It rises in the Cambrian Mountains in mid Wales, at an altitude of 2,001 feet (610 m), on the Plynlimon massif, which lies close to the Ceredigion/Powys border near Llanidloes.

[6] The name Severn is thought to derive from a British word *sabrinā, possibly from an older form *samarosina, meaning "land of summertime fallow".

[12] The Welsh form of the name is Afon Hafren (pronounced [ˈavɔn ˈhavrɛn]) first recorded in the 12th-century Historia Regum Britanniae.

[16][h] Common club-rush (Schoenoplectus lacustris) prefers to grow in shallow water such as that found in ponds, streams and river margins.

[v][w][x] The Seafarer (poem) ..."þæt he a his sæfore[y] sorge næbbe, to hwon hine Dryhten gedon wille."

As it enters the Shropshire Plain, these lower Palaeozoic rocks are replaced by Permian and Triassic age strata though largely unseen beneath a thick cover of Quaternary deposits.

It was first proposed in the 1900s that the former northerly course of the upper Severn was disrupted during the course of the ice age by the blocking of its access to the Irish Sea through Cheshire causing a large lake to develop across much of Shropshire.

[27] Over its length, there are a large number of tributaries, but the three largest feeding the non-tidal river are the Vyrnwy, the Teme and the Warwickshire Avon.

The Mule enters at Abermule, and the River Rhiw east of Berriew, followed shortly by the Camlad which rises above Churchstoke and by the Luggy Brook.

The River Stour rises in the north of Worcestershire in the Clent Hills, near St Kenelm's Church at Romsley.

The rivers Swilgate and Chelt also join the Severn's left bank, as do the Hatherley and Horsbere brooks, before it reaches Gloucester.

Bideford Brook drains the easternmost part of the Forest of Dean, entering the Severn estuary east of Blakeney.

On the opposite (southeast) bank the flow of the River Cam is usurped by the Gloucester and Sharpness Canal before reaching the estuary.

The Lyd enters the west bank of the estuary at Lydney Harbour, opposite the place where Berkeley Pill carries the waters of the Little Avon River into it.

Entering Shropshire and England, a few villages such as Shrawardine sit back from the river as it meanders eastwards towards the county town of Shrewsbury.

Several villages sit back from the river before it runs by Upton-upon-Severn and then enters Gloucestershire as it joins with the Warwickshire Avon outside of Tewkesbury.

The two major road bridges of the Severn crossing link south eastern Wales with the southern counties of England.

Other notable bridges include: The Severn Tunnel, completed in 1886 by John Hawkshaw on behalf of the Great Western Railway, lies near the Second Severn Crossing road bridge, and carries the South Wales Main Line section of the Great Western Main Line under the channel.

The vehicles were loaded onto open flat bed carriages and pulled by a small pannier tank locomotive, although sometimes they were joined to a scheduled passenger train.

The prudent owner paid to cover the vehicle with a sheet, as sparks often flew when the steam locomotive tackled the slope leading to the tunnel exit.

Between the docks and Lower Parting Llanthony Weir marks the Normal Tidal Limit (NTL) of the East Channel of the river.

[33] In Shrewsbury, boat trips around the loop of the town centre are at present provided by the Sabrina and depart from Victoria Quay near the Welsh Bridge during the summer.

A phenomenon associated with the lower reaches of the Severn is the tidal bore,[35] which forms upstream of the port of Sharpness.

It is frequently asserted that the river's estuary, which empties into the Bristol Channel, has the second largest tidal range in the world—48 feet (15 m),[36][37] exceeded only by the Bay of Fundy.

[38] During the highest tides, the rising water is funnelled up the Severn estuary into a wave that travels rapidly upstream against the river current.

[40] The sides of the estuary are also important feeding grounds for waders, notably at the Bridgwater Bay National Nature Reserve and the Slimbridge Wildfowl Trust.

[42] The river forms part of the Severn-Trent flyway, a route used by migratory birds to cross Great Britain.

In Shakespeare's Henry IV, Part 1, Henry "Hotspur" Percy recalls the valour of Edmund Mortimer, 5th Earl of March in a long battle against Welshman Owain Glyndŵr upon the banks of the Severn, claiming the flooding Severn "affrighted with [the warriors'] bloody looks ran fearfully among the trembling reeds and hid his crisp head in the hollow bank, bloodstained with these valiant combatants."

Waters links Nodens with the Severn Bore and the association of the Celtic deity with the river is explored at length by Rogers.

In Julian Barnes' 2011 novel, The Sense of an Ending, Tony, the main character, recalls "a river rushing nonsensically upstream, its wave and wash lit by half a dozen chasing torchbeams," an allusion to a visit to the Severn Bore.