Equisetum fluviatile is a rhizomatous, branched member of the horsetail family (Equisetaceae) typically found in standing water or very wet soils. Horsetails have rush-like stems with ridges of siliceous tubercles and produce spores in cones. E. fluviatile grows from light brown rhizomes up to 150 cm tall and 10 mm thick. The central cavity of the stems is more than 4/5 the stem diameter. The fertile and sterile stems are similar. The stems may be unbranched or have a few whorled branches. The sheaths on the stems have 15-20 dark brown teeth. The cones are up to 2.5 cm long and short peduncled. E. fluviatile is most similar to E. palustre but the central cavity of the stem of the latter is only one-sixth the diameter of the stem, it is more likely to be branched, and the sheaths on the stems have 10 or fewer white-margined teeth.
The cones of Equisetum fluviatile mature and shed spores in the summer.
Equisetum species reproduce sexually and asexually. Sexual reproduction is limited by ecological conditions. hygroscopic spores are produced, but they are short lived and germinate depending on humidity. Once germinated, the gametophyte produced by spores requires a recently exposed substrate to become established. Asexual reproduction is by rhizomatous growth which can be rapid in favorable moist and disturbed habitats. Reproduction also occurs by fragmentation of rhizomes and stems. Clones may spread rapidly, and since usually they are sterile, may establish characters indicating taxonomic differentiation.
Equisetum fluviatile is a host plant to Liriomyza virgo, a fly species, Dolerus cothurnatus, a sawfly species, and four macro-moth species (Bagous collignensis, B. lutulentus , Hydraecia micacea, and Hydronomus alismatis) (Biological Records Centre 2008). Water horsetail does not appear to be mycorrhizal (Peat et al. 2015). Seven fungal species are known to attack dead and dying stems of water horsetail: Ascochyta equiseti, Gorgoniceps boltonii, Gorgoniceps boltonii, Hymenoscyphus equisetinus, H. pileatus, H. rhodoleucus, Stagonospora equisiti, and Stagonospora equisiti (Peat et al. 2015).
Refer to Equisetum arvense for the known uses of horsetails and scouring rushes in general.
Equisetum fluviatile is a widespread circumpolar species. Outside of North America E. fluviatile occurs in Eurasia south to northern Italy, China, Korea and Japan. In North America this species occurs Alaska, throughout Canada, south to approximately the 44th parallel across the United States (Oregon on the west coast and Maryland on the east coast). E. fluviatile occurs in most of Alaska except for northern coastal regions sand the Aleutians. This species occurs in suitable wetland habitat on both sides of the Alaska Range in Denali.
Equisetum fluviatile is a plant of the lowlands and lower montane zones. Throughout its North American range, water horsetail occurs from 0-1500 m. It occurs at 73-991 m in Denali, and at an average of 278 m, the lowest average in altitude of all species of this genus in the park. All but one occurrence is on flat terrain (less than five degrees).
Equisetum fluviatilegrows in a wide variety of aquatic and semi-aquatic habitats, from quiet shallow water at lake and river margins, in marshes, bogs, on tidal flats, sheltered rivers and in wet ditches. It tolerates a wide range of water pH, nutrient levels, substrate types, water depths, and is often a pioneer species in freshwater successions (Botanical Society of the British Isles 2015).