Boom (navigational barrier)

A boom or a chain (also boom defence, harbour chain, river chain, chain boom, boom chain or variants) is an obstacle strung across a navigable stretch of water to control or block navigation. Booms could be military in nature, with the goal of denying access to an enemy's ships; a modern example is the anti-submarine net. Booms could also be used, especially along rivers, to force passing vessels to pay a toll.