Although they can't really trade along the White River, unless their boats can sail up and down waterfalls.
I think this is a case where the game world and lore world conflict, as is bound to happen.
Solitude in 'reality' is not a place with less than 30 people living there, it will likely have tens of thousands.
Only that is not possible in the game, so we get a condensed version.
Similarly the game world is there primarily for us. To guide us a certain way, to provide nice things to look at.
I have no doubt that lore-wise the river White would be the reason Whiterun exists, providing opportunities for trade, travel and fishery.
Possibly theyll have built sluices to deal with differences in height and waterfalls and the like, this is what many (real life) nations do to make a waterway more accessible.