First, land units can use Riverine Transport Points drawn from a global pool to move along uncontested rivers. These points represent mostly unarmed small steam boats and can be bought with money and war supplies (a generic term meaning manufactured heavy materials ).
The players also have individual Riverine Transport units at their disposal. These are even more versatile since you can use them for amphibious operations, with protection from gunboats or river ironclads if needed. Beware of forts and artilleries positions along river though, as they can sink these units rather easily. Some commanders have the unique traits of reducing this damage or even preventing it completely if they are really lucky.
But that's not all! Because military operations won't last long if supply doesn't flow correctly, here too the riverine boats play a major role. The supply lines are able to run down rivers if you have enough points in your riverine pool, and your transport counter can even be converted at an anchorage into a supply depot! Another nice option is to keep them as ships and they will distribute what supplies they have aboard to nearby troops .

2nd Division is using river boats to run down the river, while Porter Ironclads will open the way just some days before.