In 2010 and again in 2013, Bull Sharks of up to 8 feet were netted in the Potomac near Cornfield Harbor and Point Lookout State Park.
Although local media may sensationalize these events, party because National Geographic as dubbed the Bull Shark "the most dangerous shark in the world" and have investigated global cases of freshwater shark attacks.
SharkSavers actually has some great information about how sharks actually survive in fresh water.
|Potomac River Bull Shark
|Potomac River Shark Activity Map
|Potomac River Shark, 2010