What is the longest river in the United Kingdom?

The longest river in the United Kingdom is the River Severn.

The Severn runs for about 354 kilometres (220 miles), beginning on Plynlimon in mid-Wales before flowing through Wales and England. It passes near Shrewsbury, Worcester and Gloucester, then widens into the Severn Estuary and reaches the Bristol Channel. Because it crosses both countries, it is also the longest river in Great Britain.

The river has several names in its history. Its Welsh name is Afon Hafren, while Roman sources called it Sabrina. The name is connected with ancient Celtic and Brittonic forms, although its exact meaning is debated. The Severn is also famous for the Severn bore, a tidal wave that travels upstream through the estuary under suitable tidal conditions.

A common mix-up is the Thames, which is more famous and is the longest river entirely within England. The Severn, however, is longer overall and has the greater average flow. Its major tributaries include the Wye, Vyrnwy, Teme and Warwickshire Avon.