There are more common carp in the USA rivers than there are any other fish. They account for the majority of the biomass per fish sampling just about everywhere freshwater and tidal. I've seen them as far down the bay as Kent Narrows and far up smaller streams too. Late spring is their peak spawn. Find flowing water, natural or unnatural blockage and you'll find thousands of pounds of carp. Conowingo dam, locks and dams on the C&O canal, Patapsco River, .... the list is endless.