blocking rivers

[Guru94]

Has anyone of you experience with river fleets in this game? There is no tool tipp stating how much elements are required to block the river region, leaving me wonder if it is possible at all. If it is possible I think it could be a viable strategy for Prussia to block movement over the Elbe

[Captain_Orso]

You need 4 naval combat elements (sub-unit) in a river region to prevent movement across that river region with a 90% chance. For each naval combat element there is a 23% chance, which is capped at 90% regardless of the number of naval combat elements.