Hey Marty -
Bull sharks are apparently very comfortable in freshwater. I've seen them in the Mississippi River at Ft. St. Phillip (Across from Ft. Jackson) during late summer when the river clears, and they have been documented for decades in lakes in Nicaragua that are supposedly isolated from saltwater. And I recall a newspaper article in the "Shreveport Times" ("Journal"?) back in the early 1960's about a commerical fisherman catching one in a hoop net in Lake Bistineau!
Just goes to show that you should keep an eye on your toes in Louisiana waters, no matter how fresh they are!
Pete