We had a baby blue whale come up the Skagit river a few years back, we have property on the river and the little guy..(right the was like 30 feet long) stayed out in front of our place for most of a weekend ( we were camping). got some neat videos...though fishing sucked that weekend.
things go up rivers all the time..though that shark could of been tossed in by someone who bought it and it died in there tank?..
who knows....maybe they will evolve to be like the Bull shark^_^.