Geology and Natural History
The Harlem River forms a part of the Hudson estuary system, serving as a narrow strait that divides the island of Manhattan from the Bronx. Approximately 18,000 years ago the Laurentide ice sheet receded northward across the continent leaving behind a large escarpment creating the modern day Hudson River. About 6,000 years ago the Hudson River emptied into the ancient Atlantic Ocean, depositing sediments over the bedrock; this resulted in the formation of the Hudson River estuary, which is the water and land at the mouth the river that contains a mixture of salt and freshwater, including the Harlem River.
The name river is technically incorrect, as the Harlem River has neither a mouth nor a source. It simply connects two larger bodies of water, stretching from the Hudson River to the intersection of the East River at Randall’s Island, at approximately East 125th Street on the present street grid. The Harlem River is therefore affected by the actions of the ocean and neighboring rivers. The ebb and flow of the tides causes the Harlem River’s currents to fluctuate dramatically throughout the day. The effects of the tides have influenced the spread of silts, pollutants, and other particles in the water. The tides were also important in defining the usage of the Harlem River as they caused the currents to be particularly difficult to navigate in the northern portion of the waterway, allowing only smaller ships and experienced sailors.
Stretching approximately seven miles, the river originally meandered through its length, but the course of the river today is much straighter than it was in its natural state. Changes in the river’s route and shoreline character have reflected the designs of engineers, planners, politicians, and industrialists intent on subverting the limitations of the natural topography to facilitate the flow of commerce and real estate investment. Bulkheads, barriers, fills, and cuts all affected the river, changing and manipulating it into its current state. A systematic examination of the changes to the river’s function and access provides an understanding of its revamped role in the urban process.
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