Washington County, New York is part of the Great Appalachian Valley (also known simply as the 'Great Valley') which is a long narrow valley strip often between tall mountain ranges. The county transitions from the Taconic Mountains to the Adirondack Mountains, and from the Lake Champlain Valley to Hudson River Valley. Much of the county is part of the slate valley of the Upper Taconic Mountains (Taghkanic, meaning 'in the trees'). The eastern boundary of Washington County is the New York–Vermont border, part of which is Lake Champlain. This is also the border with New England proper. The northern end of the county is
New York (state) that includes within its borders parts of the Great Lakes and the Atlantic Ocean. The Hudson River begins near Lake Tear of the Clouds and flows south through the eastern part of the state, without draining Lakes George or Champlain. Lake George empties at its north end into Lake Champlain, whose northern end extends into Canada, where it drains into the Richelieu River and then ultimately the Saint Lawrence River. The western section of the state is drained by the Allegheny River and rivers of the Susquehanna and Delaware River systems. Niagara Falls is shared between New York and Ontario