Sp. Pl. 2: 971. 1753.
Plants slender, grasslike, limp, to 0.6 m; leaves and inflorescences floating or, when stranded, more or less erect. Leaves limp in water, unkeeled, flat, 0.04–0.6 m × 2–6(–10) mm; leaves of stranded plants shorter, firmer. Inflorescences: rachis unbranched, flexuous; bracts ascending, basally inflated; pistillate heads 1–3, axillary, not contiguous, sessile or most proximal short-peduncled (often long-peduncled in Alaska and nw Canada), 0.5–1.2 cm diam. in fruit; staminate head 1 or apparently so, terminal, not contiguous with distalmost pistillate head. Flowers: tepals without subapical dark spot, erose; stigmas 1, lance-ovate. Fruits dark greenish or brownish, subsessile, body ellipsoid to obovoid, not faceted, barely or not constricted at equator, 2–4 × 1–1.5 mm, tapering to beak; beak curved, 0.5–1.5 mm; tepals attached at base, reaching about to equator. Seeds 1. 2n = 30.
Phenology: Flowering summer–fall (Jun–Sep southwestward, Jul–Aug northward).
Habitat: Cool, quiet, slightly acid to somewhat basic waters of bays, pools, ditches, and peat bogs, usually in shallow water but sometimes to 60 cm depth, where less floriferous, abundant in its northern range, less so southward
Elevation: 0–3500 m
St. Pierre and Miquelon, Alta., B.C., Man., N.B., Nfld. and Labr., N.W.T., N.S., Nunavut, Ont., P.E.I., Que., Sask., Yukon, Alaska, Calif., Colo., Conn., Idaho, Ill., Ind., Maine, Mass., Mich., Minn., Mont., N.H., N.Y., Oreg., Pa., R.I., Utah, Vt., Wash., Wis., Wyo., circumboreal (not in Greenland).
This species has long been known as Sparganium minimum, but although the correct name is S. natans (C. D. K. Cook 1985).
The leaves of Sparganium natans are thinner and more translucent than those of the similar S. hyperboreum, and they lack the yellowish cast of that species. Its distalmost pistillate head is not contiguous with the staminate head, as is sometimes the case in S. hyperboreum, and its beaked fruit also distinguishes it from that species. See the discussion under S. hyperboreum for a description of S. hyperboreum × S. natans.