Crazy theory here:
Maybe the sharks get into the river by chance, and continue up the river because they need water current to flow thru their gills to get oxygen. Sharks need to be in constant motion to breathe, but they have been filmed sitting in one place where the current flows fast enough for them to breathe.
It’s easier on the shark to swim upstream than downstream because the current gives them more oxygen. Once they are ‘trapped’ in the river they naturally go upstream.
Does that make sense?
There is just as much oxygen in the water no matter which direction the fish swims. It’s the same water, carrying the same oxygen.
One thing that is interesting is that south of Alton is a set of rocky rapids called The Chain of Rocks, which is impassible part of the year, except when the river floods enough to rise above the rocks. At some point a canal was built enabling barges to bypass this spot, but I forget when the canal was dug.
For a bull shark to get to Alton in what I guess was a time before the canal, it would have had to swim over the Chain of Rocks during a flood period.