Carex aquatilis is a tall, green-flowered sedge that occurs in wetlands across Denali from the lowlands into the high mountains, often occurring in inundated habitats such as pond margins and depressions on both sides of the Alaska Range. Plants grow either singly or tufted from stout, scaly, yellow to brown cord-like rhizomes. Stems are erect, 20-120 cm tall, sharply angled and surrounded by old dried leaves and sheaths. Leaves are mostly basal, glabrous, and almost as long as the stem, flat to channeled and 2.5-8 mm wide. The lowest bract is leaf-like, 7-35 cm long and equals or exceeds the inflorescence. Plants have male and female flowers in separate spikes, but on the same stem. The terminal spikes (2-4) are erect, staminate and 1-2cm long. The lateral spikes (2-7) are erect, pistillate and 1-10 cm long. Pistillate scales are ovate to oblong-ovate, sharp-pointed, reddish to purplish brown with a pale mid-vein, usually much narrower than the perigynia. Perigynia are greenish yellow to pale brown in color, papillose and often flattened. Fruits are achenes, yellow to brown, with an abrupt beak. Water sedge could potentially be confused with large specimens of Carex bigelowii, but in that species the scales are blunt-tipped, spikes are not longer than 3 cm, and the lowest bract is not taller than the inflorescence.

Carex aquatilis flowers in mid to late summer in Denali.

Carex aquatilis is monoecious, but male and female flowers are produced in separate spikes. Species in the genus Carex are wind pollinated, but usually also self-pollinate to some degree (Bertin 2007; Friedman and Barrett 2009). Seeds are dispersed by water and grazing waterfowl. This species is sod-forming and primarily reproduces asexually through spreading rhizomes.

Carex aquatilis grows in wet meadows, swamps, fens, ponds, lakeshores, marshes, streambanks and river sloughs. It prefers soils with high organic matter.

Wet to aquatic sites.