the release of the viking give me an idea, viking langskip was powerfull ship because they navigate in deep sea and river so viking can do easy trade and raid because they didn't need port, that why they raid and trade with every european and some african nation. however, river was very important for nation not simply for water and rich ground, they was natural road, and before modern transport (plane, railroad) river was use for trade and suplly ( trade between nation, city, suply of army or city...). 

so i have the idea of navigable rivers. but i think that if it's not in game like civ for exemple it's because it would be to op or something. so i have to idea.

first one, create a new type of ship, a ship that can go between river, a trade ship where unit can go.(viking repleace this ship by langskip that play both role river and sea)

second one, we cannot navigate river but if there 2 city or district that are connected by river, there food supplies and work bonus increase. we could think of bonus from culture or law, research...  that could from exemple increase science or hapiness, and give money for 2 city connected from 2 different nation, so this could more tool for player to get bonus from land with out needed to invade your enemies. prefer to have an powerfull allies that give money as city that do revollution every 5 minute. and if i'm not wrong, amplitude dev wan't go that way, not a civlike that war is to powerfull, but a real historic 4x sim where trade and culture can be more powerfull than weapon. 

ps: i see in trailer that there different level and river can cross this level. i think that trade and supplies for 2 city shouldn't have problem, but i think ship (type doesn't matter) cna't cross this level gap, for me it's a good way to doesn't have to op river

Well , having a city up and downstream and boating could enable trade on the river.