Bull Shark

After Hurricane Katrina, many bull sharks were sighted in Lake Ponchartrain. Bull sharks have occasionally gone up the Mississippi River as far upstream as Alton, Illinois. They have also been found in the Potomac River in Maryland.
Freshwater tolerance
The bull shark is the best known of 43 species of elasmobranch in ten genera and four families to have been reported in fresh water. Other species that enter rivers include the stingrays (Dasyatidae, Potamotrygonidae and others) and sawfish (Pristidae). Some skates (Rajidae), smooth dogfishes (Triakidae), and sandbar sharks (Carcharhinus plumbeus) regularly enter estuaries. Elasmobranchs' ability to enter fresh water is limited because their blood is normally at least as salty (in terms of osmotic strength) as seawater through the accumulation of urea and trimethylamine oxide, but bull sharks living in fresh water reduce the concentration of these solutes by up to 50%. As a result, bull sharks living in fresh water need to produce 20 times as much urine as those in salt water.