Eventually you will reach a level that will have a boat on the river which will have to be taken into the equation when you create your bridge and will involve hydraulics and split joints.
Split joints allow the construction to move within a certain angle and is controlled by hydraulic components which you add to your construction in such a way that during the phase when the boat approches a hydraulic action either upwards or sideways takes place on a section of your bridge that results in the boat being able to pass by without damaging the construction of your bridge.