Going back over a Survey of History (couple of multi-page data bases, if you must know) I found the following:
1. The only definite indication of 'sea-going' vessels going far up rivers were the Viking Longships, even the largest of them (Drakkars). Although the classical Triremes and galleys from elsewhere had similar draft and size, there is no indication that anybody sent them inland anywhere.
2. Riverine warfare (examples from Russian rivers, classical to Renaissance Era China and the US Civil War) were almost always with purpose-built riverine vessels. I say almost because some of the Chinese ships/boats may have been also suited for coastal seas, but the records are not precise enough to be sure. Several US Navy Monitors were used on the biggest rivers (Mississippi) which were classified as sea-going ships, but in fact that entire class were coastal vessels that did not do well at all on the high seas.
3. The two constants about rivers, from the very earliest cities to the present day, are that first, they were the best source of fresh water for a city - anything stagnant, like a lake or oasis, could become polluted or drained too quickly; and second, they were almost the only way (along with seaports) to bring in food to the city from far away. EVERY really large city was on a river: Rome, Babylon, Aleppo, all the Egyptian and Chinese cities and capitals, later Paris and London - or on the coast with other ports near enough to feed it - Alexandria, Athens, Constantinople, etc.
Civ VI already shows the water source effect. Check
Civ VI Trade Routes show some of the Food Supply, but nowhere near enough: if the six tiles surrounding the city initially can supply 5 - 10 Food if all worked, a single Trade Route traced along a river or coast should Normally supply at least 5, not the 1 - 2 that is normal now. Overland routes should supply 0 Food normally, because there simply wasn't any way to haul multiple tons of food over land before modern wheeled and rail vehicles. This would put the emphasis on riverine/coastal boat supply that existed IRL.
From this, I suggest that the major changes RE rivers for Civ VII should be:
1. Trade Routes traced entirely along navigable rivers or any combination of rivers, lakes, and coast (i.e., All Water) from the earliest boats in the game can transport Food and Production (big timbers, raw materials in quantity) BUT not any route traced overland - those could only transport a little Production and all the early Luxury/Strategic goods, of which even a ton or two is enough to be Important until the Industrial Era when required quantities go up exponentially.
2. Military and Civilian units should get extra movement along navigable rivers, representing the extra speed possible using boats or even rafts as opposed to humping along trails on land. IF there were 'supply line' rules in the game, supply lines along rivers could be traced much further and provide faster replenishment of units far from home, compared to overland supplies and reinforcements.
(NOTE: There is already a Civ VI Mod that does the extra movement. Check.)
3. Three groups historically made the most use of rivers:
* Vikings. Especially for raiding up navigable rivers to trhe head of navigation or first fortifications, as at Paris and Cologne and in really long-distance riverine trade clear across European Russia from the Baltic Sea to the Black Sea or Caspian.
* Russian Cossacks. The initial Russian expansion into Siberia was along the great central Asian/north Asian rivers, and it was done largely by hired Cossack groups. This would definitely be a 'counter-intuitive' Unique for Cossacks, but it's worth considering just for the freshness of it.
* Chinese Dynasties. The great rivers of China (Hwang-Ho, Yellow River and their tributaries) were routes for invasion and prime locations for defensive works, and the forces on both sides frequently included sizeable fleets of armed boats. China also went further earlier than almost anyone in canalizing and 'taming' the rivers for navigation with the Grand Canal system (1600 km long!) which largely served to extend riverine trade routes far beyond what they could initially reach.
Just suggestions, but I think something along these lines is the way to go to Rationalize the benefits of rivers in Civ VII.
