Trans. Acad. Sci. St. Louis 2: 282. 1865 (as polysepalum)
Rhizomes 3-8 cm diam. Leaves mostly floating, occasionally emersed or submersed; petiole terete. Leaf blade abaxially and adaxially green, widely ovate, 10-40(-45) × 7-30 cm, ca. 1.2-1.5 times as long as wide, sinus 1/3-2/3 length of midrib, lobes divergent to overlapping; surfaces glabrous. Flowers 5-10 cm diam.; sepals mostly (6-)9(-12), abaxially green to adaxially yellow, sometimes red-tinged toward base; petals oblong, thick; anthers 3.5-9 mm, slightly shorter than filaments. Fruit green to yellow, cylindric to ovoid, 4-6(-9) × 3.5-6 cm, strongly ribbed, slightly constricted below stigmatic disk; stigmatic disk green, 20-35 mm diam., entire to crenate; stigmatic rays 8-26(-36), linear to lanceolate, terminating within 1(-1.5) mm from margin of disk. Seeds 3.5-5 mm. 2n = 34.
Phenology: Flowering spring (later in north)-summer.
Habitat: Ponds, lakes, and sluggish streams
Elevation: 0-3700 m
B.C., N.W.T., Yukon, Alaska, Ariz., Calif., Colo., Idaho, Mont., Nev., N.Mex., Oreg., Utah, Wash., Wyo.
Plants intermediate between Nuphar polysepala and N. variegata occur in eastern British Columbia.