Why a Rainstorm Can Make the Hudson Risky for Swimmers
New York’s combined sewers explain why Hudson River water quality can deteriorate after rain. A filtered pool on the East River offers one possible route back into the water, but its pilot remains unfinished.
The Hudson River is not permanently off-limits to swimmers. Its water quality has improved enormously since the industrial and sewage pollution of the 20th century, and organized open-water events still take place around New York Harbor. The trouble is timing.
After a storm, the river beside Manhattan can change quickly. The culprit is a piece of infrastructure hidden beneath the street: the combined sewer.
When one pipe does two jobs
In much of New York City, wastewater from toilets, sinks and businesses enters the same underground system as rain running off roofs and pavement. In dry weather, that mixture travels to a wastewater treatment plant. There, solids are removed and the water is treated before discharge.
Heavy rain can send more water into the network than its pipes or treatment plants can accommodate. To keep the system from backing up into streets and buildings, overflow points release part of the mixture into surrounding waterways. These events are known as combined sewer overflows, or CSOs.
New York State records roughly 398 CSO outfalls in the city. Together, they discharge an average of about 18 billion gallons annually, according to the state Department of Environmental Conservation. The overflow can contain untreated or partially treated sewage, street runoff, bacteria, debris and other pollutants.
That is why the conspicuous pipes along Manhattan’s waterfront matter. They are emergency outlets for a city whose drainage system was built around a compromise: carry sewage and stormwater together, then divert excess flow when the network is overwhelmed.
The problem with a simple 48-hour rule
Hudson River Park advises caution after significant rainfall. Its researchers report that contamination near the Manhattan shoreline will often disperse within 24 to 48 hours because of the Hudson’s strong tides and currents. That is useful guidance, but it is not a guarantee that every location is safe after exactly two days.
Bacterial contamination varies by rainfall, tide, outfall activity and location. Weekly sampling can identify broader patterns, while Hudson River Park’s dashboard uses rainfall and previous bacterial measurements to estimate daily CSO risk. Neither turns the entire river into a uniformly monitored swimming area.
The relevant indicator is usually Enterococcus, a group of bacteria associated with fecal contamination. Finding it does not identify every pathogen in the water. Instead, it signals that sewage pollution, and therefore potentially harmful microorganisms, may be present.
There are also failures that have nothing to do with rain. On July 2, 2026, a power failure at the Yonkers Joint Wastewater Treatment Plant caused partially treated wastewater to enter the Hudson. State health and environmental officials subsequently told the public to follow local closures and avoid full immersion at closed beaches. New York City said its ocean beaches remained open for the Fourth of July weekend, illustrating how an upstream incident can produce different guidance from one jurisdiction and location to another.
A pool that treats the river at its edge
New York has experimented with floating baths since at least the 1870s. The modern + POOL proposal revives the basic idea but adds a crucial layer of engineering. Its walls are intended to filter river water before that water enters the swimming enclosure.
The project is planned for Pier 35 on the Lower East Side, in the East River rather than the Hudson. A filtration demonstration began there in 2024, with the city initially targeting a pilot for final testing in summer 2025. That schedule slipped. The organization’s timeline shows permitting, design review and geotechnical work continuing into 2026. As of August 2026, it says the first construction phase is nearing completion, while shoreline resiliency work around Pier 35 is continuing through the fall.
Where it is
Pier 35, Lower East Side, Manhattan
The delays reveal what makes the concept significant. This is not simply a barrier placed around open water. It must prove that its filtration system, structure and operating procedures can consistently satisfy health requirements in a tidal urban river.
If it succeeds, + POOL will not repair New York’s combined sewer system or make every stretch of the harbor swimmable. It could provide something more focused: a controlled public entrance into the city’s waterways, protected by treatment at the exact point where people meet the river.
Sources
- New York State Department of Environmental Conservation: Combined Sewer Overflows
- New York State Combined Sewer Overflows Map
- Hudson River Park: Contaminants and water-quality monitoring
- Hudson River Park: 2024 Water Quality Report
- New York State health and environmental agencies: July 3, 2026 Hudson River statement
- New York City: Pier 35 selected for the + POOL demonstration
- + POOL project timeline
- Hudson River Maritime Museum: A brief history of Hudson River water quality
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