River is a large, natural stream of water that flows overland within a channel. Most rivers begin in mountains or hills. A river ends where it flows into another river, desert basin, ocean, or lake. A river is the main part of a river system, which also includes all the smaller streams that supply water to the river. A region of land that is drained by a river system is known as a drainage basin.
The world’s longest river is the Nile River in Africa, which flows for 4,160 miles (6,695 kilometers). The next longest is the Amazon River in South America, which is about 4,000 miles (6,437 kilometers) long. The Amazon carries more water than any other river—and more water than the Nile, the Mississippi River in the United States, and the Yangtze River in China combined.
Uses of rivers
For centuries, people have used rivers for transportation and trade. In North and South America, for example, early explorers, traders, and pioneers traveled on rivers. Later, they built towns along major rivers. Several of these towns grew into large cities. On the Mississippi River, for instance, Minneapolis, St. Louis, Memphis, and New Orleans became large cities.
Rivers are also valuable to agriculture. Farmers grow crops in the fertile land of river plains and the terraced surfaces above those plains. In dry regions, farmers use river water to irrigate their land. They dig irrigation ditches to carry water from rivers to farmland.
In addition, rivers serve as a source of power. The energy of flowing water at waterfalls and other steep places along a river can drive machines and generate electric power.
Sources of river water
Almost all river water comes from rain or melted snow. Most of the water reaches rivers indirectly. In some cases, water called surface runoff flows over the land to the river. In other cases, the water soaks into soil and rocks and becomes ground water (water beneath the surface of the earth). The ground water then moves slowly through the soil or underground rocks to the rivers. This subsurface supply of water can keep a river flowing between periods of rain. Other sources of river water include glaciers, springs, and overflowing lakes.
Where rainfall is seasonal, a river may be dry for part of the year. This kind of river is known as an intermittent river. A river that flows across a desert may also be intermittent. Such a river does not receive ground water. Rather, it gives up water to the ground beneath it. This loss, together with evaporation, may cause the river to dry up for part of the year.
Exotic rivers are large rivers that begin in a rainy area, then flow across a desert without drying up. The Nile River and the Colorado River, which is in the United States, are exotic rivers in parts of their channels.
River systems
A river system consists of the river itself and all the smaller streams that supply water to the river. A river is highest at its headwaters, where it begins. It is lowest at its mouth, where it ends. 
A river erodes (wears away) a great deal of material from the land over which it flows. This material is called the river’s load while it is flowing and sediment when it is deposited. In many rivers, most of the material is dissolved in the water and cannot be seen. This invisible load mixes with lake or ocean water at the mouth. A river also carries a visible load that consists of material ranging from tiny particles of clay to large boulders. The finer particles may determine the color of the water, which may range from red to brown or yellow.
Some rivers flow gradually from headwaters to mouth. Other rivers have irregular features, such as waterfalls, rapids, and canyons. Many rivers that empty into oceans have deep, broad mouths called estuaries.
Tributaries.
Runoff collects in tiny, temporary channels called rills. Rills often flow into streams that eventually join to form rivers. Smaller streams that flow into larger ones are called tributaries. 
Scientists often classify streams by stream order. For example, a stream with no permanent tributaries is a first order stream. Two first order streams meet to form a second order stream. Two second order streams join to form a third order stream, and so on. Low order streams are commonly called brooks or creeks, while higher order streams are commonly called rivers. One river may be a tributary of another river. For example, the Missouri River is a tributary of the Mississippi.
Channels
are passages in which river water flows. A river’s channel extends from the headwaters to the mouth. The channel bottom is the river’s bed, and the edges of the channel are the banks.
The slope of a channel tends to be steep near the source of a river and almost flat near the mouth. Channel width and depth typically increase downstream due to the increasing flow of water.
Waterfalls and rapids
occur in the upper courses (segments) of many rivers. A waterfall occurs where a river crosses a layer of hard rock that resists erosion. The water erodes softer rock downstream, creating a steep drop in the channel. Rapids occur where water tumbles over large boulders or rock ledges.
Canyons.
In the upper course of a river, the channel may occupy the full width of the valley floor. The river may undercut the sides of the valley from place to place, and it also deepens the valley. Valley sides are typically V-shaped above the valley floor. Rapid cutting can produce a canyon, a deep valley with cliffs above the floor.
Estuaries
contain a mixture of salt water from the ocean and fresh water from the river. The Amazon River is an estuary for several hundred miles or kilometers upstream from its mouth.
Estuaries formed as a result of ancient changes in sea level. During the Pleistocene Epoch, which lasted from about 2.6 million to 11,500 years ago, sea level moved down and up several times. This movement occurred because vast ice sheets accumulated on the land and then melted away several times. At the end of the epoch, the sea level was low, and rivers near the ocean had cut their valleys down to the low level. Since then, many of the ice sheets have melted. As a result, sea level has risen. Ocean water has flowed into the valleys that had been cut, forming the estuaries.
Drainage basins
A drainage basin consists of the region drained by a river system. The Amazon River has a drainage basin of about 2,700,000 square miles (7,000,000 square kilometers). The Mississippi River drains about 1,244,000 square miles (3,220,000 square kilometers) of land in North America. This land includes about 40 per cent of the United States, except Alaska and Hawaii. A basin’s waters make up a drainage net.
Divides.
The rim of a drainage basin is called a drainage divide. One major drainage divide follows the crest of the Rocky Mountains. This divide splits the continent of North America into two large drainage regions. It is therefore known as the Continental Divide. Basins to the west of the divide carry water to the Pacific Ocean. To the east is the Mississippi River basin, which carries water to the Gulf of Mexico. Other eastern basins carry water to the Atlantic Ocean and the Arctic Ocean.
Flood plains
are flat areas on one or both sides of the middle or lower course of a river. When the river overflows its banks, it floods these areas. A flood plain can be hundreds of miles or kilometers wide.
Floodwaters deposit sediment on the flood plain, particularly on the parts of the plain next to the channel. The deposits create banks called natural levees. Marshy areas called back swamps occupy the parts of flood plains beyond the natural levees.
A channel in a wide flood plain tends to curve from one side of the plain to the other. The snakelike bends in the channel are known as meanders. Meanders can form almost complete loops, with only a narrow neck of land separating the beginning and end of each loop. Eventually, floodwater flows across this neck, cutting off the loop and creating a new, straight channel for the river. The separated loop then fills with rain water or ground water. The loop may continue to exist for many years as a crescent-shaped oxbow lake.
Alluvial fans and deltas
are land masses that build up where a flow of river water decreases quickly. The decrease in the flow causes the river to deposit its visible load rapidly, forming the land mass. An alluvial fan is a fan-shaped land mass that forms where a river flows from a steep mountain slope to a flatter plain next to the slope. The decrease in elevation slows the river down, causing some of the river’s load to be deposited. River flow also slows down where a river reaches a lake or ocean, so most of the remaining load is deposited there.
A delta is a low plain that may form at a river’s mouth. The Mississippi River and the Nile River have large deltas.
