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
Lake Champlain been connected to the Erie Canal via the Champlain Canal since the canal's official opening September 9, 1823, the same day as the opening of the Erie Canal from Rochester on Lake Ontario to Albany. It connects to the St. Lawrence River via the Richelieu River, with the Chambly Canal bypassing rapids on the river since 1843. Together with these waterways the lake is part of the Lakes to Locks Passage. The Lake Champlain Seaway, a project to use the lake to bring ocean-going ships from New York City to Montreal, was proposed in the late 19th century and considered