The Phenomenon:

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:

http://physics.stackexchange.com/questions/51293/how-can-two-seas-not-mix

 And an explanation there on:

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. "

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

And the Technology:

Harvesting Power When Freshwater Meets Salty

Renewable Energy: Researchers boost power output from osmosis method that generates electricity from salinity gradients. 

Although sunlight and wind are two popular sources of renewable energy, some researchers would like to tap saltwater to produce electricity. They have designed methods that harvest the energy released when fresh and saline water mix, such as when a river meets the sea. Now one team has created a prototype device that increases the power output of such methods by an order of magnitude ( Environ. Sci. Technol. Lett. 2013, DOI: 10.1021/ez400117d).

See more on:

http://cen.acs.org/articles/91/web/2013/11/Harvesting-Power-Freshwater-Meets-Salty.html

In this regards, one may visit the following link for ex-actual practice:

Regarding the osmotic generating station, one may visit the following link:
http://www.hydroquebec.com/sustainable-development/documentation-center/pdf/file-osmotic.pdf

In which one may read:
How an osmotic generating station works When fresh water is separated from seawater by a semipermeable membrane, the fresh water moves by osmosis through the membrane into the seawater, raising the pressure on the seawater side. This is referred to as “osmotic pressure” and is used to drive a turbine. Osmotic pressure is the cornerstone of the energy generating method conceived by American Sydney Loeb in the 1970s but developed and put into practice by Norway’s Statkraft Group between 1997 and 2013 . An osmotic generating station has a limited number of components: [ A semipermeable membrane contained in modules [ Freshwater and seawater filters that optimize membrane performance [ A turbine that generates a driving force based on the osmotic pressure and permeation flow rate [ A pressure exchanger that pressurizes the seawater feed required to maintain high salinity levels downstream from the membrane.

ÙŽ    Also on Wikipedia:

https://en.wikipedia.org/wiki/Osmotic_power

Osmotic power

    One more option, as well:

https://www.technologyreview.com/s/423903/salty-solution-for-energy-generation/

     The difference in salinity between freshwater and saltwater holds promise as a large source of renewable energy. Energy is required to desalinate water, and running the process in reverse can generate energy. Now a novel approach based on a conventional battery design that uses nano materials could provide a way to harvest that energy economically.  

Best regards,

Mashallah Ali-Ahyaie

Taxonomy

• Water
• Energy