Rivers should work like in real life

river tiles make the best tiles for new outposts but by default slow everything down with their increased cost of MP to cross them. since I like high output tiles I also use max rivers setting for new games

but since I do not want the MP penalty for rivers I also use the BepInEx plugin     'Leg It!'

Humankind.LegIt.dll                                                 from https://gc2021.com/showthread.php?tid=42

which allows no extra MP cost for rivers, embarking, disembarking, and all cliffs are now scalable in one turn

NOTE - BepInEx plugins will not modify your game files. they are located in a subfolder where you Humankind.exe executable is stored at
\Humankind\BepInEx\plugins

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I do favor a later game improvement for rivers such as a tech that unlocks faster movement along rivers if researched. for this tech to be the most effective it should be in ancient or classical era before roads have already sped up land MP

another idea (found in other 4x games) is to make some river tiles rapids. river rapids tiles would only be able to be passed by a flying unit or by unlocking a later tech. this river tile improvement would have to become part of map game seed so others could play the same map with the same river rapids tile location

NOTE - you can find map game seed using filext.com to open HK save game file & then looking in the 1st few lines of a SaveDescriptor file read as text. the map game seed ends as soon a non numeric character appears in the text sequence. this map game seed can be used for new games to play the same map again. unfortunately there currently is no save map option of the HK main menu to make the preceding unnecessary