Culms hollow, ± spongy near base, trigonous in cross section, 35–125 cm; vegetative culms hollow, flattened when pressed, taller than fertile culms. Leaves: basal sheaths reddish purple, inner bands fibrillose with age; sheaths with apex of inner band pale brown to dull reddish purple, pubescent or scabrous, at least apically, rarely glabrous; ligules (6–)11–45 mm; blades 3–10 mm wide, usually sparsely to moderately long-pubescent, finely papillose abaxially, glabrous adaxially. Inflorescences 12–60 cm; spikes erect or ascending; proximal 2–5 spikes pistillate; terminal (1–)2–6 spikes staminate. Pistillate scales lanceolate to narrowly ovate, apex acute, conspicuously scabrous-awned, glabrous or scabrous near midrib. Staminate scales lanceolate to narrowly ovate, apex obtuse to acuminate, scabrous-awned, glabrous or, rarely, sparsely pubescent. Perigynia 12–21-veined, (6.5–)7–12 × 1.8–3.8 mm, glabrous; beak 2.1–4 mm, glabrous (rarely few spreading hairs on main veins), teeth spreading to outcurved, (1.2–)1.5–3 mm.
Phenology: Fruiting Jun–Aug.
Habitat: Marshes, wet prairies and meadows, open swamps, wet, open thickets, open stream, pond, and lakeshores, ditches, often in water (to 60–80 cm deep)
Elevation: 50–2800 m
Alta., B.C., Man., N.W.T., Ont., Que., Sask., Yukon, Alaska, Ariz., Calif., Colo., Idaho, Ill., Ind., Iowa, Maine, Mich., Minn., Mo., Mont., Nebr., Nev., N.Mex., N.Y., N.Dak., Ohio, Oreg., Pa., S.Dak., Utah, Vt., Va., Wash., W.Va., Wis., Wyo., Eurasia.
Carex atherodes is a major wetland species in portions of the Midwest and West and becomes increasingly uncommon and local in the eastern portions of its range. It forms large clones and can tolerate deeper water than most Carex. Glabrous forms occur and seem to be more common in the western portion of the range.
Carex atherodes rarely hybridizes with C. trichocarpa.