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.