Title: The Bronx
Content:  Bronx and Westchester County. The Hudson River separates the Bronx on the west from Alpine, Tenafly and Englewood Cliffs in Bergen County, New Jersey; the Harlem River separates it from the island of Manhattan to the southwest; the East River separates it from Queens to the southeast; and to the east, Long Island Sound separates it from Nassau County in western Long Island. Directly north of the Bronx are (from west to east) the adjoining Westchester County communities of Yonkers, Mount Vernon, Pelham Manor and, though physically separated by water, New Rochelle. There is also a short southern land boundary with Marble Hill in the Borough of Manhattan, over the 
Title: New York City
Content:  in significance as a trading port. Most of New York City is built on the three islands of Long Island, Manhattan, and Staten Island. The Hudson River flows through the Hudson Valley into New York Bay. Between New York City and Troy, New York, the river is an estuary. The Hudson River separates the city from the U.S. state of New Jersey. The East River—a tidal strait—flows from Long Island Sound and separates the Bronx and Manhattan from Long Island. The Harlem River, another tidal strait between the East and Hudson rivers, separates most of Manhattan from the Bronx. The Bronx River, which flows through the Bronx and Westchester County, is the only entirely freshwater river in the city. The 
Title: Bronx River
Content:  Plains, then south-southwest through the northern suburbs in New York, passing through Edgemont, Tuckahoe, Eastchester, and Bronxville. It forms the border between the large cities of Yonkers and Mount Vernon, and flows into the northern end of The Bronx, where it divides East Bronx from West Bronx, southward through Bronx Park, New York Botanical Garden, and the Bronx Zoo and continues through neighborhoods of the South Bronx. It empties into the East River, a tidal strait connected to Long Island Sound, between the Soundview and Hunts Point neighborhoods. In the 17th century, the river—called by the natives "Aquehung"—served as a boundary 
Title: The Bronx
Content:  former course of the Spuyten Duyvil Creek; Marble Hill's postal ZIP code, telephonic area codes and fire service, however, are shared with the Bronx and not Manhattan. The Bronx River flows south from Westchester County through the borough, emptying into the East River; it is the only entirely freshwater river in New York City. A smaller river, the Hutchinson River (named after the religious leader Anne Hutchinson, killed along its banks in 1641), passes through the East Bronx and empties into Eastchester Bay. The Bronx also includes several small islands in the East River and Long Island Sound, such as City Island and Hart Island. Rikers Island in the East 
Title: Hudson River
Content:  in New York City. South of the confluence of the Hudson and Spuyten Duyvil Creek, the east bank of the river becomes Manhattan. The river is sometimes still called the North River at this point. The George Washington Bridge crosses the river between Fort Lee and the Washington Heights neighborhood of Manhattan. The Lincoln Tunnel and the Holland Tunnel also cross under the river between Manhattan and New Jersey. South of the Battery, the river proper ends, meeting the East River to form Upper New York Bay, also known as New York Harbor. Its outflow continues through the Narrows between Brooklyn and Staten Island, under the Verrazzano Bridge, and into Lower New York Bay and the Atlantic Ocean.
Title: Bronx Kill
Content:  The Bronx Kill is a narrow strait in New York City delineating the southernmost extent of the Bronx and separating it from Randalls Island in Manhattan. It connects the Harlem River to the East River.
Title: Harlem
Content:  The Harlem River separates the Bronx and Manhattan, necessitating several spans between the two New York City boroughs. Five free bridges connect Harlem and the Bronx: the Willis Avenue Bridge (for northbound traffic only), Third Avenue Bridge (for southbound traffic only), Madison Avenue Bridge, 145th Street Bridge, and Macombs Dam Bridge. In East Harlem, the Wards Island Bridge, also known as the 103rd Street Footbridge, connects Manhattan with Wards Island. The Triborough Bridge is a complex of three separate bridges that offers connections between Queens, East Harlem, and the Bronx.
Title: Brooklyn
Content:  Brooklyn is connected to Manhattan by three bridges, the Brooklyn, Manhattan, and Williamsburg Bridges; a vehicular tunnel, the Brooklyn–Battery Tunnel (also known as the Hugh L. Carey Tunnel); and several subway tunnels. The Verrazzano-Narrows Bridge links Brooklyn with the more suburban borough of Staten Island. Though much of its border is on land, Brooklyn shares several water crossings with Queens, including the Pulaski Bridge, the Greenpoint Avenue Bridge, the Kosciuszko Bridge (part of the Brooklyn-Queens Expressway), and the Grand Street Bridge, all of which carry traffic over Newtown Creek, and the Marine Parkway Bridge connecting Brooklyn to the Rockaway Peninsula.
Title: The Bronx
Content:  Thirteen bridges and three tunnels connect the Bronx to Manhattan, and three bridges connect the Bronx to Queens. These are, from west to east: To Manhattan: the Spuyten Duyvil Bridge, the Henry Hudson Bridge, the Broadway Bridge, the University Heights Bridge, the Washington Bridge, the Alexander Hamilton Bridge, the High Bridge, the Concourse Tunnel, the Macombs Dam Bridge, the 145th Street Bridge, the 149th Street Tunnel, the Madison Avenue Bridge, the Park Avenue Bridge, the Lexington Avenue Tunnel, the Third Avenue Bridge (southbound traffic only), and the Willis Avenue Bridge (northbound traffic only). To both Manhattan and Queens: the Robert F. Kennedy Bridge, formerly known as the Triborough Bridge. To Queens: the Bronx–Whitestone Bridge and the Throgs Neck Bridge.
Title: Bronx River
Content:  The Bronx River, approximately 24 mi long, flows through southeast New York in the United States and drains an area of 38.4 mi2. It is named after colonial settler Jonas Bronck. Besides the Hutchinson River, the Bronx River is the only fresh water river in New York City. It originally rose in what is now the Kensico Reservoir, in Westchester County north of New York City. With the construction of the Kensico Dam in 1885, however, the river was cut off from its natural headwaters and today a small tributary stream serves as its source. The Bronx River flows south past 

Question: which river separates the bronx in new york city from manhattan island?
Answer: Harlem River.
the Harlem River.