Can saltwater fish swim along the borders of fresh water rivers, lakes, etc? Vice-versa with freshwater fish.?

A lot of them can and a lot cannot It depends on whether the individual species has a broad salt tolerance (euryhaline fish) or a narrow tolerance (stenohaline fish). Estuaries, the places where fresh and salt water bdies mix do have a unique mix of fish that are partially tolerant of the other type of water, both salt and fresh, but each species has a difference tolerance. Mollies for instance, can tolerate full fresh or full salt water, but most marine fish can tolerate highly brackish water, but start to thin out in numbers as the water gets fresher. Chromides are estuarine cichlids that can live in fresh or fairly salty water.

The Cheaspeake Bay is the biggest Estuary in the USA. It it mostly fresh up near the Potomac River basin and Washington DC, but gets saltier toward the Delmarva Peninsula. According to NOAA, the Chesapeake Blue Crab that is used for crab cakes can live in any part of the bay.

There are 350 species of fish that live in Chesapeake Bay. Go to the Fish - Chesapeake Bay Program website and you can click on the picture of each fish to found out how much it can travel from fresh to salty parts of the bay and vice-versa.

Smaller estuaries like tyhe Hudson river in New York Harbor usually have a bigger cutoff of fish in the opposite kind of water as there iare no good breeding or eating places of about 1/2 and 1/2 fresh and salt for a species to develop of a tolerance for both.