The West Branch Delaware River is one of two branches that form the Delaware River. It is approximately 90 mi (144 km) long, and flows through the U.S. states of New York and Pennsylvania. It winds through a mountainous area of New York in the western Catskill Mountains for most of its course, before joining the East Branch along the northeast border of Pennsylvania with New York. Midway or so it is empounded by the Cannonsville Dam to form the Cannonsville Reservoir, both part of the New York City water supply system for delivering drinking water to the City.