Herbs, annual, densely cespitose, with fibrous roots. Culms roundly trigonous, 3–25(–30) cm × 0.3–1.4 mm, glabrous. Leaves 1–3, V-shaped, 1–8 cm × 1–2 mm. Inflorescences: spike 1, loosely ovoid, compressed, 7–14 × 9–14 mm; rays 1–4, to 2(–3) cm; bracts 2–3, horizontal to ascending at 30(–45)°, V-shaped to flat, 1–12 cm × 0.5–1.5 mm. Spikelets 3–5(–8), flattened, oblong to oblong-lanceoloid, 8–18 × 2–2.6(–3) mm; floral scales (6–)10–26(–32), closely imbricate, laterally light to dark brown, medially light brown, laterally ribless, medially 2–3-ribbed, 2-keeled, oblong to ovate, 1.9–2.7 × 1.8–2.3 mm, apex obtuse. Flowers: stamens 2–3; anthers 0.4–0.5 mm, connective not prolonged; styles 0.6–1 mm; stigmas 1–1.5 mm. Achenes black, network of ridges forming isodiametric or square cells, sessile, obovoid to ovoid, 1–1.3(–1.5) × (0.6–)0.8 mm, apex apiculate, surfaces minutely punctate.
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Tufted annual 1–2(–4) dm; cauline lvs 1–3 mm wide, usually shorter than the stem; spikes all sessile, or the 1–5 rays to 10 cm; spikelets 3–10, 8–15 × 2–3.5 mm; scales ovate, 2–2.5 mm, as seen from the side in half-view usually less than 3 times as long as wide, obtuse, and with the prominent midvein incurved distally to a somewhat cucullate tip, at maturity strongly red-brown from midrib to margin (less so distally), not sulcate; stamens 2(3); achenes lenticular, elliptic-obovate, 1–1.5 mm; style undivided in the basal third, deciduous. Wet ground, especially along shores; Me. and Que. to N.D., s. to Ga. and Tex. (C. niger var. castaneus; C. rivularis)
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