The names of British rivers show marked conservatism, in other words their names persist even when the language of those who inhabit the river bank changes. The obvious example is Avon. Since afon is the modern Welsh for river it seems highly probable that those rivers with this name reflect a widely spoken Celtic language existing before the arrival of English, and the English. The Bristol Avon with its famous gorge is bridged by Brunel’s extraordinary suspension bridge at Clifton. The beautiful river at Salisbury is the Hampshire Avon. There are other rivers of this name in Devon, Warwickshire and Scotland. All are in western Britain and I think that it is true that pre-English river names become commoner the further west you travel. Similarly there are rivers Dee in north Wales and Aberdeenshire. It is thought that Deva was a Celtic goddess who gave her name to both river and the Roman fortress at Chester, once itself called Deva. If so did the goddess Verbeia, seemingly worshipped at Ilkley, give her name to the river Wharfe? This is easier to believe if you recall that the Latin ‘v’ was pronounced like English ‘w’.
A particularly interesting exception to the principle that old names are western is the Ouse. The Great Ouse drains central England into the Wash and I once used to incompetently punt on its most famous tributary, the Cam. There is also a river Ouse in Yorkshire, flowing through the city of York, and I was once familiar with a third in Sussex which enters the English Channel at the important port of Newhaven. These three rivers do not exhaust the use of the name; an Ouseburn is a tributary of the Tyne. The immediate origin of Ouse is uisce or uisge which are Gaelic language words for water. Whisky, the water of life, takes its name in the same way and I think that rivers called Usk or Esk are cognate to Ouse. But uisce is not so different from the German wasser which may hint at an older Indo-European word that survives in both language families. Vaseline may derive its name from wasser but it’s origin is petroleum.
Once you have exhausted the possibilities of naming your local river as the river or the water then looking for descriptive adjectives seems a good plan. Rivers can reasonably be clear, bright, or dark. There are at least five rivers Glen including those in Lincolnshire and Northumberland; glan is still modern Welsh for clean. The Lea (or Lee) is the second largest London river and the general view is that there was a Celtic word lug- meaning bright. The Lee crocodile was once as famous as the Surrey puma but I have not heard of its depredations recently. Dublin famously means ‘black pool’; across the Irish Sea Blackpool also means black pool. So the Irish Gaelic dubh- means dark; are there any rivers that could be so described? The river Dove in the south Peak, loved so compleatley by Izaak Walton, is an obvious candidate.
Pose two etymologists a question and you will get at least three answers. There are rivers Stour in Suffolk, Dorset and elsewhere. Does the name originate in a Celtic word for strong, or an Anglo-Saxon word with the same meaning, or might the Germanic languages have borrowed their word because of the strong rivers that already carried an older name?
Then there is the Thames, and a cluster of rivers with similar names: Tamar, Tay, Teme, Tavy, and possibly Trent and Tyne. The smart money seems to be on another Celtic word meaning dark, or perhaps a pre-Celtic word of unknown meaning. The Tame is another river from this collection, being a tributary of the Mersey. The Mersey itself should be straightforward, deriving its name from the Old English mǽres meaning a boundary. The same origin has been suggested for March in the sense of the Welsh marches or the historical Marcher Lords. There is no end to this process and I leave you to investigate your own local rivers. Does Calder mean ‘violent water’ in a Gaelic language? Do the Darent and the various Derwents derive from dair meaning an oak? But if anyone mentions a river name to you I should always reply ‘Wye?’