river

Large body of water that flows down a slope along a channel restricted by adjacent banks and levees. A river starts at a point called its source, and enters a sea or lake at its mouth. Along its length it may be joined by smaller rivers called tributaries; a river and its tributaries are contained within a drainage basin. The point at which two rivers join is called the confluence.

Rivers are formed and moulded over time chiefly by the processes of erosion, and by the transport and deposition of sediment. Rivers are able to work on the landscape through erosion, transport, and deposition. The amount of potential energy available to a river is proportional to its initial height above sea level. A river follows the path of least resistance downhill, and deepens, widens and lengthens its channel by erosion. Up to 95% of a river's potential energy is used to overcome friction.

One way of classifying rivers is by their stage of development. An upper course is typified by a narrow V-shaped valley with numerous waterfalls, lakes, and rapids. Because of the steep gradient of the topography and the river's height above sea level, the rate of erosion is greater than the rate of deposition, and downcutting occurs by vertical corrasion (erosion or abrasion of the bed or bank caused by the load carried by the river).

In the middle course of a river, the topography has been eroded over time and the river's course has a shallow gradient. Such a river is said to be graded. Erosion and deposition are delicately balanced as the river meanders (gently curves back and forth) across the extensive flood plain. Horizontal corrasion is the main process of erosion. The flood plain is an area of periodic flooding along the course of a river valley where fine silty material called alluvium is deposited by the flood water. Features of a mature river (or the lower course of a river) include extensive meanders, oxbow lakes, and braiding.

Many important flood plains, such as the Nile flood plain in Egypt, occur in arid areas where their exceptional fertility is very important to the local economy. However, using flood plains as the site of towns and villages involves a certain risk, and it is safer to use flood plains for other uses, such as agriculture and parks. Water engineers can predict when flooding is likely and take action to prevent it by studying hydrographs, which show how the discharge of a river varies with time.

Major rivers of the world include the Ganges, the Mississippi, and the Nile, the world's longest river.