A river is a natural channel of flowing water that moves across the landscape under gravity, usually from higher elevations towards lower elevations and ultimately towards another river, a lake, an inland basin, or the sea. Rivers range from tiny intermittent streams to enormous drainage systems carrying vast quantities of water. The term river generally describes a substantial natural watercourse, although terminology differs between regions and languages. Smaller channels may be called streams, brooks, creeks, burns, or rivulets. A river has a channel and usually defined banks, but its width, depth, flow velocity, sediment load, and discharge can change substantially with season, rainfall, snowmelt, drought, and human regulation.
Rivers are major components of the hydrological cycle. Water enters river systems through rainfall falling directly into channels, surface runoff, groundwater discharge, melting snow and ice, springs, wetlands, lakes, and tributary streams. The amount of water flowing past a particular point is called discharge and is commonly measured as volume per unit time. River discharge varies according to precipitation, catchment size, geology, vegetation, soil moisture, evaporation, groundwater conditions, and human activity. A drainage basin is the area from which water drains towards a particular river and its tributaries, while a watershed or drainage divide separates neighbouring drainage basins. Large rivers can therefore be understood as the downstream products of enormous interconnected catchment systems.
A river channel contains several interacting physical components. The bed forms the bottom of the channel, while banks define its sides. The wetted channel is the portion occupied by water at a particular flow level, whereas a floodplain is the relatively flat land beside a river that can be inundated during floods. River water transports dissolved substances, suspended sediment, and material moved along the bed. Flow velocity is affected by channel gradient, roughness, depth, width, discharge, and channel shape. Faster water generally has greater capacity to transport sediment, although the precise relationship depends on particle size and flow conditions. Turbulence can mix water and sediment and strongly influences erosion and deposition.
Rivers continuously modify landscapes through erosion, transportation, and deposition. Hydraulic action occurs when the force of flowing water dislodges or weakens material. Abrasion occurs when sediment carried by the river strikes and wears the channel bed and banks. Attrition causes transported particles to collide and become smaller and more rounded. Solution allows soluble minerals to be carried in dissolved form. Sediment may be deposited when flow velocity and transporting capacity decrease, such as when a river enters a lake or sea, encounters a flatter gradient, spreads across a floodplain, or loses energy around an obstruction. These processes create valleys, channels, meanders, floodplains, terraces, alluvial fans, and deltas.
River channels have characteristic patterns that vary with geology, slope, discharge, sediment supply, vegetation, and floodplain conditions. A straight channel is comparatively uncommon over long distances because small irregularities can encourage erosion on one side and deposition on another. Meandering rivers develop sweeping bends where erosion commonly occurs on outer banks and deposition on inner bends. Braided rivers divide into multiple shifting channels separated by sediment bars and are often associated with high sediment loads and variable discharge. Anastomosing rivers can contain multiple relatively stable channels separated by vegetated islands or floodplain areas. River patterns are not fixed categories, because channels can transition between forms as environmental conditions change.
Rivers have enormous ecological and human importance. River corridors provide habitats for fish, aquatic plants, insects, amphibians, birds, mammals, microorganisms, and riparian vegetation. They transport nutrients and organic matter between terrestrial and aquatic environments and connect upland landscapes with coastal ecosystems. Human societies have historically developed near rivers because rivers provide freshwater, fertile alluvial soils, transport routes, energy resources, food, and opportunities for settlement and agriculture. Rivers have also been used for hydroelectric generation, navigation, recreation, and industrial processes. However, settlements in floodplains face flood hazards, while pollution, abstraction, dams, channel modification, and catchment land use can substantially alter natural river processes and ecological communities.
Rivers can be permanent, seasonal, intermittent, or ephemeral depending on their water sources and climate. A perennial river normally carries water throughout the year, whereas intermittent and ephemeral rivers cease flowing for part of the year. Some rivers are strongly influenced by groundwater and therefore remain relatively stable during dry periods. Others depend mainly on rainfall or snowmelt and can experience dramatic fluctuations. Glacial rivers often have distinctive seasonal discharge patterns because melting increases during warmer periods. Human structures such as reservoirs, dams, weirs, canals, levees, and flood barriers can modify natural flow regimes. Water abstraction can reduce downstream discharge, while urbanisation can increase rapid runoff and produce sharper flood peaks.
Rivers are dynamic systems rather than static channels. Their form and behaviour emerge from interactions among water, sediment, vegetation, geology, climate, organisms, and human activity. A river can change course, migrate laterally, cut into its bed, deposit sediment, abandon channels, or create new floodplain features. Floods are natural parts of many river systems and can redistribute sediment, nutrients, seeds, and organic matter. River management therefore involves balancing water supply, flood risk, navigation, agriculture, ecological protection, and infrastructure requirements. Understanding a river requires considering the entire drainage basin because processes occurring far upstream can influence downstream water quality, sediment supply, discharge, and habitat.
