River Processes & Landscapes

This is a Profile of a River-Middle, Upper and Lower Stages:

In the upper course of the river, there are many features.

The river channel is narrow and fast, with interlocking spurs and v-shaped valleys

Steep drops in the river with extremely fast flows are waterfalls, with retreating gorges of recession , ending with a plunge pool .

The river cuts downwards like a saw. This happens in the upper part of the river and makes the valley go deep. Rain erodes the edges to make it wider at the top. Vertical erosion is dominant

03/06/09 By Mohammad yaseen Serry and Mohammad Osama Hassanally

Middle course of the river has more energy and a high volume of water. The gradient here is gentle and lateral (sideways) erosion has widened the river channel. The river channel has also deepened. A larger river channel means there is less friction, so the water flows faster:

As the river erodes laterally, to the right side then the left side, it forms large bends, then horseshoe-like loops called meanders.

The formation of meanders is due to both deposition and erosion.

The force of the water erodes and undercuts the river bank on the outside of the bend where water flow has most energy.

On the inside of the bend, where the river flow is slower, material is deposited.

Over time the horseshoe become tighter, until the ends become very close together. As the river breaks through and the ends join, the loop is cut-off from the main channel. The cut-off loop is called an oxbow lake . The animation demonstrates how these features form.

River Landscapes and Processes By Mohammad Yaseen Serry and Mohammad Osama Hassanally

In the lower course, the river has a high volume and a large discharge. The river channel is now deep and wide and the landscape around it is flat. However, as a river reaches the end of its journey, energy levels are low and deposition takes place.

The river now has a wide floodplain. A floodplain is the area around a river that is covered in times of flood. A floodplain is a very fertile area due to the rich alluvium deposited by floodwaters. This makes floodplains a good place for agriculture. A build up of alluvium on the banks of a river can create levees which raise the river bank.

Deltas are found at the mouth of large rivers - for example, the Mississippi. A delta is formed when the river deposits its material faster than the sea can remove it. There are three main types of delta, named after the shape they create:

Fan-Shaped

Tooth-Shaped

Birds Foot

Erosion:

This involves the wearing away of rock and soil found along the river bed and banks, mainly in the upper course. There are four main forms of river erosion:

Hydraulic action : river wears away the river bank from underneath.

Attrition : rocks being carried by the river collide and break into smaller particles.

Abrasion/Corrasion : rocks carried along by the river wear down the river bed and banks.

Solution/Corrosion : smaller particles are dissolved into the river.

Transportation :

This involves transporting the eroded material downstream.

Traction: Large boulders roll along the river bed.

Saltation: pebbles bounce along the bed.

Suspension: Sand/silt carried in the flow.

Solution: Minerals are dissolved and carried in the flow.

Large particles such as gravel and coarse sand are lifted and dropped along the river, so they bounce along the river bed in a series of bed. Larger particles like pebbles and boulders roll and slide along the river bed. Smaller particles such as clay, silt and fine sand are carried along without contact with the river bed. Dissolved materials containing minerals like calcium and sodium are carried in the water Solution

Causes:

50 days of rain

Faulty levées

Effects:

The floods occurred in 1993

23 people died, 1000s evacuated

6 million acres of farmland were submerged

Electricity cables, roads and railway lines drowned

$10 billion was needed for repairs etc

Effects:

Over 57% of land was flooded.

240 villages were submerged.

Over 1000 people were killed.

Shortages of drinking water.

Cost the country $1 billion.

Physical causes:

70% of the land is less than 1 metre above sea level.

Rivers, lakes and swaps cover 10% of the land.

Tropical Cyclones, heavy monsoon rain and coastal floods.

Snow melts in the Himalayas

Most of Bangladesh is the huge flood plain and delta of the rivers Ganges and Brahmaputra.

Human causes:

Deforestation (Nepal & Tibet)

Construction of dams in India

Global warming

Flood defences such as embankments

Urbanization creating obstacles to the free flow of water

River management

Rivers can be useful, they bring fertile soil in floods-e.g. In the meander scar and flood plain.

However, floods can destroy people and settlements.

By introducing plans, floods can be prevented, some of these are:

Questions

The point where two rivers meet and flow into the sea

The force of the water wearing away the bed and banks of the river

(i)Deforestation (ii) Construction of dams (iii) Global warming

(i) Raise levees (ii) Planting trees (iii) Cutting river channels (iv) Dredging

It is the zigzag flow of the river

2. What is Corrosion ? 1. What is a confluence? 4. Name at least 4 ways of preventing floods. 3. Name at least 3 human causes of floods in Bangladesh? 5. Explain what a meander is?