RIVERS

Processes carried out by Rivers

• Rivers gradually wear away material in the channel. The wearing away is called Erosion and makes river wider and deeper.
• The eroded boulders, pebbles and rocks are carried downstream through the process of Transportation. Material being transported is called LOAD.
• When river no longer has no more energy it gradually drops the load onto the river bed. The process is called Deposition. Heaviest material deposited first and lightest last.

Types of erosion

• Corrasion - where sand pebbles dragged along the river bed wearing it away.
• Hydraulic action - fast flowing water is forced into cracks, breaking up the bank over time up..
• Attrition - rocks and stones wear each other away and become smaller and rounder.
• Solution - when rocks such as limestone are dissolved in acid rain water.

Types of River Transport

• Saltation - similar stones and pebbles are picked up and dropped again. It results in skipping motion.
• Traction - there are large stones being dragged along the river bed.
• Solution - dissolved chemicals carried along invisible to the eye.
• Suspension - tiny particles of sediments are carried in the rivers current.

Factors that affect the flow of the river

• Velocity of the river or also the energy of river. Faster the river, the larger material that can be transported in the load. Large boulders can be transported by very fast flows.
• Volume of the running water. More running water , greater volume of load.
• Bedrock types vary. Hard ones like granite are tougher to erode and softer ones like shale are easily eroded. Some are also soluble such as Limestone.

Reasons for why deposition happen

• A decrease in Gradient
• A decrease in river flow/discharge
• river meeting sea or lake such as a delta
• river flowing slowly on inside bends

What is discharge?

• It is the volume of water flowing down the river at one time. It is measured in cubic meters per second. It is affected by melting snow and climatic variations.

Three main forms of river channels

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• Legenda: MEANDERING
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• Legenda: STRAIGHT

Three main forms of river channels

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• Legenda: BRAIDED

THREE PARTS OF THE RIVER

• Upper course
• Middle course
• Lower course

FEATURES OF THE UPPER COURSE

• The starting of the river is known as the SOURCE. It is generally in mountains or uplands. The GRADIENT is very steep. River valley floor is very narrow and RIVER CHANNEL generally occupies the complete floor. The VOLUME of the water is much lesser at this part of the course compared to other parts. The SPEED of the river is fast. The SIZE OF LOAD is large. The speed of the water allows t to carry large boulders. River winds its way around interlocking spurs. This area of the river carries out Vertical Erosion. The features formed in the upper course are POTHOLES and RAPIDS.

HOW IS A RAPID FORMED ?

HOW IS WATERFALL FORMED ?

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• Waterfalls when retreat upstream form gorges. The waterfalls are essential as they are usually used to generate hydroelectric power. They help the economy grow. They are a large tourist attraction and thus it allows the local industry to grow. it provides jobs in hotels, transport and tour companies. However very inconvenient when comes to navigation.

INTERLOCKING SPURS

MIDDLE COURSE OF RIVER

• The SPEED of the river decreases. The GRADIENT decreases. The VOLUME of the river is greater . The SIZE OF LOAD is much smaller due to erosion. The WIDTH OF CHANNEL increases. More lateral erosion takes place than vertical erosion. The features formed are meanders and ox bow lakes.

Formation of meanders and oxbow lakes

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• Water flows in helical flow causing it to be fast on outer bends compared to the inner ones. Erosion happens on the faster bend and deposition happens on slower bend. The eroded material is taken and deposited on the inner bend. The area of deposition on inner bend is a SLIP OFF SLOPE . The erosion forms a RIVER CLIFF on outer bend.

LOWER COURSE

• GRADIENT is almost flat or flat. SPEED of water is slow. Deposition is greater. SIZE OF LOAD is small. WIDTH OF CHANNEL is wide. Has the MOUTH of the river. The MOUTH is where the river flows out into the sea. Features formed in this part of the course are Floodplains, Levees and Deltas.

Formation of floodplains

• As river widens up in the lower course due to lateral erosion. When there is high discharge, river loses a lot of energy. When river overflows onto the bank the friction reduces the speed , allowing deposition to occur. Heavier load closer to the bank and silt further away from the channel. this causes area to become fertile.

Formation of Natural Leeves

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• These levees are mud walls formed naturally due to deposition. They are often strengthened to be prevented from breaking during FLOODING.

Tributaries and Distributaries

• Tributaries are smaller water bodies which join the main water channel.
• Distributaries are smaller channels that split from the main channel. Occurs generally when deposition is happening.

Formation of Delta

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• River splits into a lot of distributaries cause of load. A lot of deposition occurs making the land between the small channels very fertile. This forms a TRIANGULAR shape similar to a D. It is named after the Greek letter for D, Delta.

Factors of Discharge

• DISCHARGE

Hydrograph explination

Terms used in a Flood Hydrograph

• PARTS OF A FLOOD HYDROGRAPH

Advantages of living in floodplains

• Flat lands makes it easy to build roads and settlements. Easy to carry out agriculture.
• Soils are mineral rich and fertile cause of silt deposition making agriculture profitable.
• River valleys are natural route ways.
• Rivers are navigable, provide water supply and fish for food.

Disadvantages of a flood plain

• Often many diseases carried by insects such as Malaria and Sleeping Sickness
• Dense population will be severely affected as areas are proned to flooding.

What is a flood ?

• Flood occurs when discharge can no longer be contained in the channel, river overflows flooding.

How rainwater reaches a river ?

Case studies