Alisma gramineum Lej.
Family: Alismataceae
grass-leaved water-plantain, narrow-leaved water-plantain
[Alisma geyeri Torr.,  more...]
Alisma gramineum image
from USDA Plants website  
Etymology: Alisma: Greek name for a water plant
Plants: erect or floating, perennial, 4"-8" tall, emergent aquatic
Leaves: basal rosette, parallel veins; usually submerged, very thin and long; when emergent, on stout stalks, lance-shaped tapering to the base, 2 1/2-5 times long as wide
Flowers: white to pink, 3-parted, 1/4" wide, petals obvious longer than the sepals; inflorescence of 4 or more whorls of flowers per stalk, with stalk taller than the leaves; blooms June-Aug.
Fruits: dense ring of dry seeds, the ridge on the back of each seed grooved on either sideseed drawing
Habitat: shallow water; in muddy soil
Conservation Status: Native