Complex sentence: The identification of the Winwaed with a modern river is uncertain, but possibly it was a tributary of the Humber and there is good reason to believe it may well have been the river now known as Cock Beck in the ancient kingdom of Elmet.
Split into simpler sentences: The identification of the Winwaed with a modern river is uncertain, but possibly it was a tributary of the Humber. There is good reason to believe it may well have been the river now known as Cock Beck in the ancient kingdom of Elmet.