Most sharks are not physiologically capable of living in freshwater, although the bull shark is an exception. In order to live in freshwater, the body of water would need to be wide and deep enough to accommodate the shark, and it would have to be connected to the ocean for the shark to get there in the first place.
Bull sharks have a unique ability to maintain a continual concentration of water in their bodies, which is known as osmoregulation, even if the salinity level of the water changes. Bull sharks have been known to travel as far as 60 miles upstream in the Mississippi River, and researchers say that the Neuse River in North Carolina also harbors bull sharks who might make the journey there to pursue dolphins or even use the river as a nursery for bull shark pups. Notably, there are also five different species of river sharks that live in rivers in Australia and Asia.