas, like Gowanus, by limiting or slowing the volume of stormwater entering the sewer, the USWR works to free up capacity in the combined sewer and reduce combined sewer overflows.

The Final Environmental Impact Statement for the Gowanus Rezoning showed CSO reductions to the Gowanus Canal by 5 million gallons per year with the forthcoming Unified Stormwater Rule in place as new development installs stormwater management practices required under the new rule.

As a part of the Gowanus Points of Agreement, the City committed to report on compliance with the Unified Stormwater Rule and its impact on CSO volumes into the Canal. Updates on how this will be tracked were shared at a Gowanus Oversight Task Force Meeting on Thursday, September 28th, 2023. A video recording of that public meeting is available here.

Read more from GCC

GCC Draft Scope of Work Testimony

Memo RE Unified SW Rule

Follow-up to Dec. 2 Infrastructure Meeting

Learn More


NYC Soil & Water Conservation District
S.W.I.M. Coalition