The Rapidan River, flowing 88 miles through north-central Virginia in the United States, is the largest tributary of the Rappahannock River. The two rivers converge just west of the city of Fredericksburg. The Rapidan River begins west of Doubletop Mountain in Shenandoah National Park where the Mill Prong meets the Laurel Prong at Rapidan Camp, approximat…The Rapidan River, flowing 88 miles through north-central Virginia in the United States, is the largest tributary of the Rappahannock River. The two rivers converge just west of the city of Fredericksburg. The Rapidan River begins west of Doubletop Mountain in Shenandoah National Park where the Mill Prong meets the Laurel Prong at Rapidan Camp, approximately 3 miles south of Big Meadows. The river defines the border of Orange County with Culpeper and Madison Counties.