Stems floating or prostrate, glabrous, rooting at proximal nodes. Leaves: basal leaves seldom present, cauline leaf blades semicircular to reniform, 1-6×-lobed, parted, or dissected 1.2-7.3 × 1.9-10.8 cm, base truncate or cordate, segment margins entire or crenate, apex rounded to filiform. Flowers: receptacle sparsely hispid; sepals 5, spreading or weakly reflexed, 5-7 × 3-6 mm, glabrous; petals 5-6(-14), 7-12 × 5-9 mm; nectary scale variable, crescent-shaped, funnel-shaped, or flaplike; style 0.8-1.2 mm. Heads of achenes ovoid, 8-10 × 7-8 mm; achenes 1.8-2.2 × 1.6-2.2 mm, glabrous; beak lanceolate, straight, 1-1.8 mm. 2 n = 32. Flowering late spring-summer (May-Aug). Shallow water or drying mud; 0-1500 m; Alta., B.C., Man., N.B., Ont., Que.; Ala., Ark., Calif., Conn., Del., Idaho, Ill., Ind., Iowa, Kans., Ky., La., Maine, Md., Mass., Mich., Minn., Miss., Mo., Nebr., Nev., N.H., N.J., N.Y., N.C., N.Dak., Ohio, Okla., Oreg., Pa., R.I., S.Dak., Tenn., Tex., Utah, Vt., Va., Wash., Wis., Wyo. The Fox tribes used Ranunculus flabellaris as a cold remedy and a respiratory aid (D. E. Moerman 1986).
From Flora of Indiana (1940) by Charles C. Deam
Somewhat frequent in ponds, sloughs, and dredged ditches in the lake area, becoming infrequent to very local or absent from the southern part of the state. When its habitat becomes dry during the summer months, this species assumes a terrestrial form. This form has been named Ranunculus flabellaris f. riparius Fern. (Rhodora 38: 171. 1936.) (Ranunculus delphinifolius var. terrestris (Gray) Fern.) Its appearance is somewhat different from the aquatic form and I believe it has been the source of several reports for Ranunculus Purshii
Indiana Coefficient of Conservatism: C = null, non-native
Submersed lvs ternately decompound with linear segments 1-2 mm wide; emersed lvs (when present) broadly reniform, 3-parted, the segments 3-cleft, the division in turn 3-lobed; fls on long, stout peduncles; sep 4-8 mm; pet 6-14 mm; achenes plump, 1.3-2.5 mm, the sides rugose, the margin corky-thickened around the base and the ventral side below the middle; beak 0.7-1.5 mm; 2n=32. Quiet water and muddy shores, seldom wholly emersed; Me. to B.C., s. to Va., Ky., Ind., Ill., and La. Apr.-June. (R. delphinifolius)
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.