The river rises just to the east of the Wiltshire village of Redlynch, near Salisbury. It then flows east across the county boundary into Hampshire, where it flows north of the village of Wellow and the hamlet of Wigley. The river turns south, passing under the M27 and past Broadlands Lake and the Testwood Lakes reservoirs, before joining the Test between Totton and Redbridge. The Environment Agency records the length of the main river as 39.4 kilometres (24.5 mi), with an additional 5.6 km (3.5 mi) for the watercourse from Redlynch to the confluence north of Hamptworth.
Another similarly named waterway in Hampshire is Black Water, a small stream which flows eastwards across the New Forest, passing under Rhinefield Ornamental Drive, before joining Ober Water and Highland Water just north of Brockenhurst to form the Lymington River.