American Rivers is today naming the Nissequogue River among America’s Most Endangered Rivers® of 2026, due to the recently approved proposal to build a new dam without a comprehensive environmental impact statement. The unexpected breach of Stump Pond Dam in 2024 restored an important stretch of the Nissequogue, reconnecting habitat, allowing the floodplain to function more naturally, improving conditions for fish and wildlife, and reducing flood risks for nearby communities. Building a new dam would erase those benefits, and the environmental costs would far outweigh any recreational and flood control benefits.
“The Nissequogue River has shown us what’s possible when a river is allowed to flow freely and heal itself,” said Emilia Deimezis, associate director of river restoration for American Rivers. “In just one year, we’ve seen an incredible resurgence in native plants, fish and birds in the floodplain. Building a new dam would reverse that progress and saddle taxpayers with millions in unnecessary costs.”
The Nissequogue River is one of Long Island’s most ecologically important waterways, flowing from its headwaters to Long Island Sound. It is designated a Scenic and Recreational River by New York State and recognized as a Significant Coastal Fish and Wildlife Habitat by the U.S. Fish and Wildlife Service.
Named for the Indigenous people who lived in the region, the groundwater-fed river supports rare Atlantic white cedar, native brook trout, and important nesting and migratory bird habitat—all just 50 miles from Manhattan. The river is also a valued destination for fishing, hiking, wildlife watching, and other outdoor recreation.
The dam was originally constructed in 1798 to power a grist mill, and formed Stump Pond. Located in Blydenburgh County Park, the aging dam failed during a major storm in August 2024, draining the 118-acre impoundment overnight and transforming the site into more than two miles of free-flowing river.
“While the Stump Pond Dam—and the mill it powered—played an important role in the colonial history of Smithtown,” said Enrico Nardone, executive director of the Seatuck Environmental Association, “that history isn’t lost because Stump Pond Dam is gone. We now have a unique opportunity to reconnect the Nissequogue River and restore its original role in transporting nutrients and sediments to Long Island Sound and providing habitat for a wide diversity of native species, including ecologically important migratory fish that help to drive our coastal ecosystem.”
In just one growing season, the newly exposed floodplain has revegetated from the native seedbank, with more than 120 plant species documented, including several species identified by New York State as Species of Greatest Conservation Need. Since the dam breached, birders have documented 12 species at this site that had never previously been recorded there in Cornell University’s eBird database, reflecting an increase in bird diversity. The river has also become healthier for native brook trout with cooler water temperatures resulting in reduced stress on local populations. The restored 118-acre floodplain now helps to slow, store, and absorb stormwater—reducing flood risks for nearby communities. Residents upstream have already reported noticeably drier basements during storms.
“Save the Sound cares deeply about the Nissequogue because healthy, connected rivers are essential to clean water, wildlife, and resilient communities,” said Sara Cernadas-Martín, PhD, director of the New York Ecological Restoration Program at Save the Sound. “The recovery already underway on the Nissequogue shows that this is a once-in-a-generation opportunity to reconnect people with a healthier river, one that can support wildlife, strengthen resilience, and give future generations the chance to experience, enjoy, and learn from a thriving river.”
Despite these improvements, in March of 2026, Suffolk County approved a proposal to build a new dam, a project expected to cost more than $10 million in construction alone, with additional long-term maintenance obligations.
With the Suffolk County’s environmental review process now concluded, and the proposed dam moving into permitting, American Rivers and its partners are calling on New York State agencies to require a comprehensive environmental analysis through the permitting process, including a robust alternatives analysis to identify approaches that would protect the river’s restored habitat, floodplain function, and natural flow.
Learn more about America’s Most Endangered Rivers® of 2026, the urgent threats they face, and the solutions needed to protect and restore these vital waterways.