When fresh water enters the surface of an embayment it moves persistently seaward. Enroute it entrains seawater from below to form a halocline, in which the salinity increases with depth and to seaward. Wind mixing creates a nearly homogeneous zone in the upper part of the halocline. Below the halocline there is a nearly homogeneous lower zone in which sea water intrudes at a rate sufficient to supply the demand for entrainment....
Tully, John P.. 1958. "On structure, entrainment, and transport in estuarine embayments." Journal of Marine Research 17, (1). https://elischolar.library.yale.edu/journal_of_marine_research/939