Rhizomes present; cauline stems terete, with dark spots, 6–110 cm long. Turions absent. Nodal glands absent. Leaves submersed and floating, although either type may be absent on certain individuals, alternate, more or less spiraling. Submersed leaves petiolate; ligules conspicuous, free from blade, light brown, not fibrous, not shredding apically, persistent, convolute, 1.5–11.7 cm long, the apex acute; petiole terete, 0.9–11.5 cm; leaf blade lax, light green to dark green, ovate to oblanceolate, arcuate, 5–12.5 cm long, 1.5–5.8 mm wide, with 19–49 veins, without lacunae each side of midrib, the base cuneate to obtuse, not clasping, without basal lobes, the margins entire or crispate, the apex acute to round apiculate, without a bristle tip, not cucullate. Floating leaves petiolate; petiole continuous in color to apex, terete, 2.3–22.6 cm long; leaf blade adaxially light green, lanceolate to round elliptic, 4.3–9.2 cm long, 2.5–3.5 mm wide, the veins 27–49, the base obtuse to cordate, the apex acute to obtuse, without terminal bristle. Inflorescences unbranched, distal only; distal peduncles cylindrical, turgid, axillary or terminal, erect, 4.5–22.3 cm long; spikes emersed, cylindrical, 34–65 mm long, the nodes 10–15, the internodes 3–4 mm long. Flowers whorled; perianth spathulate, clawed, the claw 0.2–0.5 mm long, the blade 2.5–2.8 mm long, 2.8–3.2 mm wide; anthers 0.8–1.2 mm long; carpels 4. Fruiting gynophore absent. Fruits obovate, red brown, turgid, abaxially angled, laterally angled, 5–6.7 mm long, 4.5–5.2 mm wide; abaxial angles keeled; lateral angles ridged, without sharp points; beak erect, 0.5–0.8 mm long; tubercules absent; embryo ca. one full spiral. 2n= 52.

Flowering summer and fall. Waters of lakes, ponds, streams, and rivers; 100--2000 m; B.C., Man., N.B., Nfld., N.S., Ont., Que., Sask.; Ala., Ark., Calif., Conn., Ga., Idaho, Ill., Ind., Iowa, Kans., Maine, Md., Mass., Mich., Minn., Mo., Mont., Nebr., N.H., N.J., N.Y., N.C., N.Dak., Ohio, Okla., Oreg., Pa., R.I., S.Dak., Tenn., Vt., Va., Wash., W.Va., Wis., Wyo.

Potamogeton amplifolius is common throughout much of North America. Its submersed leaves are larger than most other species, are usually arcuate, and have more veins than do any other species.

One hybrid, Potamogeton amplifolius ´ P. illinoensis = P. ´ scoliophyllus J. O. Hagström, has been described.