Culms to 2 times wider than thick, 35–60(–70) cm × 0.5–1.3 mm, hard, usually with to 10 blunt ribs, at 20X often finely ridged and minutely granular. Leaves: distal leaf sheaths brown to reddish. Spikelets 3–12 × 3–4 mm; proximal scale amplexicaulous, 3.5–4 mm, midrib region very broad and fleshy, apex entire; subproximal scale with a flower; floral scales 30–40, 8–10 per mm of rachilla, orange-brown to stramineous, midrib region stramineous, broadly ovate, papery or sometimes membranous, 3–4 × 2 mm, apex entire, rounded. Flowers: perianth bristles present, sometimes rudimentary, brown, length variable, obscurely retrorsely spinulose; anthers brown, 1.7–2.2 mm. Achenes subdeltoid in outline, sometimes broadly obpyriform, equilaterally trigonous, angles prominent, 0.9–1.2 × 0.7–1.1 mm, apex truncate. Tubercles sessile, depressed-pyramidal, often apiculate, as wide as achene, 0.3–0.5 × 0.7–1.1 mm.
Phenology: Fruiting summer.
Habitat: Fresh, oligotrophic, acid, sandy or peaty, often drying shores, ponds, ditches
Elevation: 10–300 m
Ala., Fla., Ga., Ind., Mass., Mich., N.Y., N.C., R.I., S.C., Tex., Va.
Eleocharis melanocarpa is reported from Rhode Island; I have not seen a voucher specimen.