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 physiographic section of the larger Saint Lawrence Valley, which in turn is part of the larger Appalachian physiographic division. Lake Champlain is one of numerous large lakes scattered in an arc through Labrador, in Canada, the northern United States, and the Northwest Territories of Canada. Although it is smaller than each of the Great Lakes (Ontario, Erie, Huron, Superior, and Michigan), Lake Champlain is a large body of fresh water. Approximately in area, the lake is long and across at its widest point, and has a maximum depth of approximately . The lake varies seasonally from about above mean sea