It would be nice to have few larger rivers in the game...that you can navigate...like amazon, nile, etc. and the rest should be smaller...Canals are just too much....When you irrigate..you practically make canals...how else would you irrigate...We need to keep the game playable and interesting, and not have too many details to worry about..
About the rivers..
If you want to navigate the rivers with the ships , rivers would have to be a separate square...which would create some problems in the game making...