Does sea water and river water mix?

yes it will mix. sea water is more denser than fresh/river water. For a fraction of second, the sea water/salt water will go down. Then after that it will mix within a second. To have proof of concept, you can take sea water as blue color and river water as red color. Do this simple experiment to validate the concept. read more

Tides take sea water into the estuary and up the river. On the flood tide, water goes in. On the ebb tide, water comes out. The time of day makes no difference. Sea water and fresh water are mixing all the time in a river’s lower reaches and estuary (the last part of the river, then the part where it becomes the sea). read more