I'm not sure how Severn works for prime number round direction. Seven is indeed a prime number but that leaves an R which I don't see being explained by 'direction'
This is certainly presenting a problem !
I have Mersey already
There are heaps of rivers with tidal bores ,I can see from the link.
I get Irwell and Orwell ( with bore being the well.)
I looked for synonyms os “bore” and Irk came up and there is a River Irk
Mmmm... still pondering !
Thanks to you all !