An estuary is where the salt & fresh types of this are mixed
The defining feature of an estuary is the mixing of fresh and salt water, not its... [DOC] [TLE] Estuary - Wikipedia [PAR] In this type of estuary, river output greatly exceeds marine input and tidal effects 
... Fresh water floats on top of the seawater in a layer that ... waves at the interface
, mixing the seawater upward with the freshwater.