Plant: perennial from rhizomes, monoecious; overwintering bulblets sometimes present; stem > 1 m, whitish when dry Leaves: submersed leaves 1-3 cm, whorled, 3-6 per node, with thread-like pinnate divisions; midrib and divisions linear, divisions < 15 mm, < 26 per leaf; emergent leaves simple, opposite, leaf- or bract-like, entire to pinnately divided Inflorescence: spike, 3-8 cm, emergent; leaves 1-3 mm, < flowers, bract-like, oblanceolate to ovate, entire to coarsely toothed Flowers: unisexual; lower pistillate; middle sometimes bisexual; upper staminate; calyx lobes 4; petals generally 4, ephemeral on staminate flowers, minute or 0 on pistillate flowers; stamens generally 8; ovary 4-chambered, stigma plumose Fruit: Fruit: segments 4, nut-like; Seed 1 per chamber Misc: Ponds, ditches, streams, lakes; < 2600 m.; Jun-Sep