Carex fluviatilis Boott var. unisexualis (C. B. Clarke) Kükenthal.

Rhizome long creeping, slender, clothed with brown sheaths disintegrating into fibers. Culms (10-)15-50 cm tall, compressed trigonous, with pale brown sheaths at base. Leaves shorter than culm, blades linear, 1-2.5 mm wide, flat or plicate, apex acuminate. Involucral bracts setaceous or glumelike. Plants dioecious (rarely monoecious); female spicate inflorescence cylindric, 2.5-8 × 0.7-1 cm; male inflorescence rather narrow, 5-6 mm wide; spikes 15-30, unisexual, rarely androgynous; female spikes oblong-ovate, 5-8 × ca. 4 mm wide; male spikes oblong, ca. 6 × 2-3 mm. Female glumes pale green, ovate, 2-3 mm, sparsely ferruginous puncticulate, margins white hyaline, green 1-veined costa excurrent into an awn at acute apex. Utricles greenish or pale, ferruginous dotted, longer than glume, ovate, plano-convex, 2-3 × 1-1.5 mm, membranous, many veined on both faces, base subrounded, spongy, shortly stipitate, upper margins serrulate winged, apex gradually narrowed into a scabrous beak, orifice 2-toothed. Nutlets dark brown, loosely enveloped, ovate or elliptic, plano-convex, ca. 1.2 mm, lustrous, base shortly stipitate, apex rounded; stigmas 2. Fl. and fr. Apr-Jun.

Lakesides, ponds, swamps, grasslands. Anhui, Hubei, Hunan, Jiangsu, Jiangxi, Yunnan, Zhejiang [Japan].