Plants perennial, mat-forming; rhizomes evident, long, 0.5-1.5 mm thick, soft to firm, cortex often breaking loose, longer internodes 5-30 mm, scales often fugaceous, 4-8 mm, membranous, not fibrous. Culms terete, often with 8-12 blunt ridges when dry, 8-80 cm × 0.3-1.4 mm, firm to soft, internally spongy. Leaves: distal leaf sheaths persistent, not splitting, proximally red (to stramineous), distally green to stramineous, usually inflated, often callose, membranous to papery, apex often red-brown, broadly obtuse to subacute, tooth sometimes present, to 0.1 mm. Spikelets ovoid to lanceoloid or nearly cylindric, 3-18 × 2-3(-4) mm, apex acute (to obtuse); proximal scale amplexicaulous, entire; subproximal scale with flower; floral scales often spreading in fruit, 15-50, 4-5 per mm of rachilla, medium brown to sometimes red-brown, midrib regions mostly stramineous to green, in proximal part of spikelet ovate, apex rounded, in distal part lanceolate, apex entire, acute, 2-3.5 × 1.5-1.7 mm, mostly carinate. Flowers: perianth bristles 4 or absent, light brown to stramineous, stout, usually equal, equaling achene to slightly exceeding tubercle; stamens 3; anthers dark yellow to stramineous, 1-1.8 mm, apiculate; styles 2-fid. Achenes not persistent, dark yellow, stramineous, or dark brown, obovoid to obpyriform, biconvex, angles obscure, 0.9-1.6 × 0.7-1.2 mm, apex rounded, neck absent to long, smooth at 30X, or sometimes finely rugulose at 10-30X with 20 or more horizontal ridges in a vertical series. Tubercles brown to whitish, pyramidal, much higher than wide to, lower than wide, 0.35-0.65 × 0.2-0.6 mm. 2n = 16, 18, 19, 20. Fruiting summer. Non-calcareous or calcareous fresh or brackish shores, marshes, meadows, fens, disturbed places; 0-2300 m; Alta., B.C., Man., N.B., N.W.T., Ont., Que., Sask.; Alaska, Ariz., Ark., Colo., Conn., Del., Idaho, Ill., Ind., Iowa, Kans., Ky., Maine, Mass., Mich., Minn., Miss., Mo., Mont., Nebr., N.H., N.J., N.Mex., N.Y., N.C., N.Dak., Ohio, Okla., Oreg., Pa., R.I., S.Dak., Tenn., Tex., Utah, Vt., Va., Wash., W.Va., Wis., Wyo. Eleocharis erythropoda is extremely variable. Intermediates with E. palustris variant b are common in the region of sympatry in the East, and with E. macrostachya variant b in the West, and E. erythropoda is sometimes difficult to distinguish from E. uniglumis. Eleocharis calva Torrey is an invalid name.