Distribution: Oregon south to Baja California, east to the Atlantic Coast.
Habitat: Fresh water.
Flowers: June - August
Conservation Status: Not of concern
Monoecious, submersed annual, the slender, branching stems 3-6 dm. long and scarcely 1 mm. thick.
Leaves opposite, 1-3 cm. long and 0.5 mm. broad, the base expanded for 3-5 mm. and somewhat sheathing, the blade tapered from well above midlength to a tip.
Flowers tiny, in the axils of leaves clustered toward the branch tips; staminate flowers few, a single stamen enclosed in a transparent bract; anther 4-celled; pistillate flowers naked, the pistil 1, the stigmas 2-4, sessile, elongate.
Achene spindle-shaped, 3 mm. long, completely filled by the single seed, which is straw-colored and prominently pitted.