The area where freshwater meets saltwater or an ocean is usually referred to as an estuary. A prime example of this would be the Chesapeake Bay in the United States. Here many freshwater rivers meet the ocean. This area of interchange is referred to as brackish water. Brackish water is not fully saline or as salty as ocean water, say 34 ppt salt but is less saline, 8-24 ppt would be typical brackish water environment.
The area where a river enters an ocean usually has a low current as opposed to areas further upstream where currents have be rather strong do to mountainous topography. The area that separates true freshwater from brackish water is often referred to as the “fall line”. This is where tidal influences end.
Some freshwater fish species are highly intolerant of salinity and will die if exposed to saline waters. These fish species typically live above the fall line in rivers. Some freshwater species are more tolerant of higher salinity levels and will live below the fall line. The complete mirror of this can be seen in fish who live in the ocean. Some ocean fish need high salinity levels to survive and thus stay in the open ocean. Other species can tolerate lower salinity levels and can be found in the lower reaches of rivers below the fall line. Some are so well adapted to low salinity levels they may never venture far from the lower reaches of rivers or the estuarine areas.
There are two special kinds of fish, one anadromous fish such as trout, salmon, certain shad and striped bass spend most of their adult lives in the ocean or estuarine areas but ascend rivers to spawn in low salinity freshwater rivers. The rivers then act as a protective area for the newly born young fish, who after reaching a certain size migrate down river back to the ocean to take advantage of the increased food supply oceans provide. The other special type of fish are catadromous fish. The best example of this fish is the American eel. This species is born in the Sargasso Sea in the middle of the Atlantic Ocean. When the hatch, the follow ocean currents to the North American coast. Once there they ascend rivers all over North America. The females may go hundreds of miles inland to grow for more than 10 years. Males tend to stay closer to the ocean. When they reach a mature size, the males and females migrate back out to the Sargasso Sea where they spawn and die.
Each species is adapted to certain parts of a river system and that is where they will stay. Sometimes floods will force freshwater fish down river further than they wish and they may actually die. This is very rare though. Times of heavy rain like during hurricanes can actually lower salinity levels so low in estuarine environments like the Chesapeake Bay that is allows fish species that normally wouldn’t live in that area to use it until salinity levels increase again.

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