Pontoon bridge

A pontoon bridge is a bridge that floats on water. It is a floating bridge with barges or boats ('pontoons') to support the bridge deck. 
Pontoon bridges are usually temporary structures, but some are used for long periods. Permanent floating bridges are useful for sheltered (quiet) water-crossings. It may be too expensive to suspend a bridge from anchored piers. Pontoon bridges may need a section that is elevated, or can be raised or removed, to allow ships to pass.
Pontoon bridges are often used in wartime as river crossings. They are usually temporary, and are sometimes destroyed after crossing (to keep the enemy from using them), or collapsed and carried (if on a long march). They were used to great advantage in many battles. Pontoon bridges have been in use since ancient times. It is thought that the first London Bridge was a Roman pontoon bridge. Roman legions had men and equipment to build pontoon bridges, and often made them. The first pontoon bridge was made by the Zhou Dynasty.