There are three noble rivers, the Thames, the Severn and the Humber, and these it stretches out as though they were three arms. Into them goods from across the ocean are carried, merchandise coming from all countries by this same sea-traffic. [I.2]

The SEVERN appears in the opening description of Britain as one of the three great rivers of Britain. Geoffrey next provides an etymology, explaining that it was named for Habren, the daughter of Estrildis, mistress and later queen to Locrinus:

Some time later, when Corineus was at long last dead, Locrinus deserted Gwendolen and took Estrildis as his Queen. Gwendolen was most indignant at this. She went off to Cornwall and there she assembled all the young men of that region and began to harass Locrinus with border forays. At last, when both sides had gathered an army together, they joined battle near the River Stour. There Locrinus was struck by an arrow and so departed from the joys of this life. With Locrinus out of the way, Gwendolen took over the government of the kingdom, behaving in the same extravagant fashion as her father had done. She ordered Estrildis and her daughter Habren to be thrown into the river which is now called the Severn; and she published an edict throughout the length and breadth of Britain that this river should be called after the girl's name. Gwendolen's intention was that this everlasting honour should be done to Habren because her own husband had been the girl's father. It thus comes about that right down to our own times this river is called Habren in the British language, although by a corruption of speech it is called Sabrina in the other tongue. [II.5]

The Welsh name for the river is Hafren.

The Severn also appears in the foundation story for GLOUCESTER [IV.15], and in the account of the Christianization of Britain:

The Severn divides these last two provinces [Loegria and Cornwall] from Kambria or Wales, which last was placed under the City of the Legions. [IV.19]

It is not surprising that the river appears twice in Merlin's prophecies, given that the prophecies often feature bodies of water:

A Mountain Ox shall put on a Wolf's head and grind its teeth white in the Severn's workshop.

A Wolf will act as standard-bearer and lead the troops, and it will coil its tail round Cornwall. A soldier in a chariot will resist the Wolf and transform the Cornish people into a Boar. As a result the Boar will devastate the provinces, but it will hide its head in the depths of the Severn. [VII.4]

The river also appears in a catalogue of British mirabilia :

Arthur also told Hoel that there was a third pool in the parts of Wales which are near the Severn. The local people call in Lin Ligua. When the sea flows into this pool, it is swallowed up as though in a bottomless pit; and, as the pool swallows the waters, it is never filled in such a way as to overflow the edges of its banks. When the tide ebbs away, however, the pool belches forth the waters which it has swallowed, as high in the air as a mountain, and with them it then splashes and floods its banks. Meanwhile, if the people of all that region should come near, with their faces turned towards it, thus letting the spray of the waters fall upon their clothing, it is only with difficulty, if, indeed, at all, that they have the strength to avoid being swallowed up by the pool. If, however, they turn their backs, their being sprinkled has no danger for them, even if they stand on the very brink. [IX.7]

This account seems to be drawn from chapter 69 of the Historia Brittonum, an account of Llyn Lliwan and the phenomenon of the tidal surge known as the Severn Bore. Geoffrey does not however reproduce the account of the Bore itself, chapter 68 in the Historia Brittonum, where the phenomenon of the Dau Ri Hafren, or Two Kings of the Severn, is described.

The river's last appearance in the Historia is as the important frontier that it undoubtedly was in Geoffrey's day, as earlier.

Gormund then fought Keredic and chased him over the Severn into Wales. [XI.8]