Where is deposition in a river

Base level is where a stream meets a large body of standing water, usually the ocean, but sometimes a lake or pond.

Streams work to down cut in their stream beds until they reach base level. The higher the elevation, the farther the stream is from where it will reach base level and the more cutting it has to do. As a stream gets closer to base level, its gradient lowers and it deposits more material than it erodes. On flatter ground, streams deposit material on the inside of meanders. When a stream flows onto its floodplain, its velocity slows and it deposits much of its load.

These sediments are rich in nutrients and make excellent farmland. A stream at flood stage carries lots of sediments. When its gradient decreases, the stream overflows its banks and broadens its channel.

The decrease in gradient causes the stream to deposit its sediments, the largest first. These large sediments build a higher area around the edges of the stream channel, creating natural levees. When a river enters standing water, its velocity slows to a stop. The stream moves back and forth across the region and drops its sediments in a wide triangular-shaped deposit called a delta.

If a stream falls down a steep slope onto a broad flat valley, an alluvial fan develops. Alluvial fans generally form in arid regions. The energy in a river causes erosion.

The bed and banks can be eroded making it wider, deeper and longer. Headward erosion makes a river longer. This erosion happens near its source. Surface run-off and throughflow cause erosion at the point where the water enters the valley head. Vertical erosion makes a river channel deeper. This happens more in the upper stages of a river the V of vertical erosion should help you remember the V-shaped valleys that are created in the upper stages. Lateral erosion makes a river wider. This occurs mostly in the middle and lower stages of a river.

There are four main processes of erosion that occur in rivers. These are:. Hydraulic action. The pressure of water breaks away rock particles from the river bed and banks. The force of the water hits river banks and then pushes water into cracks. Air becomes compressed, pressure increases and the riverbank may, in time collapse.

Where velocity is high e. Near waterfalls and rapids, the force may be strong enough to work on lines of weakness in joints and bedding planes until they are eroded. The sediment carried by a river scours the bed and banks. Where depressions exist in the channel floor the river can cause pebbles to spin around and turn hollows into potholes.

Eroded rocks collide and break into smaller fragments. The edges of these rocks become smoother and more rounded. Attrition makes the particles of rock smaller. It does not erode the bed and bank. Pieces of river sediment become smaller and more rounded as they move downstream. Carbon dioxide dissolves in the river to form a weak acid. This dissolves rock by chemical processes. This process is common where carbonate rocks such as limestone and chalk are evident in a channel.

Meanders support the formation of flood plains through lateral erosion. When rivers flood the velocity of water slows. As the result of this the river's capacity to transport material is reduced and deposition occurs. This deposition leaves a layer of sediment across the whole floodplain. After a series of floods layers of sediment form along the flood plain.