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 George (New York) Mountain in the southern quarter of the lake. The northern end of the lake that is located near Ticonderoga is considered the southern end of the Champlain Valley, which includes Lake Champlain, as well as the cities Plattsburgh, New York and Burlington, Vermont. There are six known invasive species in Lake George. The Asian clam first found in 2010 is the biggest threat, along with the Eurasian watermilfoil. Other invasive species are the Chinese mystery snail, curly-leaf pondweed, spiny water flea, and zebra mussel. The lake was originally named the "Andia-ta-roc-te" by local Native Americans. James Fenimore Cooper in his