Leafl. Bot. Observ. Crit. 1: 206. 1906.
Stems 0.5–2 m; herbage sparsely stellate-hairy. Leaf-blades deeply (3-), 5-lobed, or 7-lobed, 5–20 cm wide, lobes triangular-ovate, broadest at base, base truncate to cordate, margins coarsely dentate. Inflorescences clusters forming interrupted spikes to corymbose racemes distally; involucellar bractlets linear-subulate, 3–6 (–8) × 1 mm, 1/3–2/3 as long as calyx. Flowers: calyx 5–8 (–11) mm, lobes triangular-ovate to obtuse, 2.5–4 (–5.5) mm, ± as wide as long, slightly exceeding tube; petals pinkish white to rose-purple, 1.8–2.5 cm. Schizocarps 10–12 mm diam.; mericarps 8–12, 6–10 mm. Seeds (2 or) 3 or 4, dark-brown, 2 mm, puberulent. 2n = 66.
Phenology: Flowering Jun–Aug.
Habitat: Stream banks, meadows, open woodlands, disturbed places, foothills to mountain slopes
Elevation: 1400–2900 m
Alta., B.C., Colo., Idaho, Mont., Nev., N.Mex., Oreg., Utah, Wash., Wyo.
Iliamna rivularis is variable in stature, pubescence, and leaf characters. Plants of Idaho and Wyoming with distally smaller (4–10 cm), three- to five-lobed blades, truncate bases, and inconspicuously toothed margins have been recognized as var. or subsp. diversa, but fall within the morphological range of the species as a whole.
Iliamna rivularis is in the Center for Plant Conservation’s National Collection of Endangered Plants as I. rivularis var. rivularis.
"broadest" is not a number.