Rhizomes present; cauline stems terete, without dark spots, to 100 cm long. Turions absent. Nodal glands absent. Leaves submersed, alternate, more or less distichous. Submersed leaves sessile; ligules conspicuous, adnate to blade, light greenish brown to white, fibrous, shredding apically, persistent, connate, 0.5–2 cm long, adnate ca. 1/4 of length, the apex obtuse; leaf blade coarse, dark green to reddish brown, linear to linear lanceolate, not arcuate, 2–7(–12) cm long, 3–4(–8) mm wide, with 20–60 veins, without lacunae each side of midrib, the base obtuse, not clasping, with basal lobes, margins minutely serrulate, the apex acute, without a bristle tip, not cucullate. Inflorescences rarely branched, distal only; distal peduncles cylindrical, turgid, axillary, erect, 3–5(–7) cm long; spikes emersed, moniliform, 7–20 mm long, the nodes 4–5, internodes 4–5 mm long. Flowers alternate to whorled; perianth spathulate, clawed, the claw 1–1.2 mm long, the blade 2–2.3 mm long, 2.4–2.6 mm wide; anthers 0.4–0.7 mm long; carpels 4. Fruiting gynophore present, ca. 0.5 mm long. Fruits obovate, brown, turgid, abaxially angled, laterally angled, 3–5 mm long, 2–3.3 mm wide; abaxial angles keeled; lateral angles keeled, without sharp points; beak erect, 0.7–0.9 mm long; tubercules absent; embryo less than one full spiral. 2n= 52.

Flowering late summer--early fall. Shallow to deep water of ponds, lakes, and slow-flowing rivers; 5--3300 m; Alta., B.C., Man., N.B., Nfld., N.W.T., N.S., Ont., PEI., Que., Sask., Ala., Alaska, Calif., Conn., Del., Idaho, Ill., Ind., Maine, Mass., Mich., Minn., Mont., N.H., N.J., N.Y., Ohio, Oreg., Pa., R.I., Utah, Vt., Wash., Wis., Wyo.

Potamogeton robbinsii is our most easily recognized species when it is fertile. It is the only species with a branched inflorescence. The species, however, occurs in fairly deep water, forming large colonies that essentially cover the substrate. Only rarely do the plants flower. It also is the only species with truly auriculate leaves, the blades forming small lobes projecting past the stem on each side of the stem. Other species my have slightly rounded bases of blades, but none others have lobes that actually protrude past the stem.

The species has a fairly large disjunction, being known in the northern part of our area, and Tensas River, Baldwin County, Alabama. The Alabama population has been collected on at least two occasions over 40 years, once as recently as 1970.