Police in Old Bridge confirmed the death. The dolphin was first spotted in the South River in Old Bridge on Wednesday.
Over the last few days people gathered on a nearby bridge to watch the unusual sight.
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Experts from the Marine Mammal Stranding Center had hoped it would swim back out into the ocean.
NewsCopter 7 captured images of the dolphin in the river near the Old Matewan Road overpass.
An Eyewitness News viewer captured up-close video of the dolphin:
The South River runs off the Raritan River, which connects to Raritan Bay -- but this dolphin was nowhere close to the bay.
The dolphin made an appearance above water every 1 1/2 to 2 minutes.
Teams with the stranding center initially tried using a boat to coax the roughly 400-pound dolphin toward the Raritan Bay, and the dolphin appeared to finally be doing that Saturday morning. But volunteers who were following the dolphin from a distance soon saw it swim into a shallow area.
They alerted center staffers, who saw the dolphin was having seizures and struggling to breathe. They took it out of the water, and it soon died.
A necropsy will be conducted to determine what caused the dolphin's death.
(Some information from the Associated Press.)