The Nymphaeus was a river of Latium. Pliny the Elder seems to be the only ancient author who mentions it, and he has it flowing into the Tyrrhenian Sea somewhere between Astura and Circeii. The compilers of the Barrington Atlas show it flowing across the Pomptinus Ager. Nineteenth-century sources that include W. Smith, and perhaps whence the Barrington Atlas Directory, associate this river described by Pliny with a stream called "Ninfa" that is said to flow from near the Medieval site of Ninfa (close to Norma in Lazio) to the sea. It is unclear whether this waterway is now extant and observable in the landscape.
Description via Pleiades. Coordinates and heritage data from the Atika atlas.
Atika pings you when you walk or drive past heritage like this. Works offline, no account needed to explore.