A dolphin found its way into the headwaters of the Gowanus Canal in Brooklyn on Friday, apparently trapped where nature has long struggled to survive.

As crowds of onlookers braved the bitter cold to catch a glimpse from the Union Street bridge, curiosity turned to concern as the dolphin appeared to be having difficulty swimming in the narrow 1.8-mile-long waterway.

An expert from a marine animal rescue organization said he had never seen a dolphin so far up the canal, away from where it empties into New York Harbor.

In the afternoon, Julika Wocial, a marine biologist from the rescue group, the Riverhead Foundation, said it was worrisome that the dolphin was by itself, “because they are very social animals; they usually travel in pods.” She said it was an adult common dolphin, 6 to 10 feet long and about 200 pounds.