Process of change at work within a river. The material of the river-bed is being continually worn away, and transported and deposited elsewhere; how much and how far depends on the river's speed and turbulence. The movement of eroded material is indicated by arrows. (1) Straight river with symmetrical bed; the eroded material is deposited on the banks when the water level is high. (2) Winding river with an asymmetrical bed; material is worn off the concave bank, usually the outer side of a curve, and deposited on the convex.
Young rivers (A) close to their source tend to be fast-flowing, high-energy environments with rapid downward and headward erosion, despite the hardness of the rock over which they may flow. Steep-sided "V-shaped' valleys, waterfalls, and rapids are characteristic features. Mature rivers (B) are lower-energy systems. Erosion takes place on the outside of bends, creating looping meanders (1) in the soft alluvium of the river plain. Deposition occurs on the inside of bends and on the river bed. At a river's mouth (C), sediment is deposited as the velocity of the river slows. As the river becomes shallower more deposition occurs, forming islands and braiding the main channel into multiple, narrower channels. As the sediment is laid down (2), the actual mouth of the river moves away from the source into the sea or lake, forming a delta.
A river is a body of inland water that flows downhill under gravity in a natural channel into the sea, a lake, or, as a tributary, into another river. The main sources of rivers are springs, lakes, and glaciers. Near the source a river flows swiftly, the rocks and other abrasive particles eroding a steep-sided V-shaped valley (see erosion). Variations in the hardness of the rocks over which it runs may result in waterfalls. In the middle part of its course the gradients become less steep, and lateral (sideways) erosion becomes more important than downcutting. The valley is broader, the flow less swift, and meandering more common. Toward the river mouth, the flow becomes more sluggish and meandering prominent: the river may form oxbow lakes. Sediment may be deposited to form a delta.
Discharge and speed of a river
The discharge of a river is the volume of water flowing past a point in a given time. It is usually expressed as cubic meters per second (cumecs), and is calculated by mu tip lying the cross-sectional area of the river by the speed of the water. The speed of a river is controlled by the slope of the river, the depth of the river and the roughness of the river bed.
Rivers transport sediment as they flow, by the process of traction (rolling), saltation (jumping), suspension (carrying). and solution. A greater discharge increases the amount of sediment that can be transported during flood conditions, but as a river returns to normal flow it deposits sediment. A river adjusts its channel shape to be able to transport sediment most efficiently. This can result in the erosion of a river channel or the building up of floodplains, and sand and gravel banks.
Rivers flood when their channels cannot contain the discharge. Flood risk can be reduced by straightening the channel, dredging sediment or making the channel deeper by raising the banks.
Water gaps and wind gaps
A water gap is a short, narrow gorge cut through a ridge or region of high ground by a stream or river. If the river no longer passes through it, the gorge is termed a wind gap.