Well, historically, a great many battles were fought at river crossings. In fact it would be interesting to get a % on it, especially during major invasions... The only big problem for EB is that the engine doesnt really represent the situation very well. Although bizarrely at least one of the custom battles in the RTW Alexander campaign does it rather excellently. the biggest issue being that the AI just resorts blindly to one massive frontal assault with its best units or sits on top of the nearest hill, not the opposite bank while defending.

Personally in my fevered imaginings I would like to see siege type options brought up at these points. ie if the defender has been there a while he can fortify the position a little (although that option would be good anywhere on the map. Im not talking about actual forts because then you have no option to withdraw and just get stuck in them and cant get out without sallying, which is a nonsense IMO.) Also the attacking side would be able to build a pontoon bridge or rafts to cross the river elsewhere with certain units etc etc. Real life rivers are often far wider comparatively than represented in the game and in many cases could not be crossed on foot at all.