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. Surface water evaporates in lagoons creating denser saltier water which sinks to the lagoon bottom. Surface salt water flows in to replace the denser water and the denser bottom water flows out of the lagoon at the bottom. This is the reverse of estuarine circulation.