River

Definition A river is a natural flow of freshwater that flows on or through land towards another body of water downhill. This flow can be into a lake, an ocean, or another river. A stream may also be referred to as a watercourse. This means that all water that flows in rivers must ultimately come from precipitation. The headwaters of a river are the smaller streams that feed a river, and make up the river's source. All of the land uphill of a river that feeds it with water in this way is in that river's drainage basin or watershed. A common misconception holds that all or most rivers flow from North to South, but this is not true. When a river carves a plateau or a similar high-elevation area, a canyon can form, with cliffs on either side of the river. was carved by the Colorado River. A river in a permeable area does not exhibit this behavior and may even have raised banks due to sediment. The monitoring of the sediment yield of a river is important for ecologists to understand the health of its ecosystems, the rate of erosion of the river's environment, and the effects of human activity. These places may have floodplains that are periodically flooded when there is a high level of water running through the river. These events may be referred to as "wet seasons' and "dry seasons" when the flooding is predictable due to the climate. These islands exist in almost every river. This may be because an arid climate is too dry depending on the season to support a stream, or because a river is seasonally frozen in the winter (such as in an area with substantial permafrost), or in the headwaters of rivers in mountains, where snowmelt is required to fuel the river. This can happen in karst systems, where rock dissolves to form caves. These rivers provide a habitat for diverse microorganisms and have become an important target of study by microbiologists. These rivers do not typically host any life, and are often used only for stormwater or flood control. siphon too much water away from a river for other uses, the riverbed may run dry before reaching the sea. When viewed from above, a delta can appear to take the form of several triangular shapes as the river mouth appears to fan out from the original coastline. ==Classification==

In hydrology, a stream order is a positive integer used to describe the level of river branching in a drainage basin. Several systems of stream order exist, one of which is the Strahler number. In this system, the first tributaries of a river are 1st order rivers. When two 1st order rivers merge, the resulting river is 2nd order. If a river of a higher order and a lower order merge, the order is not incremented. Order increments only when two rivers of the same order merge. Stream order is correlated with and thus can be used to predict certain data points related to rivers, such as the size of the drainage basin (drainage area), and the length of the channel. == Ecology ==

Models River Continuum Concept in England provide organic matter for organisms to process. The ecosystem of a river includes the life that lives in its water, on its banks, and in the surrounding land. The width of the channel of a river, its velocity, and how shaded it is by nearby trees. Creatures in a river ecosystem may be divided into many roles based on the River Continuum Concept. "Shredders" are organisms that consume this organic material. The role of a "grazer" or "scraper" organism is to feed on the algae that collects on rocks and plants. "Collectors" consume the detritus of dead organisms. Lastly, predators feed on living things to survive. Further downstream, the river may get most of its energy from organic matter that was already processed upstream by collectors and shredders. Predators may be more active here, including fish that feed on plants, plankton, and other fish. Smaller rivers can only sustain smaller fish that can comfortably fit in its waters, whereas larger rivers can contain both small fish and large fish. This means that larger rivers can host a larger variety of species. ==Human uses==

Infrastructure in the Hubei province of China from flooding. Modern river engineering involves a large-scale collection of independent river engineering structures that have the goal of flood control, improved navigation, recreation, and ecosystem management. Many of these projects have the effect of normalizing the effects of rivers; the greatest floods are smaller and more predictable, and larger sections are open for navigation by boats and other watercraft. Dams are very common worldwide, with at least 75,000 higher than in the U.S. Globally, reservoirs created by dams cover . History Pre-industrial era The first civilizations of Earth were born on floodplains between 5,500 and 3,500 years ago. The desert climates of the surrounding areas made these societies especially reliant on rivers for survival, leading to people clustering in these areas to form the first cities. It is also thought that these civilizations were the first to organize the irrigation of desert environments for growing food. River transportation is also important for the lumber industry, as logs can be shipped via river. Countries with dense forests and networks of rivers like Sweden have historically benefited the most from this method of trade. The rise of highways and the automobile has made this practice less common. Another example is the restoration of the Isar in Munich from being a fully canalized channel with hard embankments to being wider with naturally sloped banks and vegetation. This has improved wildlife habitat in the Isar, and provided more opportunities for recreation in the river. In Genesis, the flood's role was to cleanse Earth of the wrongdoing of humanity. The act of water working to cleanse humans in a ritualistic sense has been compared to the Christian ritual of baptism, famously the Baptism of Jesus in the Jordan River. Along with mythological rivers, religions have also cared for specific rivers as sacred rivers. The Ancient Celtic religion saw rivers as goddesses. The Nile had many gods attached to it. The tears of the goddess Isis were said to be the cause of the river's yearly flooding, itself personified by the goddess Hapi. Many African religions regard certain rivers as the originator of life. In Yoruba religion, Yemọja rules over the Ogun River in modern-day Nigeria and is responsible for creating all children and fish. Some sacred rivers have religious prohibitions attached to them, such as not being allowed to drink from them or ride in a boat along certain stretches. In these religions, such as that of the Altai in Russia, the river is considered a living being that must be afforded respect. Rivers are some of the most sacred places in Hinduism. There is archeological evidence that mass ritual bathing in rivers at least 5,000 years ago in the Indus river valley. While most rivers in India are revered, the Ganges is most sacred. The river has a central role in various Hindu myths, and its water is said to have properties of healing as well as absolution from sins. Hindus believe that when the cremated remains of a person is released into the Ganges, their soul is released from the mortal world. == Threats ==

