Oenanthe fluviatilis, the river water-dropwort, is a flowering plant in the carrot family, Apiaceae, which is endemic to north-west Europe.

These extend upwards through the water column, becoming thicker as they ascend, to a diameter of about an inch (2.5 cm), with a central hollow about half that size.

The lower, underwater leaves are once to twice pinnate, with distinctive cuneate (diamond shaped) segments which are deeply divided into almost linear lobes (especially so in faster-flowing water), with petioles as long as the leaf.

In the summer, the upper, aerial, leaves sometimes form: they are 1-3 times pinnate with ovate, lobed segments about 1 cm long, and have petioles about one third the length of the blade.

[1][2] The underwater foliage is highly distinctive, with its diamond-shaped leaves and the ability of the plant to form a carpet on the bed of the stream, which remains green throughout the winter.

Flowering plants could be confused with fine-leaved water-dropwort, although that species has more highly divided leaves (4-pinnate) and larger fruits (more than 5 mm long).

[2][1] Barring a few false starts and minor confusions, river water-dropwort was overlooked as a species until the 1840s, largely because it rarely flowers.

Coleman initially reported his observations to Charles Babington who, in 1843, cautiously added it to his Manual of British Botany as Oenanthe phellandrium var.

[9] River water-dropwort is entirely confined to North-west Europe, being recorded only in Ireland, England, Germany, Austria, France, Denmark and the Low Countries.

In this chalk stream it grows in a community of Ranunculus penicillatus, Sparganium emersum, Schoenoplectus lacustris and Groenlandia densa, and it is associated with a wide variety of rare and protected species.