Shenandoah River  The Shenandoah River is a tributary of the Potomac River, long with two forks approximately long each, in the U.S. states of Virginia and West Virginia. The principal tributary of the Potomac, the river and its tributaries drain the central and lower Shenandoah Valley and the Page Valley in the Appalachians on the west side of the Blue Ridge Mountains, in northwestern Virginia and the Eastern Panhandle of West Virginia. The Shenandoah River is formed northeast of Front Royal near Riverton, by the confluence of the South Fork and the North Fork. It flows northeast across Warren County, passing underneath Interstate 66 from its formation. Beyond the I-66 bridge the river flows through a set of bends before turning to the northeast again, crossing into Clarke County below I-66. Five miles (8 km) downriver from the Clarke County border, the Shenandoah passes under U.S. Route 50 and then passes through a triple bend. below the Route 50 bridge, the river passes underneath State Route 7 and then continues northeast another where it crosses into Jefferson County in West Virginia. Once in West Virginia the river completes six large bends before joining with the Potomac from the southwest near Harpers Ferry. The confluence is on the West Virginia-Maryland border and from the Virginia-West Virginia border. The Shenandoah valley is underlain by limestone. The fertile soil made it a favored place for early settlement. It continues to be a major agricultural area of Virginia and West Virginia. Some karst topography is evident, and the limestone is honeycombed with caves. Several have been developed as commercial tourist attractions, including Luray Caverns, Shenandoah Caverns, and Skyline Caverns. On the riverbank a few miles above Harper's Ferry is said to be a cave with an opening just large enough for a mounted rider to squeeze through. It widened in the interior to a spacious room where hundreds of Col. John Mosby's raiding troops are said to have hidden from pursuing Union cavalry. Since 2005, the Shenandoah River has experienced several springtime fish kills that have affected several of its native fish species. In 2005, redbreast sunfish and smallmouth bass along a stretch of the South Fork Shenandoah River began dying of lesions caused by bacteria and fungi. Although the fish kill eventually wiped out 80% of the adult redbreast sunfish and smallmouth bass, juvenile populations appeared to be unaffected.