Leaves mostly in 3s, spreading to recurved, linear, oblong, to ovate, 5--13 ´ (1.1--)2--5 mm, flat. Inflorescences: staminate spathes cylindric, 8.2--13.5 mm; peduncles abscissing just before or during anthesis; pistillate spathes cylindric, 8.3--17.5 mm. Flowers unisexual; staminate flowers: stamens 7--9(--18), pedicels detaching before or during anthesis; all filaments connate proximally, forming column; anthers 1.7--3 mm; pollen in tetrads; pistillate flowers: styles 2.6--4 mm. Seeds fusiform, 4--5.7 mm, basal hairs absent. 2n = 24 (Britain). Flowering summer. Waters, mostly calcareous, of lakes and rivers; 0--2000 m; B.C., Man., N.B., N.S., Ont., Que., Sask.; Ala., Ark., Calif., Colo., Conn., Del., Fla., Idaho, Ill., Ind., Iowa., Kans., Ky., Maine, Md., Mass., Mich., Minn., Mo., Mont., Nebr., Nev., N.H., N.J., N.Mex., N.Y., N.C., N.Dak., Ohio, Oreg., Pa., R.I., S.Dak., Tenn., Utah, Vt., Va., Wash., W.Va., Wis., Wyo.; Europe; Asia; Australia.