Byram River in the context of "Port Chester Harbor (Long Island Sound)"

Westchester County is a county located in the southeastern portion of the U.S. state of New York, bordering the Long Island Sound and the Byram River to its east and the Hudson River on its west. The county is the seventh most populous county in the State of New York and the most populous north of New York City. According to the 2020 United States census, the county had a population of 1,004,456, its highest decennial count ever and an increase of 55,344 (5.8%) from the 949,113 counted in 2010. Westchester covers an area of 450 square miles (1,200 km), consisting of six cities, 19 towns, and 23 villages. Established in 1683, Westchester was named after the city of Chester, England. The county seat is the city of White Plains, while the most populous municipality in the county is the city of Yonkers, with 211,569 residents per the 2020 census. The county is part of the Hudson Valley region of the state.

The annual per capita income for Westchester was $67,813 in 2011. The 2011 median household income of $77,006 was the fifth-highest in New York (after Nassau, Putnam, Suffolk, and Rockland counties) and the 47th highest in the United States. By 2021, the county's median household income had risen to $105,387. Westchester County ranks second in the state after New York County (the New York City borough of Manhattan) for median income per person, with a higher concentration of incomes in smaller households. Simultaneously, Westchester County had the highest property taxes of any county in the United States in 2013.

The Connecticut panhandle is the southwestern appendage of Connecticut, where it abuts New York State. It is contained entirely in Fairfield County and the Western Connecticut Planning Region, and includes all of Greenwich, Stamford, New Canaan, and Darien, as well as parts of Norwalk and Wilton. It has some of the most expensive residential real estate in the United States.

The irregularity in the boundary is the result of territorial disputes in the late 17th century between the British colonies of New York and Connecticut. In an agreement on November 28, 1683, that established the New York–Connecticut border as 20 miles (32 km) east of the Hudson River, New York gave up its claim to this area of 61,660 acres (249.5 km) east of the Byram River, whose residents considered themselves part of Connecticut. In exchange, New York received an equivalent area consisting of a 1.81-mile-wide (2.91 km) strip of land known as the "Oblong" running northwards from Ridgefield, Connecticut to the Massachusetts border, alongside the New York counties of Westchester, Putnam, and Dutchess. New York was also given undisputed claim to Rye, New York.

The Kensico Reservoir is a reservoir in the New York City water supply system. Spanning the towns of North Castle and Mount Pleasant, New York, it was formed by the Kensico Dam in 1885, which impounded waters from the Bronx and Byram rivers.

In addition to being a catchment for its own drainage basin in Westchester County, the reservoir serves as a mixing basin for and stores waters from both the Catskill Aqueduct and Delaware Aqueduct, as well as the drainages of the West Branch Reservoir and Boyds Corner Reservoir (that lie within the Croton River watershed), which get mixed at West Branch with those of the Delaware Aqueduct before the combined flow is carried to Kensico.