Bar-built or restricted-mouth
estuaries occur when sandbars or barrier
islands are built up by ocean waves and currents along coastal areas
fed by one or more rivers or streams. The streams or rivers flowing
into bar-built estuaries typically have a very low water volume during
most of the year. Under these conditions, the bars may grow into
barrier beaches or islands and the estuary can become permanently
blocked. The area between the coast and the barrier beaches or islands
are protected areas of calm water called lagoons.
Barrier beaches or islands break the
impact of destructive ocean waves before they can reach the estuary
and mainland, consequently protecting them. The barrier beaches take
the brunt of the waves' force and are sometimes completely washed away,
leaving the estuary and coast exposed and vulnerable. During heavy
rains, large volumes of water flowing down the river or stream can
also completely wash away small bars and reopen the mouth of the estuary
(Ross, 1995; Sumich, 1996).
Bar-built estuaries are common along
the Gulf Coast of Texas and Florida,
in the Netherlands, and in parts of North Carolina. Good examples are
Pamlico Sound in North Carolina, Matagorda Bay in Texas, and the Nauset
Barrier Beach System on Cape Cod, Massachusetts.
the animation below, ocean waves move sediments
shoreward, as the sediments build up,
they form sandbars.
on image to PLAY