Bristol Channel, inlet of the Atlantic Ocean, Great Britain, lying between southern Wales and south-western England. Approximately 137 km (85 mi) long and between 8 and 69 km (5 and 43 mi) wide, the channel follows an irregular coastline with a total length of about 360 km (225 mi). The eastern extremity of the channel merges with the estuary of the River Severn, the progressive narrowing causing a tidal bore, producing the highest tidal waves in Britain. The Channel is navigable by ocean-going vessels as far east as Portishead, the modern port of Bristol. On the Welsh side, Swansea and Cardiff are major ports with an important industrial heritage. The main islands are Lundy, at the mouth of the channel, Steep Holm, and Flat Holm.