The East River is a tidal strait of the North Atlantic Ocean in New York City connecting Upper New York Bay to Long Island Sound. Despite its name, the East River is not a river at all. The East River makes numerous appearances throughout the TMNT franchise.
In the Archie comics issue And Deliver us from Evil, the prehistoric Manmoth is found in the depths of the East River.
In the end of the episode "Donatello Makes Time" of the 1987 animated series, Raphael suggests Donatello dropping Professor Lloyd Cycloid's time stopping device there.
The East River figures prominently in the Tales of the TMNT 2024 series. It was the home of the East River Three until the trio was mutated and told to leave by their friends and family. The Jersey Bight, a gigantic pearl, kept their previous home in the East River safe, but was pulled out of the East River by a trawler. The East River three make a plan to retrieve the pearl, but this plan involves flooding much of New York City.