Like I understand that people have to have water, and in many settings, you have a city that pulls water out of a drainage, and then is adding treated wastewater back into the system a bit further down.

In the case of NY, all their water is pulled from high up in the Delaware river basin and is discharged into the Atlantic, never returning any treated wastewater back to the Delaware. This as a result, since the lower Delaware is tidal, is pushing the salt line further and further up river every year.