Procumbent or floating and ascending aquatic biennial to perennial, with fibrous roots. Stems hollow, striate, slightly grooved, usually tinged reddish purple, up to 150 cm long, often rooting at nodes. Aerial lvs 2-4-pinnate, petiolate; ultimate segments usually pinnatifid or crenate, up to 10-(15) mm long; lvs associated with infl.-branches often reduced and 1-pinnate with narrower segments; submerged lvs, when present, 3-4-pinnate, with linear to filiform segments. Umbels terminal or lf-opposed, shortly pedunculate, 1-5 cm diam.; rays (3)-5-12; rays and pedicels somewhat thickened or not at fruiting; bracts 0-1-(3), linear; bracteoles 5-9, linear-lanceolate or subulate. Fls numerous, white or tinged pink, c. 2 mm diam. Fr. ovoid-cylindric, 3-5 mm long; ribs prominent, the lateral broader; styles slender, < ⅓ length of fr.