Hudson River Sees Rare Algal Bloom as Heat and Drought Team Up Again

New York’s Hudson River faces an uncommon—and unwelcome—surge in toxic algal blooms, offering a cautionary tale about climate, infrastructure, and the price of neglect.

On an unseasonably balmy September afternoon, boaters and anglers on the Hudson River north of New York City found themselves adrift in a pea-green soup. For almost 60 miles, from Beacon to just above Germantown, a thick scum of cyanobacteria—commonly known as blue-green algae—had taken over the river. Such blooms are the bane of still ponds and shallow lakes, not of powerful tidal rivers that wrap themselves twice daily in saltwater from the Atlantic. Yet, this year, the Hudson’s green affliction proved the worst in forty years.

Scientists point to a trifecta of hotter water, prolonged drought, and routine sewer overflow as the culprits behind this year’s bloom. Stuart Findlay of the Cary Institute, a nonprofit research centre, wryly noted that prior evidence of blooms in the Hudson was faint and rare, “one year out of five you may get any evidence at all, much less of this extent.” According to federal monitoring, 2025’s summer was New York’s driest since 1999, receiving just 60% of its usual rainfall. That, paired with an average September temperature close to 71°F—matching records last set in 2018—left river currents sluggish and the water ripe for stagnation.

While cooler October weather has brought some relief, experts caution that the current respite is temporary. Shannon Roback, science director at Riverkeeper, warns that warming trends and increasingly erratic rainfall, both hallmarks on climate scientists’ ledgers, will nudge such harmful blooms from freak event to regular hazard. As the Hudson warms and its flows weaken, cyanobacteria will seize opportunities again.

The implications for New York City and its neighbors upstream are sobering. While the city itself, blessed (for once) by a saline tide at its southern reach, is spared from these freshwater blooms, its drinking water remains among America’s cleanest. The story is different for the millions who recreate, fish, or draw water from portions of the river north of the city. Cyanobacterial blooms can exude toxins that threaten the nervous systems of humans and animals—prompting health advisories and strangling local economies built around water sports in the Lower and Mid-Hudson Valley.

Economically, the timing bodes ill. The Hudson Valley’s tourist and real estate booms are in part due to the river’s allure. A vibrant swath of weekenders, restaurant owners, and river guides ride the river’s summer reputation. News of toxic algae is a deterring buzzkill, likely to worry not just swimmers but property owners and investors, too. Downstream, New York’s billion-dollar efforts to clean up the Hudson after decades of industrial pollution risk being undone by upstream failings.

There are political implications, as well. The bloom throws a harsh light on upstate New York’s aging sewer infrastructure and the persistent lack of investment in nutrient removal. Despite decades of environmental mandates, most municipal treatment plants along the river do not extract nitrogen and phosphorus—key algal nutrients—from their discharge, since retrofits are exorbitant and politically unpopular. During ordinary rainstorms, many upstate towns see their combined sewer and storm systems overflow directly into the river. Each freshet carries a toxic cargo, fueling blooms and eroding public confidence in government stewardship.

River trouble, globally shared

The Hudson’s malaise is not unique. Across much of the temperate world—from the Thames in England to the Loire in France, and the once-pristine lakes of Canada—similar blooms are making surprise appearances in rivers thought too dynamic for such slow-moving crises. What was once a problem for stagnant ponds now affects working waterways. The United States spends billions annually contending with blooms in the Ohio, Mississippi, and Great Lakes. These events, once rare, are becoming alarmingly routine as climate change and urban expansion overpower past engineering triumphs.

What lessons, then, should New York and its peers draw? Climate adaptation is one, as is honest reckoning with the true costs of deferred infrastructure. Wastewater plants equipped to strip nitrogen and phosphorus exist—they merely require the political will (and not-insignificant checks) to build and operate. New York’s experience with the Clean Water Act and subsequent Superfund cleanups shows that environmental investments pay off, but only if half-measures are shed. Agricultural run-off, a predictable aggravator, still escapes meaningful national pushback.

Neither optimism nor resignation is justified. The surge of toxic blooms on the Hudson is an unmistakable signal: climate change’s most salient risks may not manifest as mythic hurricanes or biblical floods, but as chronic, costly nuisances that undermine health, recreation, and property. The city’s saltwater bulwark offers some protection, but New Yorkers are not hermetically sealed off from upstream trouble. River health, quintessentially, is interconnected.

As New York’s state government begins drafting its next round of environmental spending, it would do well to remember that the price of inaction is rarely paid by those who have the most means to avoid it. The Hudson’s algal blooms may be a minor footnote to the city’s daily grind, but upstate and downriver, livelihoods and reputations suffer. Neglect has accumulated bit by bit, with each unmodernized water plant and short-term compromise. If the river is to remain an asset rather than a cautionary tale, it will take more than autumn’s cool reprieves to push back the green tide. ■