The Hoosic is a three-state river. It forms in the Green Mountains of Vermont, the Taconics of New York, and the sides of Mount Greylock – the highest mountain in Massachusetts. The river runs seventy miles from its headwaters south of the Cheshire Reservoir in Massachusetts to the community of Stillwater, NY, on the Hudson River. It’s watershed covers 720 square miles. Major tributaries include the North Branch, the Green River, the Little Hoosic, the Walloomsac, the Owl Kill, and the Tomhannok.

The Taconic range on the west and the Hoosac Range (in Massachusetts) and the Green Mountains (in Vermont) on the east constitute the high headwaters areas of the watershed. In general, the Taconics and the Mt. Greylock massif are high-limestone mountains. The plant communities they support are unusually rich, and include some species at the northern limits of their ranges. The Green Mountains are generally granitic. Plant communities there are relatively less diverse.

The twenty-nine Massachusetts miles have been adopted by the six communities through which they run as state-designated Local Scenic Rivers. Two tributaries, Hopper and Money Brooks on the west side of Mt. Greylock, are state-designated Natural Scenic Rivers. The Tomhannock Reservoir and much of its watershed is also protected; it is the water supply for the city of Troy, NY.