All names of physical features interest me. Towns and villages may come and go but rivers have always been useful for transport especially when land was impenetrable, so river names are often very ancient.
Sorry, I forget who listed all these Welsh river names that are so similar. This is just what I wanted to go with my favourite theory that special sorts of rivers' names come from the ancient Sanscrit proto- European root , meaning 'surging, strong' and are 'T' names of which there are a great many.
Other names to go with the Welsh ones are Thames, Tay, Tees, Tweed. The ancient settlers of the Islands were pre Celtic but the rivers names persisted partly because people thought rivers were powerful living things.
(I got a lot of this stuff from one of my favourite books)
tavas, तवस् a. Ved. 1 Old. -2 strong, great. -n. Strength, power (बल); सोमस्य मा तवसं वक्ष्यग्ने