Pontoon bridge in 638. The army of the Umayyad Caliphate built a pontoon bridge over the Bosporus in 717 during the Siege of Constantinople (717–718). The Carolingian army of Charlemagne constructed a portable pontoon bridge of anchored boats bound together and used it to cross the Danube during campaigns against the Avar Khaganate in the 790s. Charlemagne's army built two fortified pontoon bridges across the Elbe in 789 during a campaign against the Slavic Veleti. The German army of Otto the Great employed three pontoon bridges, made from pre-fabricated materials, to rapidly cross the Recknitz river at the Battle on the Raxa
Pontoon bridge consisted of one or more pontoons, which were maneuvered into position and then anchored underwater or on land. The pontoons were linked together using wooden stringers called "balks". The balks were covered by a series of cross planks called "chesses" to form the road surface, and the chesses were secured with side guard rails. A floating bridge can be built in a series of sections, starting from an anchored point on the shore. Modern pontoon bridges usually use pre-fabricated floating structures. Most pontoon bridges are designed for temporary use, but bridges across water bodies with a constant water level can