Old Croton Dam

The Old Croton Dam is a historic dam located in Yorktown, Westchester County, New York, now lying submerged beneath the waters of the New Croton Reservoir. The dam was built on the Croton River between 1837 and 1842, and was the first substantial masonry dam in the United States. Construction was delayed by a January 1841 storm that washed away most of the dam, with heavy downstream damage and loss of life.
History.
The gravity dam constructed with a rubble core and granite ashlar foundation, it was high and long. The dam impounded water from the Croton River watershed, forming a reservoir several miles long to the northeast along the path of the Croton River. Water flowed to New York City through the Old Croton Aqueduct, which starts just upstream of the dam and carries water down the Croton River valley toward the Hudson River, and roughly follows the Hudson south. 
The dam and aqueduct constitute a major part of the original New York City water supply system. The New Croton Aqueduct opened in 1890, augmenting the original until supply from the Delaware and Catskill Aqueducts was sufficient to take it off line in 1955. When the New Croton Dam was completed in 1906, the old dam was submerged to a depth of .
The dam site was added to the National Register of Historic Places in 1973.