Halocline?
It is. Many factors affect the extent and type of mixing, including the size and shape of the estuary, wind direction, and tidal range. Salt and fresh water do not tend to mix well as fresh water is less dense. Unfortunately, it's 20 years since I studied this stuff, and that's all I can remember!I suspect that the physics is pretty complicated. Even if the fresh water is moving, I would guess that some of the salty stuff still manages to get upstream. It's a long time since I took my A-levels. We need @McWobble
When i was snorkling at a nature reserve in mexico the water had an oily appearance where you could see the two different waters mixing.Can be quite disarming to dive through..
The Amazon's discharge into the ocean is such that this halocline is found up to 125 miles out to sea.