Rhizomes present. Cauline stems terete, without spots, 28-120 cm; nodal glands absent. Turions absent. Leaves both submersed and floating or the floating absent, ± spirally arranged; submersed leaves lax, sessile or petiolate, stipules persistent, conspicuous, free from blade convolute, light brown to red-brown, not ligulate, not fibrous, not shredding at tip, 1--8 cm, apex acute, petioles, if present, 0.5--4 cm, blades red-brown to light green, elliptic to lanceolate or rarely linear, often arcuate, 7--19-veined, 5--20 ´ 0.2-4-.5 cm, base acute, margins entire, often crisped, apex not cucullate, acute-mucronate, lacunae present, 2--5 each side midrib; floating leaves petiolate, petiole continuous in color to apex, 2--9 cm, blades light green above, elliptic to oblong-elliptic, 13--29-veined, 4--19 ´ 2--6.5 cm, base cuneate, apex round-mucronate. Inflorescence unbranched, emersed; peduncles not dimorphic, terminal or axillary, erect to ascending, cylindric, 4--30 cm; spikes not dimorphic, cylindric, 2.5--7 cm. Fruits sessile, grayish-green to olive-green, obovate to ovate, laterally compressed, abaxial keel present, lateral ridges present, 2.5--3.6 ´ 2.1--3 mm, lateral ridges without points; beak present, erect to slightly recurved, 0.5--0.8 mm; sides without basal tubercles; embryo with ca. full spiral. 2n = 104 [A. Löve & D. Löve 1981]

Flowering and fruiting summer--autumn. Alkaline waters of streams, rivers, lakes, ponds, and sloughs; 5--2350 m; B.C., Man., N.B., N.W.T., Ont., Que.; Ala., Ark., Calif., Colo., Conn., Del., Fla., Ga., Ill., Ind., Iowa, Kans., Ky., Md., Mass., Mich., Minn., Mo., Mont., Nebr., N.J., N.Mex., N.Y., N.C., Okla., Ohio, Oreg., Pa., S.C., S.Dak., Tex., Utah, Vt., Va., Wash., W.Va., Wis., Wyo.; Mexico; West Indies; Central America; South America.

This species and Potamogeton gramineus are often difficult to separate. Certainly, in the extreme of each they are easily separated, but they continually grade into each other. Features to look for are acute-mucronate apex of submersed leaves of P. illinoensis and acuminate for P. gramineus. Also, number of veins seems to work as well.

Three hybrids, Potamogeton illinoensis ´ P. nodosus = P. ´ faxonii T. Morong, P. amplifolius ´ P. illinoensis = P. ´ scoliophyllus J. O. Hagström, and P. gramineus ´ P. illinoensis = P. ´ spathuliformis (J. W. Robbins) T. Morong, have been described.