Not every river that exists nowadays and allows boats to travel up has been a navigable waterway in history. e.g. nowadays you can ship up the St. Lawrence-River into the Great Lakes - because of 15 locks and canals that have been build. It is similar at the eastern seaboard of the US - in part the threat of U-Boats in the World Wars lead to the expansion of canals so that a lot of shipping can move "inland" instead of along the sea coast. A similar expansion of canals happened in the decade before railways became more than a toy for the amusement of passengers on short routes. However most of those canals were finished building long after the War of Independance, most in the 19th century, e.g. https://en.wikipedia.org/wiki/Chesapeake_and_Ohio_Canal https://en.wikipedia.org/wiki/Erie_Canal https://en.wikipedia.org/wiki/Cape_Cod_Canal here the canal after it stopped being in use was filled and the land used to build the railway straight to a city without razing houses https://en.wikipedia.org/wiki/Wabash_and_Erie_Canal https://en.wikipedia.org/wiki/Washington_City_Canal https://en.wikipedia.org/wiki/Morris_Canal or as a larger plan https://en.wikipedia.org/wiki/Intracoastal_Waterway All that did not exist in the game normal timeframe of 1492 - 1792 and was built later. The natural rivers that existed at the time of colonization were not all "navigable". like the Niagara Falls on the St. Lawrence riverway or the constant shifting shoals and sandbanks of the Missisippi we need to discriminate between rivers (too small to ship anything or ridden with currents, water falls etc.and that should not even be on the map), rivers (that allow for some transportation with small canoes that have flat hulls like the natives used) that should be on the map only as lines that give bonuses and rivers (large, that can be shipped by specialized boats, e. g. river barges or coastal ships with flat hulls). Not every river that exists and that some explorer rowed a canoe up needs to be a large river. That should be reserved for the largest of them.