Question

why doesn't sea and river mix together​

Answer 1

Rain replenishes freshwater in rivers and streams, so they don't taste salty. However, the water in the ocean collects all of the salt and minerals from all of the rivers that flow into it.

Answer 2

Answer:

In some estuaries, the mixing of fresh water from rivers and saltwater from the sea is extensive; in others it is not. ... Because fresh water flowing into the estuary is less salty and less dense than water from the ocean, it often floats on top of the heavier seawater

If the river water is flowing into denser ocean water, it will tend to ride on top of the ocean water until it finally blends in. If the rate of flow is high enough, you can have a stream of freshwater extending some distance into the ocean.