Sandy Bay Mountain stands on the borders of three major rivers watersheds. The north side of Sandy Bay Mountain drains into the South Branch of the Penobscot River, then into the West Branch, the main stem of the Penobscot, and into Penobscot Bay. The east side of the mountain drains into the West Branch of Sandy Stream, then into the Moose River, the Kennebec River, and the Gulf of Maine. On its southwest side, the mountain drains into Ruisseau Noir, then into the Rivière du Loup, the Chaudière River, the Saint Lawrence River, and finally the Gulf of Saint Lawrence.