Severn (river, Great Britain), river, southwestern Great Britain. It rises in the Plynlimon Mountains, central Wales, and flows into England, then south until it forms a large estuary that widens into the Bristol Channel. The river is 354 km (220 mi) long and is navigable from the Bristol Channel to Welshpool, Wales. The Upper Avon and the Wye rivers are among the tributaries; the river is an excellent source of salmon with its tributaries abounding in trout. Canals connect the Severn with the Thames, Trent, and Mersey rivers. A tunnel under the estuary, about 6 km (about 4 mi) long, connects Bristol with southern Wales.