Key takeaways
- Harmful algal blooms (HABs) are buildups of a type of toxic bacteria that can make people, pets, and wildlife sick.
- In the fall of 2025, there was an enormous and unprecedented HAB in the Hudson River, from roughly Poughkeepsie to Catskill.
- The HAB pushed toxin levels in untreated water above EPA and state safety thresholds at all five Hudson River drinking water treatment plants, which together serve more than 100,000 people. Treated water remained at safe levels.
- Pollution from sewage and stormwater, drought, and warm water are three of the main drivers — and all three are expected to get worse with climate change.
- Riverkeeper is pushing New York State to cut nutrient pollution, fund HAB monitoring and response, and improve public communication about HABs.
What is a harmful algal bloom?
Harmful algal blooms (HABs) in the Hudson River watershed happen when a certain type of bacteria grows quickly and forms a green layer on the surface of the water. This “cyanobacteria” — also known as blue-green algae — produces toxins that can damage the liver and nervous system, and cause gastrointestinal illness and skin/eye irritation in people and animals.
Because exposure can come from swimming, drinking water, or even inhalation, HABs keep people away from the water and require stronger, more expensive treatment of drinking water.
What causes harmful algal blooms?
The 2025 HAB on the Hudson was likely caused by a combination of large amounts of nutrients (phosphorus and nitrogen) from sewage, stormwater runoff, and agriculture, plus slower river currents and higher water temperatures during a warmer-than-usual summer. These conditions are likely to become more common with climate change.
What happened on the Hudson River in 2025?
HABs are common in lakes and have been documented in some creeks throughout New York, but have rarely happened in the Hudson River. From September through October 2025, a widespread, persistent, and toxic HAB occurred in the middle of the Hudson, from roughly Poughkeepsie to Catskill. Riverkeeper and the Cary Institute, who've tracked cyanobacteria in the river for decades, confirmed this was the largest and most impactful HAB the Hudson has seen in living memory.
Similar HABs on other large rivers in the U.S. have also become more common in recent years. While the 2025 HAB on the Hudson was unprecedented, scientists warn that the Hudson could become prone to HABs as the climate changes.
Are harmful algal blooms dangerous?
The cyanobacteria from HABs produce toxins that are dangerous for humans and other animals. Consuming these toxins can damage the liver and nervous system, and cause gastrointestinal upset. During the 2025 bloom, evidence suggests that people and dogs were exposed to cyanobacteria while swimming and boating on the river, highlighting the need for clearer, faster public notification when there is a HAB.
Did the Hudson River HAB affect drinking water?
The Hudson River is the source of drinking water for more than 100,000 people in the "Hudson 7" communities of Poughkeepsie, Rhinebeck, Esopus, Hyde Park, and Lloyd. During the 2025 HAB, toxin levels in untreated water at all five treatment plants were higher than the levels the EPA uses to trigger a public warning.
When a plant operator first spotted what looked like cyanobacteria in their intake, they called Riverkeeper for help. Riverkeeper's science team sampled the water, measured toxin levels, and helped operators coordinate with the New York State Department of Health (DOH). The operators adjusted their treatment processes in real time to keep toxins out of tap water. And it worked — toxin levels were brought down to safe levels before reaching customers at the tap.
The HAB also increased the risk of disinfection byproducts (DBPs) — harmful, cancer-linked compounds that can form when disinfectants react with organic matter in the water. Several Hudson River treatment plants reported an uptick in DBPs during the HAB, along with complaints about changes in tap water taste and odor, which can also be caused by the presence of cyanobacteria.
Will this happen again?
Most likely, yes — and probably more often. Droughts are expected to become more frequent and severe as the climate changes, bringing more extreme weather, longer HAB seasons, and warmer water temperatures. At the same time, nutrient pollution tends to rise along with population numbers because of aging sewer infrastructure and agricultural runoff. We don’t expect HABs to affect the Hudson every year, but these trends mean that HABs are likely to become a more regular feature of life on the Hudson unless we address the pollution driving them.
Reducing nutrient pollution — through wastewater treatment upgrades, fixing sewage leaks and overflows, dealing with aging septic systems, and keeping agricultural and urban runoff out of the river — can meaningfully lower the risk of future blooms.
What is Riverkeeper doing about harmful algal blooms?
Riverkeeper is pushing state agencies and lawmakers to take action on several fronts:
- Cut nutrient pollution at the source. We're calling on the New York State Department of Environmental Conservation (NYS DEC) to analyze its water quality data for the Hudson, and reduce nutrients that end up in the river from wastewater plants and other major sources. We are calling on the agency to develop a "pollution diet" (known as a Total Maximum Daily Load) for the Hudson River and finish long-overdue TMDL plans for the Mohawk and Wallkill rivers.
- Secure state funding for clean water infrastructure. We're urging the NYS Legislature and Governor Hochul to keep funding the Clean Water Infrastructure Act and the Environmental Protection Fund, which support the Hudson River Estuary Management Program, the Mohawk River Basin Program, and local wastewater upgrades, especially as federal funding and enforcement face cuts.
- Expand long-term monitoring. Funding for certain NYS DEC and Cary Institute ecological monitoring of the estuary runs out in 2026. We're calling for new investment to keep that monitoring going and to expand the Hudson River Environmental Conditions Observation System (HRECOS), a network of real-time sensors that could help drinking water plants respond quickly to emerging threats.
- Support drinking water operators. We're advocating for the NYS DEC and DOH to continue to support source water monitoring and HAB response for the Hudson 7 and other drinking water suppliers impacted by HABs.
- Close gaps in public communication. Riverkeeper is working with state and local partners to build a clearer communications protocol so that when a HAB is detected, people get consistent, timely warnings before they're exposed while swimming, boating, or letting pets in the water.
What should I do if I spot a harmful algal bloom?
If you suspect that you’re seeing a HAB, avoid swimming, boating, or playing in the water, and don’t drink it. For more details on what a HAB looks like and to report it, visit What to do if you spot an algae bloom.
Riverkeeper's Water Quality Portal lets you explore water quality data across the Hudson River watershed, with monthly data on algae indicators.