Procumbent to ascending, aquatic or amphibious perennial, rooting at lower nodes. Stems hollow, grooved, striate, up to 180 cm long. Basal lvs 1-pinnate, petiolate; leaflets in 2-8 pairs, ovate to lanceolate, serrate, sometimes lobed, particularly the terminal leaflet, 1-8-(12) cm long, sessile; stem lvs similar to basal, but uppermost often reduced to 3-5 leaflets. Umbel 2-4 cm diam., compound, subsessile or shortly pedunculate, lf-opposed; rays 4-14; bracts 0-2, linear to lanceolate; bracteoles 4-8, ovate-triangular. Fls numerous, white, 1-2 mm diam. Fr. ovoid, dark brown, 1.5-2.5 mm long; ribs light brown, slender or slightly thickened.