What you are describing is actually known as a tidal strait and is formed from a mixture of salt and fresh water. read more
In the case of the East River, part of its water comes from the Hudson River which starts up in the mountains , picks up water from drainage and become the Harlem River as it enters the City and then mixes with freshwater from the Bronx to become what is known as the East River. So the East River is fresh and salt water, mostly salt. read more
Yes, the East River is salt water because it is not a real river. It is a tidal strait. read more
The East River is a salt water tidal estuary in New York City. The waterway, which is actually not a river despite its name, connects Upper New York Bay on its south end to Long Island Sound on its north end. read more