Mare's Tail
Hippuris vulgaris

Family: Plantain (Plantaginaceae)
AKA:
Photo taken on: July 7, 2010
Location: Pioneer Trail, GV, CO
Life Zones: Plains to subalpine
Habitat: Ponds, stream sides

Grows to 12" with whorled leaves around un-branched stems. The leaves underwater are long and limp, above water they are short and stiff. It looks like Horsetail but it has tiny greenish flowers and is not jointed like the Horsetail.

Flora of North America reference.