Culms 35-100 cm, angles glabrous or scabrous. Leaves: basal sheaths leafless, usually red-brown; blades amphistomic, 5-8 mm wide, papillose on both surfaces. Inflorescences: peduncle of proximal spike not more than 4 cm; proximal bract longer than inflorescence, 4-8 mm wide. Spikes erect; proximal 2-6 spikes pistillate, 4-6 cm × 4-7 mm, base cuneate or attenuate; terminal 1-3 spikes staminate. Pistillate scales pale red-brown with narrow red-brown marginand broad pale midvein. Perigynia pale brown, veinless, obovoid, 2.6-3.2 × 1.5-2.2 mm, apex rounded; beak pale brown, thickened, not more than 0.2 mm. 2n = 76, 77.

Fruiting Jul-Aug. Marshes, shallow water along shores, usually in neutral or calcareous substrates; 0-1000 m; Man., N.B., N.S., Ont., Que.; Conn., Ill., Ind., Iowa, Kans., Maine, Mass., Mich., Minn., Mo., Nebr., N.J., N.Y., N.Dak., Ohio, Pa., S.Dak., Vt., Va., Wis.

Populations of Carex aquatilis var. substricta from Minnesota to Massachusetts tend to have very pale scales with a very broad, pale brown midvein, distinctly obovoid perigynia, and broad pistillate spikes that are rarely staminate at the apex. These populations tend to occur on neutral or calcareous substrates (fens) rather than the more acidic marshes favored by var. aquatilis. The two varieties intergrade where they come into contact.

Infrequent but locally plentiful in the northernmost tier of counties in marshes and sloughs and on lake or river borders. Most of the reports of C. aquatilis from Indiana were doubtless based upon specimens of this species.