They then settle into the lower layer and are transported back into the estuary. The finest particles become trapped at the boundary between fresh and slat water forming a 'turbid' layer. Tides move in and out of the estuary and can temporarily move the mixed and turbid layers either inshore or out to sea. The combined effects of tides, mixing, and intermittent strong river discharge can be to trap pollutants in the estuary. B. Lagoons - often associated with estuaries and always with barrier islands.
Lagoons have to river input.
They are formed from sea water flooding behind a barrier or by a barrier building out and cutting off a coastal sea area. Located on broad low gradient coastal plains and broad shelves of passive margins. Tides dominate lagoonal flow. Restricted mouths of lagoons can cause strong tidal forces. Tide water must enter and leave through narrow inlets or breaks in the barrier.