A pontoon bridge or floating bridge is a bridge

Bridge

A bridge is a structure built to span physical obstacles such as a body of water, valley, or road, for the purpose of providing passage over the obstacle...

 that floats on water and in which barge- or boat-like pontoon

Pontoon (boat)

A pontoon is a flotation device with buoyancy sufficient to float itself as well as a heavy load. A pontoon boat is a flattish boat that relies on pontoons to float. Pontoons may be used on boats, rafts, barges, docks, floatplanes or seaplanes. Pontoons may support a platform, creating a raft. A...

s support the bridge deck and its dynamic loads. While pontoon bridges are usually temporary structures, some are used for long periods of time. Permanent floating bridges are useful for sheltered water-crossings where it is not considered economically feasible to suspend a bridge from anchored pier

Pier

A pier is a raised structure, including bridge and building supports and walkways, over water, typically supported by widely spread piles or pillars...

s.