It begins in the town of North Castle, New York (east of Armonk) in a series of ponds at about 600 feet (180 m) altitude.

Flowing northeast into the town of Bedford, it abruptly turns south and passes through the Mianus River Gorge, a nature preserve, before entering Stamford, Connecticut, where it is dammed into the Mianus Reservoir, officially known as the Samuel J. Bargh Reservoir (256-foot (78 m) altitude) of the Greenwich Water Company.

From there, it flows south through Stamford and Greenwich into Long Island Sound, at sea level.

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