Perennials (perhaps flowering first year) to 200+ cm (aquatics). Leaves sessile; submersed blades multifid (2-3+-pinnatisect), ultimate lobes filiform, mostly 0.1-0.3 mm diam.; aerial blades ovate to lanceolate, 10-45+ × 5-20+ mm, bases cuneate, margins pectinately incised to serrate or entire, not ciliate, apices obtuse to acuminate or attenuate, faces glabrous. Heads usually borne singly. Peduncles (10-)20-100 mm. Calyculi of 5-6 usually spreading, oblong to obovate bractlets 5-8 mm, margins entire, not ciliate, abaxial faces usually glabrous. Involucres ± hemispheric, 7-12 × 12-15 mm. Phyllaries 7-8+, ovate, 7-10 mm. Ray florets 8; laminae yellow, 10-15 mm. Disc florets 10-30+; corollas pale yellow, 5-6 mm. Cypselae (outer and inner ± alike) yellowish to greenish brown, nearly terete or weakly 4-angled, ± linear, 10-15 mm, margins not barbed or ciliate, apices truncate, faces smooth or ± striate, glabrous; pappi of 2(-6) divergent to patent, retrorsely barbed (on distal ¹/4 or so) awns 13-25(-40) mm. 2n = 26. Flowering Jul-Sep. Still or slow-moving waters; 0-300+ m; B.C., Man., N.B., N.S., Ont., Que., Sask.; Conn., Idaho, Ill., Ind., Iowa, Maine, Mass., Mich., Minn., Mo., Mont., N.H., N.J., N.Y., Ohio, Oreg., Pa., R.I., Vt., Wash., Wis. M. L. Roberts (1985) presented a strong case for treating Bidens beckii as Megalodonta beckii.