Well,whenever soldiers had to cross a river,they'd primarily move quickly in an attempt to capture any available bridges. With the advances in technology,tanks especially needed to have bridges that could support their weight. Some notable illustrations of the strategic importance of bridges is seen in Operation Market Garden and also the Battle of Remagen. In the Eastern front,crossing the Volga River was done with boats,under heavy fire from the opposite bank. However,sometimes bridges were destroyed or too heavily defended. The Allies (the U.S and Britain) had a style of temporary bridge called the Bailey Bridge,that could be carried on trucks. They'd able to be quickly thrown across the length of the rivers and then they could cross in that fashion. However,the main importance was bridges. If there was no option,theyd try and swim!