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. … In other words, the ocean today probably has a balanced salt input and output (and so the ocean is no longer getting saltier).
Source
https://oceanservice.noaa.gov/facts/riversnotsalty.html
In addition to what others have said: the waters are different temperatures and densities, which keeps them from mixing quickly. Salt water will float on fresh water. Warmer water will float on cooler water.They will mix eventually but it takes time. Areas where fresh and salt water meet are called “brackish” and the place is called an “estuary”. They have a specific ecology of their own and species that live under those conditions of low salt.
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