I just happened to see a documentary last night titled Dynamic Salt. There is quite a bit of research in progress right now in the US and Europe to develop large scale test systems. Simply put when salt water meets up with fresh water the salt seeks to migrate into the fresh water and when it does it produces electricity. This migration is called osmosis. I've used it in curing my hams and bacon. You coat the outside of a piece of meat with salt and in a few days it goes all the way through the meat. The fresh water coming out of rivers into the ocean would be likely locations for power plants. This osmosis is why humans need salt as it migrates through the body it takes nutrients with it. It produces electricity in our bodies as well. That is why "grounding" is becoming a big deal in health. The technique is called "blue energy" production: https://www.electrochem.org/redcat-blog/what-is-blue-energy/
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