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A perennial stream or perennial river is a stream or river (channel) that has continuous flow in parts of its stream bed all year round during years of normal rainfall.^[1] "Perennial" streams are contrasted with "intermittent" streams which normally cease flowing for weeks or months each year, and with "ephemeral" channels that flow only for hours or days following rainfall. During unusually dry years, a normally perennial stream may cease flowing, becoming intermittent for days, weeks, or months depending on severity of the drought. The boundaries between perennial, intermittent, and ephemeral channels are not defined, and subject to a variety of identification methods adopted by local governments, academics, and others with a need to classify stream-flow permanence.

As stream flow decreases in dry weather, visible flow above the stream bed may not be readily evident, especially in streams with coarse substrate (gravel and rocks), where water is flowing beneath and between these particles (hyporheic flow). From a biological perspective, a stream may be considered flowing if sufficient water is available to support flow-dependent aquatic life, including fish and gill-breathing amphibians, benthic insects, crustaceans, and mollusks, many of which survive in shallow hyporheic flow beneath rocks or logs. This extreme low flow may not be detectable on typical USGS stream-flow gauges, but is vital to stream ecology.^[2]

Etymology

The word “perennial” comes from the 1640s, "evergreen," formed in English from Latin perennis "lasting through the year (or years)," from per "through" (from PIE root *per- (1) "forward," hence "through") + annus "year" (see annual(adj.)). Botanical sense of "Remaining alive through a number of years" is attested from 1670s; figurative meaning of "enduring, permanent" is from 1750. Related: Perennially. For vowel change, see biennial. The noun meaning "a perennial plant" is from 1763.^[3]

Indicators

Biological Indicators

Benthic Macroinvertebrates

Macroinvertebrate refer to easily seen invertebrates, larger than 0.5 mm and can be collected from stream and river bottoms.^[4] The larval stages of most aquatic insects are good indicators of perennial streams. Larva such as caddisflies, mayflies, stoneflies, and damselflies^[5] require a continuous aquatic habitat until they reach maturity. Crayfish and other crustaceans plus snails and aquatic worms also indicate a good chance that the stream being assessed is perennial. Another great indicator of a perennial stream would be the presence of a clams. Bivalves (clams) require a persistent aquatic environment for survival.^[6]

Vertebrates

Fish and amphibians are secondary indicators when it comes to assessing for a perennial streams because some fish and amphibians can inhabit areas without persistent water regime. When assessing for fish, all available habitat should be assessed; from pools, riffles, root clumps and other obstructions. Fish will seek cover if alerted to human presence but should be easily observed in perennial streams. Amphibians also indicate a perennial stream and range from tadpoles, frogs, salamanders, and newts. These amphibians can be found in stream channels, along streambanks, and even under rocks. Frogs and tadpoles usually inhabit shallow and slow moving waters near the sides of stream banks. Frogs will typically jump into water when alerted to human presence.^[6]

Geological Indicators

Well defined river beds composed of riffles, pools, runs, gravel bars, a bed armor layer, and other depositional features, plus well defined banks due to bank erosion are good identifiers when assessing for perennial streams.^[7] Particle size will help identify a perennial stream. Perennial streams cut through the soil profile which removes fine and small particles. By assessing areas for relatively coarse material left behind in the stream bed and finer sediments along the side of the stream or within the floodplain will be a good indicator of persistent water regime.^[5]

Hydrological Indicators

A perennial stream can be identified 48 hours after a storm, if a stream is still flowing and there is no visible determination of flow above the channel then it is a good indicator that base flow is occurring in the stream. Another perennial stream identifier is when there is an abundance of red rust material, in a slow moving wetted channel or stagnant area, which are iron oxidizing bacteria that form when oxygen depleted ground water is exposed to the surface. Looking for leaf litter is an additional indicator when trying to access if a stream is perennial. Perennial streams should have no accumulation of leaf litter in the bed due to continuous transport of plant material through the channel. In addition nearby plants, in an active flood plain of a perennial streams, will have fine sediment deposited on plants. Also, an organic drift line composed of organic debris in piles or lines will be present on the active floodplain indicating recent high flows.^[5]

See also

• 
  Winterbourne, a stream that flows only in winter

References

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