Plants slender, to more than 2 m long; leaves and inflorescences usually floating. Leaves limp, unkeeled, flat to plano-convex, 0.2–0.8(–2.5) m × 2–5(–10) mm. Inflorescences: rachis unbranched, flexuous, its fertile part usually erect at water surface; bracts ascending, lower bracts inflated near base; pistillate heads 2–5, at least some supra-axillary, not contiguous, sessile or most proximal peduncled (often prominently so in deeper water), 1–3 cm diam. in fruit; staminate heads (1–)2–4, contiguous and appearing as one elongate head, not contiguous with distalmost pistillate head. Flowers: tepals without subapical dark spot, erose-tipped, stigma 1, lance-ovate. Fruits reddish to brownish, dull, short-stipitate, ellipsoid to fusiform, not faceted, body constricted at equator, 3–7 mm, tapering to beak; beak straight, 1.5–2 mm; tepals attached at base, reaching about to equator. Seeds 1. 2n = 30.
Phenology: Flowering summer–fall (Jun–Oct southwestward, Jul–Aug northward).
Habitat: Acid, oligotrophic waters of lakes, ponds, ditches, and streams, usually in shallow waters but to 2.5 m deep
Elevation: 0–4000 m
Greenland, St. Pierre and Miquelon, Alta., B.C., Man., N.B., Nfld. and Labr., N.W.T., N.S., Ont., P.E.I., Que., Sask., Yukon, Alaska, Ariz., Calif., Colo., Conn., Idaho, Maine, Mass., Mich., Minn., Mont., Nev., N.H., N.Mex., N.Y., Oreg., Utah, Vt., Wash., Wis., Wyo., circumboreal.
Sparganium angustifolium is sometimes abundant, its leaves then covering the surface. It is a relatively invariable species that forms fertile hybrids with S. emersum (C. D. K. Cook and M. S. Nicholls 1986), from which it is distinguished by its contiguous staminate heads and flat to plano-convex leaves. See the discussion under S. emersum.