Plants densely cespitose. Culms (5–)20–80 cm. Leaves: sheath summits usually U-shaped, sometimes prolonged to 2.5(–4) mm beyond collar; distal ligules 1–3(–5) mm; blades 3–4(–5) per fertile culm, 10–20 cm × (1.5–)2–3(–5) mm. Inflorescences erect, dense, headlike, green to brown, (0.8–)1.5–2.2 cm × 7–20 mm; proximal internode 1.5–5 mm; 2d internode 1–3.5 mm; proximal 2–3 bracts leaflike, ascending to spreading, much longer than inflorescences, usually less than 1.8 mm wide, bases ± surrounding culms. Spikes (5–)7–10(–14), individually distinct or indistinct, broadly ovoid to ovoid, (4.5–)7–10 × 4.5–7(–9) mm, base and apex truncate to acute. Pistillate scales gold to brown, with whitish, green, or tan midstripe, lanceolate to ovate, 2.4–4.3 mm, shorter, narrower than perigynia, margin sometimes white, 0.25 mm wide, apex acuminate to awned. Perigynia ascending to ascending-spreading, cream colored to light brown, conspicuously 0–9-veined abaxially, conspicuously 0–8-veined adaxially, ovate to lanceolate, plano-convex or flat, (2.8–)3.5–4(–4.8) × (0.8–)1–1.5(–1.8) mm, 0.35–0.45 mm thick, margin flat, including wing (0.1–)0.2(–0.5) mm wide, wing ciliate-serrulate on distal body; beak gold to red-brown at tip, cylindric, unwinged, abaxial suture usually with conspicuous white margin, distance from beak tip to achene 1.9–2.5 mm, entire for at least 0.4–0.9 mm. Achenes sessile or shortly stipitate, ovate to obovate, (1–)1.2–1.6 × 0.7–1 mm, 0.3–0.4 mm thick. 2n = 68.

Fruiting late spring–summer. Seasonally moist meadows, marshes, pond and lake margins; 100–4000 m; Alta., B.C., Man., Sask., Yukon; Alaska, Ariz., Calif., Colo., Idaho, Mont., Nev., N.Dak., Oreg., S.Dak., Tex., Utah, Wash., Wyo.; Mexico (n Baja California).

Carex athrostachya intergrades with C. unilateralis.