In the 20th century, the Turnpike was re-designated as U.S. Route 250. In the 21st century, it remains Highland County's major east-west roadway, and crossing into West Virginia, becomes a National Scenic Byway. According to the U.S. Census Bureau, Highland County has a total area of , of which is land and (0.2%) is water. The county's western border is naturally defined and lies along the Eastern Continental Divide in the Allegheny Mountains. Likewise, the eastern border lies along the ridge line of Shenandoah Mountain. The northern and southern boundaries of the county are defined more artificially, cutting across numerous mountain ridges and valleys. The county is bordered to the west by Pocahontas County in the state of West Virginia, to the north by Pendleton County, West Virginia, to the east by Augusta County, Virginia, and to the south by Bath County, Virginia. The county is northwest of Richmond, Virginia. The average elevation of Highland County is , which is the 16th highest average elevation among counties in the Eastern United States. The highest elevation in the county is in the Allegheny Mountains and the lowest elevation is near the Cowpasture River southwest of Patna. The western regions of the county are generally higher in elevation than are the eastern and especially southeastern regions. As the county lies within the Ridge-and-Valley province of the Appalachian Mountains, it features numerous valleys and mountain ridges that are generally oriented in a "northeast to southwest" direction. The valleys are from west to east the Alleghany Valley, the Bluegrass Valley, the Monterey Valley, the Bullpasture Valley, and the Cowpasture Valley. The majority of the county and all the southern portions of the county form part of the James River watershed, while northern sections drain into the Potomac River. West of Monterey, the divide is roughly centered along the path of U.S. Route 250; however, in eastern Highland County, the James watershed extends further northward. The westernmost valley, the Alleghany Valley, is narrow and situated between Allegheny Mountain to the west and Lantz Mountain to the east. It is drained by Back Creek, a tributary of the Jackson River, in the southern section of the county and is drained by Straight Fork to the north. The Laurel Fork runs to the west of Alleghany Valley, joining the Straight Fork in Pendleton County, West Virginia to the north to form the North Fork South Branch Potomac River.