Distribution: Throughout much of temperate North America
Habitat: Standing to slow-moving water, from the lowlands to mid-elevations in the mountains
Flowers: June - September
Conservation Status: Not of concern
Aquatic, monoecious, rootless, perennial herbs, the slender stems freely branching, up to 4 m. in length.
Leaves in whorls of 5-12, 1-2 times palmately and dichotomously dissected into linear or filiform, finely-toothed segments, often strongly dimorphic in different parts of the plant.
Flowers minute, apetalous, solitary in the leaf axils, both the staminate and pistillate flowers with a perianth of 8-15 greenish segments; stamens 10-16, the filaments very short; style 1, persistent; stigma short, entire.
Flattened achene, 4-6 mm. long, with a single, terminal, straight spine up to 12 mm. long, and 2 divergent, basal spines 1-6 mm. long.