How can two seas not mix?

How can two seas not mix? How can two seas not mix? I think this is commonly known and the explanation everyone gives is "because they have different densities". See more on: https://lnkd.in/eJy-m_6

And an explanation thereon:

http://traveller24.news24.com/Explore/pics-14-of-natures-hypnotising-ocean-phenomena-only-mermaids-really-understand-20151222

" 7. Halocline  Salinity separation phenomena  - Halocline is a zone where there are extreme vertical changes in the salinity in the ocean. 
In lower latitudes the halocline can represent a decrease in salinity with increased depth; high altitudes it represents the opposite.

The halocline normally develops in regions where there is a great amount of freshwater input from rivers producing surface waters of low salinity, a zone where salinity increases rapidly with depth (the halocline), and a deeper zone of greater saline, dense waters. "

(Photo credit - Ocean Networks Canada)

And more images, from Google search for images:

https://www.google.com/search?q=fresh+and+saline+waters+side+by+side+on+the+ocean.&espv=2&biw=1164&bih=645&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjU_pjt4sTLAhUGORQKHbxSD4gQ_AUIBygC#imgrc=_

And one may go through:

http://www.britannica.com/science/density-current

Density current

 
PhysicsWritten by: Claudia Cenedese

Best regards,

Mashallah