How does river water not get salty even though it is connected to salty ocean water?

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How does river water not get salty even though it is connected to salty ocean water?

In: Chemistry

River water is “uphill” from the ocean. Ocean water won’t flow uphill to mix with a river. I will insert some other visual description on how water flowing uphill would be a rare thing.

The flow of the water moves toward the ocean. How can salty water move up the river if the river is flowing the other way?

There are often areas where the force of high tide coming in pushes some salt water up the other way, but it doesn’t go very far, and it all comes back at low tide.

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

The diffusion rate of salt in water is slower than the flow of the water from the river into the ocean. The line is not sharply-defined – an intermediate area is called an estuary, where the flow diminishes and tidal flows interfere, the water is brackish (partly salty).

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.