Plants extensively colonial; rhizomes long. Culms trigonous in cross section, 18-65 cm, smooth distally. Leaves: basal sheaths pale brown; ligules as wide to wider than long; blades pale to mid green, flat to W-shaped, 4-11 mm wide, glabrous. Inflorescences 8-24 cm; proximal bract 11-37 cm, exceeding inflorescence; proximal 2-4 spikes pistillate, erect or often the proximal ascending to spreading; terminal 1(-2) spikes staminate. Pistillate scales lanceolate-acuminate, 2.4-6.9 × 0.3-0.9 mm, shorter than or the proximal longer than perigynia, margins serrulate-ciliate, apex with prominent scabrous awn. Staminate scales acute to acuminate, smooth except at tip. Perigynia ascending, 7-11-veined, narrowly elliptic, 4.2-7 × 1.3-1.8 mm, apex tapered; beak 1.4-2.4 mm, bidentulate, smooth, teeth straight, 0.2-0.5 mm. Stigmas 3. Achenes pale brown, trigonous, smooth. Fruiting Jun-Jul. Shallow cold streams, springheads, seeps, margins of fens, and seepy stream, pond, and lakeshores in open or lightly shaded sites, in highly calcareous soils; 100-600 m; Ont.; Conn., Mass., Mich., Mo., N.Y., Pa., Vt., Va., Wis. Carex schweinitzii is very local and uncommon but often forms large colonies where present. It hybridizes occasionally with C. hystericina. The single, old Missouri collection (G. Yatskievych 1999+) seems so far disjunct as to suggest that there may have been a label mixup.