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 Seaway advancing beyond the early planning stages. The Great Depression cut the project's planning budget, while World War II and completion of the St. Lawrence Seaway delayed matters. The growth of road and air transportation reduced the need for a canal, but the project was still under serious consideration as late as 1962. As proposed, ships would have used a dredged channel in the Hudson River, transferred to an upgraded Champlain Canal, navigated Lake Champlain, traversed an upgraded Chambly Canal and St Ours Canal, and traveled a dredged route up the Richelieu River to Montreal. Today, the seaway's planned route is