The river names are read from adj-defs.csv, until now they seem to be unused. For the crossing difficulty, a new column can be added in the same file, after the river names. However this is to be implemented in 1.09
Thanks, gunman, this seems to be easier than I expected.
I've checked the vanilla adj-defs.csv file and found - to my surprise - all river names. I'd add that the makers of the E3 map disregarded these names which are now entirely missing. Seems that I'll have to add them one by one.