system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how is salt water separated from fresh water in rivers? They eventually lead to the ocean so where does the salt end and the fresh begin?
gpt
Rivers flow into the ocean.  The force of the flow is stronger than the tidal forces of the ocean until you get right to the edge between the two, and that's where the salt water takes over.  That line can move back and forth a bit between high and low tides, but in general that's why there is a separation.  A river is flowing outward and it pushes the salt water away.