Culms dark green, 1-3 m × 2-6 mm, very firm, rarely soft, air cavities in distal 1/4 mostly 0.5 mm wide. Leaves: sheath fronts coarsely pinnate-fibrillose. Inflorescences 2 times branched, branches mostly stiffly ascending. Spikelets 3-40, 6-18 × 3-4 mm. Flowers: perianth bristles 6, equaling achene body; styles 2-fid. Achenes plano-convex. 2n = 36, 38. Fruiting summer. Fresh, often calcareous to brackish marshes, fens, lakes, slow streams, often emergent in water to 1.5 m; 0-2700 m; Alta., B.C., Man., N.B., N.W.T., N.S., Ont., P.E.I., Que., Sask., Yukon; Alaska, Ark., Colo., Conn., Del., Idaho, Ill., Ind., Iowa, Kans., Ky., Maine, Md., Mass., Mich., Minn., Mo., Mont., Nebr., N.H., N.J., N.Y., N.C., N.Dak., Ohio, Okla., Pa., R.I., S.Dak., Tex., Vt., Va., Wash., Wis., Wyo.; probably Eurasia.