Herbs, to 6 dm. Leaves emersed, rarely floating, petiolate; blade ovate to elliptic, to 15 ´ 10 cm. Inflorescences to 1 m. Flowers chasmogamous; sepals 1.5--2.5(--6) mm; petals white, 1--3 mm, margins slightly erose, apex mostly acuminate; anthers rounded, 0.4--0.6 mm; style ± curved, 0.2--0.4 mm, ¼ length ovary. Fruiting heads 2--4 mm diam; achenes obliquely ovoid, 1.5--2.2 mm, abaxially rounded, with 1 abaxial groove; beak ascending. 2n =14. Flowering spring--fall. Shallow ponds, stream margins, marshes, and ditches; 50--1500 m; N.B., Ont., Que.; Ala., Ark., Colo., Conn., Del., D.C., Ga., Ill., Ind., Iowa, Kans., Ky., La., Maine, Md., Mass., Mich., Minn. Miss., Mo., Nebr., N.H., N.J., N.Y., N.C., N.Dak., Ohio, Okla., Pa., R.I., S.C., S.Dak., Tenn., Tex., Vt., Va., W.Va., Wis.; Mexico.