The deep-freeze weather hadn't seemed to faze the dolphin as it swam in the Gowanus Canal, which runs 1.5 miles through a narrow industrial zone near some of Brooklyn's wealthiest neighborhoods.
Marine experts had hoped high tide, beginning around 7:10 p.m., would help the dolphin leave the canal safely. But the dolphin was confirmed dead shortly before then, said the Riverhead Foundation for Marine Research and Preservation.
Experts aim to conduct a necropsy to determine why the dolphin died, but it may well have been ill when it got into the canal, said Robert DiGiovanni, a senior biologist with the foundation, which specializes in cases involving whales, dolphins, seals and sea turtles.
The New York Police Department said the marine foundation's experts had planned to help the dolphin on Saturday morning if it didn't get out of the canal during high tide. DiGiovanni said the experts had decided to hold off intervening Friday because of the stress the dolphin might have experienced in being captured.
"We erred on the side of saying, 'OK, if this is an animal that were just lost or disoriented, this would be the least invasive course of action, to give it the most chance of success,"' he said.
Earlier, with the dolphin swimming about and surfacing periodically, bundled-up onlookers took cellphone photos, and a news helicopter hovered above the Gowanus Canal.
The canal was named a Superfund site in 2010, meaning the government can force polluters to pay for its restoration. For more than a century before, coal yards, chemical factories and fuel refineries on the canal's banks discharged everything from tar to purple ink into the water, earning it the local nickname The Lavender Lake for its unnatural hue.
While the dolphin was churning up sediment and mud, it's unclear whether that contributed to its death, DiGiovanni said.
The dolphin, which appeared to be about 7 feet long, likely entered the canal from the Atlantic Ocean through the Lower and Upper New York Bays and then the Gowanus Bay, which leads to the canal. It's about 20 miles from the canal to open ocean.
Experts don't know why the dolphin wandered into the canal, but in general that can happen when one gets sick or disoriented, DiGiovanni said.
It's not uncommon for sea creatures to stray into city waters, though they don't often swim away alive.
A dolphin was found dead last August near Long Island, south of the canal. Another washed up in June in the Hudson River near Manhattan's Chelsea Piers sports complex.
In 2007, a baby minke whale that briefly captivated the city wandered into the Gowanus Bay and swam aimlessly before dying. Two years later, a 20-foot-long humpback whale took a tour of the city's waters before leaving New York Harbor safely.
AP Radio reporter Jackie Quinn in Washington contributed to this report.