"Potamic" is an adjective meaning "of or relating to rivers." It comes from the Greek word for river, "potamos." (As seen in "hippopotamus, or "river horse.") Despite this ancient root, the word was not seen in English until the 19th century. "Potamology" (the study of rivers) comes from the same root.
Depending on which source you believe, either 19th-century German geographer Carl Ritter or German geo-philosopher Ernst Knapp of about the same time named the first of his stages of civilization "the potamic," for early societies "developed in extensive river valleys, such as those of the Nile, the Tigris and Euphrates, and the Ganges." (The other two were the thalassic, for cultures influenced by the ability to cross large inland seas like the Mediterranean, and the oceanic, influenced by the ability to traverse entire oceans.) This classification has been used by other historians, and the word "potamic" also seems to have been adopted by people in geosciences, engineers interested in irrigation and such river-related works, and biologists studying estuaries and other wetland environments.
Despite the fact that Google searching shows "potamic" as a farly common misspelling of the name of the Potomac River, this river's name comes from an Indian word and not the Greek root. (I should hope no one would be so uncreative as to name the waterway "the river-like river"!)
The E2 node Potomac River