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 of the 1812-era fort with a massive new Third System masonry fortification, known as Fort Montgomery. Portions of this fort are still standing. In the early 19th century, the construction of the Champlain Canal connected Lake Champlain to the Hudson River system, allowing north-south commerce by water from New York City to Montreal and Atlantic Canada. In 1909, 65,000 people celebrated the 300th anniversary of the French discovery of the lake. Attending dignitaries included President William Howard Taft, along with representatives from France, Canada, and the United Kingdom. In 1929, then-New York Governor Franklin Roosevelt and Vermont Governor John Weeks