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.