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A river is a natural flowing watercourse, usually a freshwater stream, flowing on the Earth's land surface or inside caves towards another waterbody at a lower elevation, such as an ocean, sea, bay, lake, wetland, or another river.

Topography

Sources of rivers

The Continental Divide of the Americas showing the major drainage basins in North America.

Rivers are part of the water cycle, the continuous process by which water moves about Earth. This means that all water that flows in rivers must ultimately come from precipitation. The sides of rivers have land that is at a higher elevation than the river itself, and in these areas, water flows downhill into the river. The headwaters of a river are the smaller streams that feed a river, and make up the river's source. These streams may be small and flow rapidly down the sides of mountains. All of the land that is uphill and feeds a river with water in this way is said to be in that river's drainage basin or watershed. A ridge of higher elevation land is what typically separates drainage basins; water on one side of a ridge will flow into one set of rivers, and water on the other side will flow into another. One example of this is the Continental Divide of the Americas in the Rocky Mountains. Water on the western side of the divide flows into the Pacific Ocean, whereas water on the other side flows into the Atlantic Ocean.

This melting toe of the Athabasca Glacier in Jasper National Park, Alberta, Canada will eventually feed water into the ocean.

Not all precipitation flows directly into rivers; some water seeps into underground aquifers. These in turn can still feed rivers via the water table, the groundwater beneath the surface of the land stored in soil. Water flows into rivers in places where the river's elevation is lower than that of the water table. This phenomena is why rivers can still flow even during times of drought. Rivers are also fed by the melting of snow glaciers present in higher elevation regions. In summer months, higher temperatures melt snow and ice, causing additional water to flow into rivers. Glacier melt can supplement snow melt in times like the late summer, when there may be less snow left to melt, helping to ensure that the rivers downstream of the glaciers have a continuous supply of water.

The flow of rivers

As rivers flow downstream, they eventually merge together to form larger rivers. A river that feeds into another is a tributary, and the place they meet is a confluence. Rivers must flow to lower altitudes due to gravity. The bed of a river is typically within a river alley between hills or mountains. Rivers flowing through an impermeable section of land such as rocks will erode the slopes on the sides of the river. When a river carves a plateau or a similar high elevation area, a canyon can form, with cliffs on either side of the river. A river in a permeable area does not exhibit this behavior, and may even have raised banks due to sediment. Rivers also change their landscape through their transportation of sediment, also known as alluvium when applied specifically to rivers. This debris comes from erosion performed by the rivers themselves, debris swept into rivers by rainfall, as well as erosion caused by the slow movement of glaciers. The particle size of the debris is gradually sorted by the river, with heavier particles like rocks sinking to the bottom, and finer particles like sand or silt carried further downriver. This sediment may be deposited in river valleys or carried all the way to the sea.

Rivers rarely will run in a straight direction, instead preferring to bend or meander. This is because any natural impediment to the flow of the river may cause the current to deflect in a different direction. When this happens, the alluvium carried by the river can build up against this impediment, serving to further redirect the course of the river. The flow is then directed against the opposite bank of the river, which will erode into a more concave shape to accommodate the flow. The bank will still serve to block the flow, causing it to reflect back in the other direction. Thus, a bend in the river is created.

The Nile in Egypt is known for its fertile floodplains, which flood annually.

Rivers may run through low, flat regions on their way to the sea. These areas may have floodplains, areas 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. The alluvium carried by rivers, laden with minerals, is deposited into the floodplain when the banks spill over, providing new nutrients to the soil, allowing them to support human activity like farming as well as a host of plant and animal life.

The terminus

The Lena river delta in Russia is formed from sediment carried by the river.

While rivers may flow into lakes or man-made features such as reservoirs, the water they contain will always tend to flow down towards the ocean. However, if human activity siphons too much water away from a river for other uses, the riverbed may run dry before reaching the sea. The outlets mouth of a river can take several forms. Tidal rivers (often part of an estuary) have their levels rise and fall with the tide. Since the levels of these rivers are often already at or near sea level, the flow of alluvium and the brackish water that flows in these rivers may be either upriver or downriver depending on the time of day.

Rivers that are not tidal may form deltas that continuously deposit alluvium into the sea from their mouths. Depending on the activity of waves, the strength of the river, and the strength of the tidal current, the sediment can accumulate to form new land. 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.

Ecology

Models

River Continuum Concept

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 a number of 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.

The river can then be modeled by the availability of resources for each creature's role. A shady area with deciduous trees might experience frequent deposits of organic matter in the form of leaves. In this type of ecosystem, collectors and shredders will be most active. As the river becomes deeper and wider, it may move slower and receive more sunlight. This supports invertebrates and a variety of fish, as well as scrapers feeding on algae. 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.

Flood Pulse Concept

The flood pulse concept focuses on habitats that flood seasonally, including lakes and marshes. The land that interfaces with a water body is that body's riparian zone. Plants in the riparian zone of a river help stabilize its banks to prevent erosion, filter alluvium deposited by the river on the shore, including processing the nitrogen and other nutrients it contains. Forests in a riparian zone also provide important animal habitats.

Movement of organisms

The flow of a river can act as a means of transportation for plant and animal species, as well as a barrier. For example, the Amazon River is so wide in parts that the variety of species on either side of its basin are distinct. Some fish may swim upstream in order to spawn as part of a seasonal migration. Species that travel from the sea in order to breed in freshwater rivers are called anadromous. Salmon are an anadramous fish that may die in the river after spawning, contributing nutrients back tot he river ecosystem.