I was and am only concerned to rebutt the suggestion that there ever was or is such a thing as a PRN over all physically navigable waters able to float a log raft in the Stone Age.
I have not sought to research or deal with the last 700 odd years of development of river rights and wrongs. Still less have I sought to research any individual waters - as I said, I take the BCU's list of waters with an exercisable GRN at face value.
I assume that as a reputable body well advised by expert lawyers and with local documentation to hand for each river, that they have it right. That's part of your problem arguing for wider and far more extensive PRN rights, you have to overcome your own sporting bodies' own researches. Hardly surprising they are keeping their heads well down. It's the advice I would have given them if ever asked :wh.
If you re-read my post analysing the Commentator Roger de Hoovenden's writings you will note that it is obvious that substantial navigation took place not only in the original Great rives but in the lesser rivers as well, "wood and the like" other things being carried.
The difference and definition, in so far as material, is the King's protection of Great rivers. A Great river is one that has the King's protection versus the rest that don't. You don't have to have the King's protection for a minor or lesser river to be subject to an equivalent local and particular right of navigation.
The category of Great river may itself not have been closed. Others may well have come to be added over the centuries to the primary pair (Thamesis and Meadway) evidenced in Magna Carta. The simplest way to add to the category of Great river would be the King's simply saying so, exercising the prerogative to protect such and such additional river as vital to the Crown, commerce and the security of the nation.
How can such particular rights of navigation arise in the lesser waters ? Long standing custom and continuous active user is of the essence of Common Law. See the equivalent legal debate over antient rights of way on land that was part and parcel of the legal background and controversy supporting the Kinder Scout and other instances of NOT civil trespass that finally led to the right to roam acts*.
So if the local authorities or landowners adjoining a minor navigable water let people navigate up and down - to their commercial advantage - without objection or interruption, confining their financial claims to mooring fees etc. then do it long enough to become firmly established common law custom and practice in the locality and on that particular river and there you have it.
Though equally it should be remembered that common law rights of this sort do require (as with the antient rights of way on land) to be continuously exercised, even if infrequently, or they are liable to lapse.
[Edit]Silting up is the classic example...nature itself preventing navigation and then, by lapse of time and impossibility of user, the right itself lapses[/Edit]
Another way of creating such local rights is if the local authorities - or equivalent way back when - may simply have said so. "Warewivelhamptonshire welcomes commercial traffic" or equivalent, to their local advantage.
Then there is statute. The specific creation of a new right of Navigation by statute, or variation of existing common law rights. Instances might be such as where a formerly un-navigable river was dredged and portions straightened to allow commercial or vital military traffic, or to extend that commercial traffic further upstream than the historic rights went. Or more simply, the creation of a new canal running where no water ever did before, to define its purpose and existence.
There are hundreds of such Navigation Statutes that apply to each particular water, to create, vary, extinguish or otherwise affect the rights of navigation in that particular water. Each to be considered on its merits. Researching them is many lifetime's work and for a legal historian like the Reverend C, if he thinks that the BCU have missed one. Not my cup of tea
.
I digress.
I trust that is sufficient answer for your present purposes ?
* In which I believe the Rev. C played no small part as an activist. Now trying to repeat the trick on the rivers ? but without the same legal merits I fear.