The Afon Hafren, known in English as the River Severn, is the longest river in Great Britain at 220 miles (354 km). It rises in the Cambrian Mountains of mid Wales on the Plynlimon massif at an elevation of 610 metres (2,000 ft), near the Ceredigion/Powys border close to Llanidloes. From its source in the Hafren Forest, the river flows through Shropshire, Worcestershire, and Gloucestershire before entering the Severn Estuary near Chepstow and ultimately the Bristol Channel.

The river carries the most voluminous flow of any river in England and Wales, with an average flow rate of 107 cubic metres per second measured at Apperley, Gloucestershire. This substantial volume makes the Severn a particularly reliable summer touring river, remaining paddleable when many other rivers in the region may be running too low. The upper reaches near the source are not navigable at low flows; one source-to-sea expedition found that paddling became practical from a point near Garthmyl on the B4385, rather than from the source itself.

The river passes through or near several historically significant locations, including Ironbridge, Shrewsbury, Worcester, and Gloucester. The county towns of Shrewsbury, Worcester, and Gloucester all lie directly on its course, offering potential access and resupply points for multi-day touring paddlers.

The river's Welsh name, Afon Hafren, derives from the legendary figure Habren (or Hafren), a character from early British mythology who was said to have been drowned in the river, after which it bore her name. The Roman name for the river was Sabrina. This deep cultural and mythological heritage gives the river a distinctive historical character that extends well beyond its physical dimensions.

The Severn is primarily regarded as a touring river suited to paddlers with appropriate experience, given its length, variable conditions, and the tidal nature of its lower reaches approaching the estuary. The river becomes tidal in its lower section, which introduces additional considerations for trip planning in that stretch.