Alisma L.
Family: Alismataceae
Alisma image
from USDA Plants website  
1a. Achenes with 2 abaxial grooves; leaves of mature plant submersed, ribbon-like, if emersed, lanceolate to narrowly elliptical, less than 2 cm wide ... A. gramineum
1b. Achenes with 1 abaxial groove; leaves of mature plant emergent or floating, elliptical to ovate, up to 15 cm wide ... 2

2a. Petals 3.5--6 mm long, petals distinctly longer than the sepals; mature fruiting heads 4--7 mm wide; achenes 2--3 mm long ... A. triviale
2b. Petals 1--3 mm long, petals and sepals more or less equal in length; mature fruiting heads 2--5 mm wide; achenes 1.5--2.5 mm long ... A. subcordatum
Species within Adams County 2016