REGIONAL DRAINAGE BASINS
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All water flows downhill, from the mountains to the sea, and the land area upon which precipitation falls is divided by the streams and topography into watersheds. A watershed is the drainage area that contributes water flow to a given point. Small streams merge together into larger and larger water courses, and so watersheds are often fan-shaped, with the bottom point of the fan being the river mouth (Small Watershed Example).

The Catskill high peaks are the headwaters of the Delaware River system, as well as the headwaters for major tributaries to the Mohawk and Hudson Rivers, and the northwestern Catskills also contributes to the Susquehanna River drainage. All around the world, mountainous areas are critically important for their supply of pure drinking water. The City of New York harnesses much of the clean Catskill surface water to supply high quality drinking water to over nine million people.