A structure that is built in order to extend the banks of a stream or intracoastal river beyond the coastline. This is done to direct the direct the flow of a stream or tide and keep sediments moving so they are not deposited and fill in the channnel. It also serves as protection from large waves that might prevent boats from travelling inland along the stream.

Jetties are built in parallel pairs along both banks of the channel. Because they are perpendicular to the coast they tend to interrupt longshore drift and thus widen beaches.