We love to play confusing tricks upon foreigners with regard to surnames and placenames. One example of the former: there are people whose name is Fanshaw, but who sign themselves Featherstonehaugh. I sometimes wonder how they get on when entering America and facing those grim and defensive immigration officials.
With some regularity, a controversy arises every eighteen months about how the name of the River Cherwell, which joins the Thames at Oxford, should be pronounced.
Oxonians make the first syllable rhyme with bar (compare also the County of Berkshire ... I'm not sure about the Berkley, er, Hunt ...). But rustic folks living up the Cherwell valley make that first syllable rhyme with sir. Old maps, old documents, which vary the orthography between Charwell and Cherwell, make clear that the former is, historically, correct.
I have little doubt that explanation is to be found in Mgr Knox's words about the Barsetshire town of Greshamsbury. " ... the inhabitants no longer called it Greemsbury, but pronounced it as it was spelt; for with the coming of education they had learned how to write and forgotten how to talk ...".
In Devon, to give another example, Crediton is now called, as spelt, Crediton; but it used to be pronounced Kirton.
You see, in Oxford we have two rivers, and so it is often necessary to specify which is intended. But where there is only one river, it would be excessive to name it: one would just call it "the River" or, if tidal, "the Water". Similarly, if there is only one church, it is "the Church". Only if there is the possibility of confusion would one say "S Mark's Church" or "S Peter's" or "the Presbyterian Church".
This is why so many of our River "names" are simply 'Celtic' or even 'pre-Celtic' words meaning River (Avon ...).
Some river names on modern maps seem even to arise from mistakes by those busy and pompous people, Antiquarians (the Adur ...)!