Nightbringer | The Arthurian Online Encyclopedia

Abbey of Beale Adventure

Arthur founded this abbey fairly early in his reign, on or near the Humber River, at the site of his victory over the five invading kings of DenmarkIrelandSoleise, The Vale, and the Isle of Longtains.

Judging from the text, it would be a place where the river is crossable on horseback in calm weather. Perhaps Selby or Cawood?