The Lost River and the Cacapon River in West Virginia are actually the same. The stream is known as the Lost River from where it originates in several springs near Brock's Gap, south of Mathias, to where it disappears about four miles southwest of Wardensville. It flows in a northeasterly direction through the rich but narrow Lost River Valley, which includes Lost River State Park, before passing through the gap at McCauley. All of the Lost River is located in Hardy County.

The river is "lost" at Sandy Ridge, between Baker and Wardensville, where, when the water level is low, the main flow sinks under some large rocks into a subterranean passage. It remains underground for a little more than two miles. If it has rained recently, the surface stream bed may still have some water in it, but the major flow remains underground. In flood times, the above-ground river becomes a roaring stream and passes through the chasm it has cut at Sandy Ridge. When the river emerges, it is called the Cacapon River. The Cacapon continues to follow a northeasterly course across the eastern portion of Hampshire County and the west-central part of Morgan County to the Potomac River. The total distance of the Lost River is 33 miles from its source to where it emerges. The Cacapon and Lost River channel distance from source to mouth is 110.9 miles. The Lost River and Cacapon drain parts of three counties.