General:
Amphibious perennial herb, usually with stolons and rooting from nodes, roots slender, fibrous; stems decumbent to prostrate, creeping, to 50 cm long, simple to few branched, smooth or sparsely stiff short-hairy, somewhat hollow.

Leaves:
Tufted at nodes, simple, lanceolate or oblanceolate to thread-like, 0.7-6.5 cm long, 0.4-10 mm wide, usually entire, sometimes shallowly-toothed, bases and tips pointed to thread-like; long- to short-stalked.

Flowers:
Inflorescence of solitary, terminal flowers or few-flowered cymes near branch tips, on stalks to 10 cm long; bracts lanceolate or oblanceolate, receptacle smooth; petals 5 or 6, distinct, yellow, 2.5-7 mm long, 1-4 mm wide, egg-shaped, nectary on upper surface, the nectary scale smooth, shallowly cup-shaped, wider than long; sepals 5, spreading or weakly bent back, 1.5-4 mm long, 1-2 mm wide, smooth or appressed stiff-hairy; stamens 10-50; pistils 5-50.

Fruits:
Achenes, several to many in a globe-shaped head 2-4 mm long, 3-4 mm wide, the achenes egg-shaped, 1.2-1.6 mm long, 1-1.4 mm wide, smooth, smooth to net-veined; beaks persistent, lanceolate to linear, straight or curved, 0.1-0.6 mm long.

Notes:
Separation of this species into infraspecific taxa, based mainly on leaf width and flower size, is arbitrary at best. See Calder and Taylor (1968) and Whittemore (1997) for alternative treatments.

Source:  The Illustrated Flora of British Columbia