A Hydronym is the name of an individual pond/lake/stream/river/sea/ocean, as ‘The Round Pond’, ‘Lake Superior’, ‘The Tyburn Brook’, ‘The Yangtze’, ‘The South China Sea’, ‘The South Pacific’.
I came across the word when looking up the River Rhine (Wagner had come up in a quiz we did last night) and it appears that it is just the ancient Celtic name and means ‘flowing’. There is a river Reno in Italy which shares the same name origin. And the Greek roi meaning flow may be related. See also Rhône.
Names of rivers tend to be conservative : new arrivals may speak a different language but often just go with the flow and retain local names of physical features such as hills and rivers. And quite often the local name is not really a name so much as the literal description. For example, ‘Avon’, just means river, as in modern Welsh Afon.
Particularly interesting is ‘Ouse’. This is thought to be from Celtic usso = water. The word is cognate with, that is to say has the same root, as ‘water’, and Russian voda, from which vodka, ‘little water’, is derived. And these are cognate with Gaelic uisce, which is seen in the word whiskey, using beatha =‘ water of life’.