Copepod (Crustacea) species diversity was remarkably low on Plummers Island, Maryland, and its immediate environs over a year-long sampling period in 1996 and 1997, compared to that previously and contemporaneously reported nearby within the Potomac River Basin; only two species were found on the Island and two others in the adjacent side channel of the Potomac River. Subsequent sampling in 2004 resulted in records of eight species, seven of which were not found in 1997; six of these were collected on the Island. One explanation for the paucity of species in 1996–1997 is the extreme floods of January and September 1996; the January flood reached a height not recorded since 1972 and destroyed canal locks. These floods scoured much of the Potomac floodplain and may have caused severe local population losses. The scarcity of aquatic microhabitats on the Island undoubtedly also contributed to the low local diversity.