A possible river is the Taratyr, the mouth of which (Aber Taratyr) is mentioned as the chief meeting place of synods to decide on the punishment/excommunication of the Kings of South Wales by the Book of Llan Dâv (See Other Works). The book mentions it as being a tributary of the Wye and was, from the general list of people who met there, presumably near the lower end of the river.
The "Landscape Origins of the Wye Valley" project (Homepage) identifies this river with one running through Bolstone, on the basis of a mill at the river's junction with the Wye called "Abbot Tarretts" (1639 CE), but earlier Aburtaretts (1505 CE), Abbertaret (1230 CE), and Abethtarada (1191 CE), giving a hypothetical Aber Tar(r)ada* (rather more speculatively, one might also note the nearby Gannah Farm).
While Bolstone is a long way up the Wye, and the Wye bore is now rare and insignificant, Thomas A. Walker, chief Contractor on the Severn Tunnel project, notes in his book describing the 1870/80s construction*...
The tide rises at Chepstow Bridge to the height of 50 feet, and runs up the river Wye at high spring-tides for a distance of nearly 20 miles, measured along the winding course of the stream.
This would carry the tide almost around Bolstone at this time, though whether the bore made the same journey is hard to tell. The Wye lacks the strong tapering and depth changes necessary to generate the stronger Severn bore, though if the Severn Bore began further downstream in the Severn during the Dark Ages (and the possible location of Wonder 6 at Caldicot might suggested it did), it is possible it diverted more strongly down the Wye. Still, Bolstone seems a long way downstream, if, indeed, it is the Aber Taratyr.