The Saranac River flows easterly through the central part of the county, discharging into Lake Champlain at Plattsburgh, and the Great Chazy River flows northeastward through the upper part of the county, discharging into Lake Champlain at Coopersville. The Ausable River forms a large part of the southern county line. The county's highest point is Lyon Mountain, in the town of Dannemora, at 3,830' (1167m) ASL. According to the US Census Bureau, the county has a total area of , of which is land and (7.1%) is water.