I've been reading quite a bit, and one thing I have become aware of is that the very part of the river Brue that ceased to exist once the river had been re-routed was central to the ancient XII hides of Glastonbury, the land said to have been granted to Joseph of Arimathea by King Arviragus right back in the first century, and which certainly seems to have been confirmed by charters issued by the early Saxon kings. Exactly why the river was so diverted is something of a mystery.
The 'seven holy islands' were clearly very important to the Abbey and the monks, at least in the early (pre-Benedictine) days. They were Glastonbury itself, then Beckery, Meare, Godney, Bleadney, Martinsey and Nyland (Andrewsey). All of them had chapels built upon them, hidden away in the marshes and often treacherous to reach. They are all near to the original course of the Brue, but their significance must have been reduced considerably by the time the river's course was changed.
Ray Gibbs' book 'The Legendary XII Hides of Glastonbury' is one work I have been reading. I had noticed that the Axe, as it leaves Bleadney, has high banks built up whilst the land falls away to the east, towards Wells. I figured that the river must have flowed that way originally, and Gibbs says yes it did, and once joined by the Axe it could have been 40 or 50 feet wide and still deep enough to take boats. Apparently sea-going craft could have come as far as Bleadney, where there was once a substantial dock.
Gibbs calls the Cheddar valley the 'Vale of Sorrows', and suggests that in early days people were taken for burial by boat, according to an old Celtic rite, and the Lady of the Lake was the presiding Goddess in these parts. I have no idea myself whether this was true into the Christian era and the early days of the Abbey, though certainly the landscape in those days was far more watery.
The re-direction of the Brue has been put down to reasons of commerce and power politics, part of the on-going feuding between Glastonbury and Wells. So far I have found no evidence for this, but whether or not it is true, Glastonbury would scarcely have so radically altered the land/water-scape unless its spiritual significance had shifted. Dunstan's institution of the Benedictine rule would have provided such a shift, back in the tenth century. It was seen as 'modernisation', and one effect was to have all the monks under one roof and no longer encouraged to spend time in isolated hermitages. By the thirteenth century the holy islands and their hidden chapels would already have been largely a matter of legend.
This, at least , is where I have got to so far.