If there is a bridge or ford crossing a river on the campaign map, and an army is standing close to it when they are attacked, the resulting battle will occur at the crossing, so the battlefield will feature the river and at least one bridge or ford. Remember, if you are the defender you may have to defend two crossings or you could use the second one to your advantage!
Be careful when fighting in woods or forests, cavalry will be far less effective due to the restrictions on movement provided by tree cover. Similarly, siege weapons can be fired when deployed in a wood or forest, but their effectiveness will be greatly reduced due to the proximity of the trees.
Coastal areas and islands may have areas of shallow water nearby. Your ships cannot cross shallow water for fear of running aground, it is only possible to row around it. It is possible to identify shallow water by the dark rock patches or coral reefs under the surface when near a coastline.