now runs dry in the deserts of Mexico, rather than running to the sea, due to diversion of water for agricultural uses. Freshwater fish make up 40% of the world's fish species, but 20% of these species are known to have gone extinct in recent years. Human uses of rivers make these species especially vulnerable. Rivers that flow freely from headwaters to the sea have better water quality, and also retain their ability to transport nutrient-rich alluvium and other organic material downstream, keeping the ecosystem healthy. It has been found in the bodies of humans and animals worldwide, as well as in the soil, with potentially negative health effects. Research into how to remove it from the environment, and how harmful exposure is, is ongoing. Fertilizer from farms can lead to a proliferation of algae on the surface of rivers and oceans, which prevents oxygen and light from dissolving into water, making it impossible for underwater life to survive in these so-called dead zones. Urban rivers are typically surrounded by impermeable surfaces like stone, asphalt, and concrete. Cities often have storm drains that direct this water to rivers. This can cause flooding risk as large amounts of water are directed into the rivers. Due to these impermeable surfaces, these rivers often have very little alluvium carried in them, causing more erosion once the river exits the impermeable area. It has historically been common for sewage to be directed directly to rivers via sewer systems without being treated, along with pollution from industry. This has resulted in a loss of animal and plant life in urban rivers, as well as the spread of waterborne diseases such as cholera. In modern times, sewage treatment and controls on pollution from factories have improved the water quality of urban rivers. is expected to reduce the level of waters in the Western United States. Climate change can change the flooding cycles and water supply available to rivers. Floods can be larger and more destructive than expected, causing damage to the surrounding areas. Floods can also wash unhealthy chemicals and sediment into rivers. Droughts can be deeper and longer, causing rivers to run dangerously low. This is in part because of a projected loss of snowpack in mountains, meaning that melting snow cannot replenish rivers during warm summer months, leading to lower water levels. Lower-level rivers also have warmer temperatures, threatening species like salmon that prefer colder upstream temperatures. Attempts have been made to regulate the exploitation of rivers to preserve their ecological functions. Many wetland areas have become protected from development. Water restrictions can prevent the complete draining of rivers. Limits on the construction of dams, as well as dam removal, can restore the natural habitats of river species. Regulators can also ensure regular releases of water from dams to keep animal habitats supplied with water. Limits on pollutants like pesticides can help improve water quality. == Extraterrestrial rivers ==

Today, the surface of Mars does not have liquid water. All water on Mars is part of permafrost ice caps, or trace amounts of water vapor in the atmosphere. However, there is evidence that rivers flowed on Mars for at least 100,000 years. The Hellas Planitia is a crater left behind by an impact from an asteroid. It has sedimentary rock that was formed 3.7 billion years ago, and lava fields that are 3.3 billion years old. These images reveal channels formed in the rock, recognized by geologists who study rivers on Earth as being formed by rivers, Titan's rivers flow with liquid methane and ethane. There are river valleys that exhibit wave erosion, seas, and oceans. Scientists hope to study these systems to see how coasts erode without the influence of human activity, something that is not possible when studying terrestrial rivers. == See also ==