Intermittent river

According to the U.S. Environmental Protection Agency definition, an intermittent river, or intermittent stream, is any river or stream that only flows during certain times of the year, and may not have any flowing surface water during the dry season. Distinction: intermittent vs ephemeral stream Intermittent rivers do not rely on, but may be supplemented by, stormwaters or other runoff from upstream sources. as compared to ephemeral streams, which may or may not have a defined channel, and rely mainly on storm runoff, as their aquatic bed is above the water table. An ephemeral stream does not have the biological, hydrological, and physical characteristics of a continuous or intermittent stream. Recent litigation was brought by eighteen states' attorneys general because of a change to the interpretation of what is to be considered by the EPA and Army Corps of Engineers as "waters of the United States" during May 2020. == Causes of intermittence ==

Intermittent streams contain water during periods when groundwater levels are above or at the level of stream's channel, allowing for surface flow. The mechanisms which control surface flow of intermittent streams are climatically and geographically specific. For example, intermittent streams fed by snowmelt and glacial meltwater cease to flow when they either freeze or there is not enough inputs to sustain surface water. In arid and semiarid regions of North America, most formerly perennial rivers are now intermittent. This is a direct consequence of the extensive networks of dams and aqueducts that were built for human withdrawal of water that used to flow into wetlands, deltas, and inland sinks. This phenomenon can be observed in the Colorado River, whose flow has decreased significantly since 1905. In recent years, several U.S. states and Mexico have used significant amounts of water for agricultural and urban uses, which caused flows reaching the Colorado River delta to drop to near zero. == Distribution ==

Intermittent rivers are found on every continent, and may even be more common than perennial rivers. More than 30% of the total length and discharge of the global river network is estimated to be intermittent rivers. However, due to some low-order streams being difficult to categorize or track, this total could be over 50% when taking those into account. In the face of global climate change, this total is further increasing, as many of the world's rivers that were once perennial are now intermittent in regions suffering from severe climatic drying or water appropriation. == Types ==

Arroyos Intermittent streams can be found in many different climate regions. For example, arroyos are intermittent streams that erode deep vertical channels through fine sediment in arid and semiarid regions in the American Southwest during precipitation events. Many incised arroyos that are destructive to stream beds and adjacent man-made structures were formed as a result of drainage channelization and overgrazing during the late nineteenth century along with the influx of American settlers in the Southwestern United States. Glacial streams Glacial streams are considered intermittent streams as the flow intermittence fluctuates with solar energy input. Most glacial streams are alpine headwater streams that receive water from the glacial meltwater. The streams become dry or freeze starting from autumn and last until early spring; the flow of the glacial streams is highest during summer. The intermittency of the glacial streams also fluctuates at different times of the day. Bourne Winterbourne == Ecology ==

The inhabitants of intermittent rivers can change with the water level. As a result of contrasting conditions throughout the year, invertebrate assemblages of the same intermittent stream can be notably distinct from one another. How biodiversity of these habitats changes with conditions has been debated in literature. Current findings suggest that while lotic biodiversity generally decreases with increasing flow intermittence, increased lentic and terrestrial biodiversity during those periods can compensate. Rewetting causes changes both in the dissolved nutrients in the stream, and in species compositions. Additionally, when drying, these riverbeds often leave behind organisms, such as fish, which were unable to relocate in response to lowering water levels. These organisms are often used as a food source for a variety of terrestrial animals, such as birds, mammals, and reptiles. Types of fish Different types of fishes inhabit intermittent rivers. The Brassy minnow (Hybognathus hankinsoni) is native to the intermittent Niobrara River, Wyoming. Redband trout (Oncorhynchus mykiss gairdneri) is native to intermittent desert streams of southwestern Idaho. The West Fork Smith River provides vital habitat to different species, including coho salmon, returning to spawn in Oregon. Cobitis shikokuensis (Hina-ishi-dojo) in intermittent rivers move into the hyporheic zone when water flows are low. When the water returns, C. shikokuensis emerge out of the hyporheic zone to recolonize the flowing river system. During stream drying, Campostoma spadiceum (Highland stoneroller) move into pool habitats when riffle areas become too shallow for survival. Food web The food web of intermittent streams differs from perennial streams in that species number and abundance change drastically among the flowing, contraction/fragmentation, and dry phases. Intermittent streams tend to have a food web based heavily on detritus and follow the bottom-up trophic model. Both the ratios of predator to prey and the number of trophic levels depend on the size of the intermittent stream. == Conservation ==

Intermittent rivers face many threats. Diversion of river water for large-scale consumption, such as industrial use or for farming, can alter the ecology of intermittent rivers. Disturbances caused by humans can result in short-term (pulse) and long-term (press) effects on intermittent stream habitats. == See also ==