Mostly the Harlem River is important because it provides passage between the Hudson and East Rivers at the north end of Manhattan. The only other passage between the Hudson and East Rivers is at the south end of Manhattan, where the Hudson and East Rivers meet at New York Bay.

Ships traveling down the Hudson from upstate can pass through the Harlem River to the East River, instead of having to travel all the way down the length of Manhattan on the Hudson River, around New York Bay, and then back up the East River.

This passageway is so important to New York that the city literally moved part of Manhattan to the Bronx (and then later moved part of the Bronx to Manhattan) to better accommodate boat travel on the Harlem River. The Marble Hill neighborhood used to be at the northernmost tip of Manhattan, just north of the Inwood neighborhood. The part of the Harlem River that flowed past Marble Hill was extremely narrow, making it very dangerous for ships to sail from the Hudson River to the East River via the Harlem River.

So in 1895, a deep, wide trench was completed (called the Harlem River Ship Channel), cutting Marble Hill off of Manhattan and making it into an island. In 1914, the narrow, dangerous stretch of river that separated Marble Hill from the Bronx was filled in, making Marble Hill geographically part of the Bronx (and making Inwood the northernmost neighborhood in Manhattan).

To this day, the Marble Hill neighborhood is still officially a part of Manhattan, even though it's in the Bronx now. Although it has a Bronx zip code and area code, it is still part of the borough of Manhattan and New York County (New York County is the county name for Manhattan).

In the 1930s, the river's path was again reconstructed, though this time with the opposite effect: part of the Bronx (just west of Marble Hill) was cut off and reattached to the tip of Upper Manhattan. This piece of land is now the Inwood Hill Park Nature Center.

Lenox Hill Hospital in Manhattan, New York City

New York has a few layers of rock that hold up the state. These layers are granite gneiss, taconic orogeny, and Manhattan Prong. Manhattan Prong is made up of sandstone and marble.

The Bronx and Manhattan were never connected. They have always been separated by the Harlem River, which branches off of the Hudson River, flows down over the top of Manhattan, and into the East River.

However, there is a part of the Bronx -- the Marble Hill neighborhood -- that used to be in Manhattan. The Marble Hill neighborhood used to be on the northermost tip of Manhattan, just north of the Inwood neighborhood. The part of the Harlem River that flowed past Marble Hill was extremely narrow, making it very dangerous for ships to sail from the Hudson River to the East River via the Harlem River.

So in 1895, a deep, wide trench was completed (called the Harlem River Ship Channel), cutting Marble Hill off of Manhattan and making it into an island. In 1914, the narrow, dangerous stretch of river that separated Marble Hill from the Bronx was filled in, making Marble Hill geographically part of the Bronx. However, to this day, the Marble Hill neighborhood is still officially a part of Manhattan, even though it's in the Bronx now.

If you're interested in learning more about the history of the Marble Hill area, I have included a link to the Wikipedia page about the neighborhood.