2.2 What are the major fluvial processes? Part B

Direction of fluvial erosion Vertical erosion / Downcutting How can a river erode the land? Three directions of fluvial erosion The higher the energy of the river, the more the erosive power. Direction of fluvial erosion Vertical erosion / Downcutting Lateral erosion Headward erosion

Three directions of fluvial erosion Vertical erosion / Downcutting A river removes rocks from its bed. The valley will be deepened.

Three directions of fluvial erosion Lateral erosion Original valley A river removes rock along the banks. The valley is widened.

Three directions of fluvial erosion Headward erosion Loose rock debris is brought down by the overland flow behind the river source. The river source extends backward. The valley is increased in length.

Ways of fluvial erosion How can a river erode the land? Four ways of fluvial erosion Ways of fluvial erosion Abrasion Attrition Hydraulic action Corrosion / Solution

Four ways of fluvial erosion If the animation cannot work properly, click here to download the latest Adobe Flash Player.

How can a river erode the land? What factors control the rate of erosion of a river? Angularity, amount and weight of load Speed of water flow Erosive power of river Resistance of rock Amount of channel water Channel gradient Rate of fluvial erosion Water quality (acidity, temperature) Rock structure Mineral composition

How can a river erode the land? How do human activities cause more fluvial erosion? Industrial activities: discharge of pollutants water acidity fluvial erosive power Deforestation: interception overland flow amount of channel water fluvial erosive power Cutting a slope: slope gradient and infiltration overland flow amount of channel water fluvial erosive power

In what ways does a river carry its load? Apart from erosion, river energy can carry the eroded materials as ‘load’. The downstream movement of eroded material is called ‘transportation’. The higher the energy of a river, the ( less / more ) efficient its transportation. The Huang He

In what ways does a river carry its load? Four ways of transportation Large particles hop and bounce along the river bed Small particles are carried in the water without touching the river bed Dissolved minerals carried by water Suspended load Suspension Bed load Solution Dissolved load Very large particles roll or slide on the river bed Bed load Traction Saltation

In what ways does a river carry its load? What factors control the amount of load in a river? (Credit: Tsui Chung Pong David) The Huang He carries a large amount of load. The load of the river is mainly silt from the Huangtu Gaoyuan (Loess Plateau). It gives the yellowish colour to the river.

In what ways does a river carry its load? What factors control the amount of load in a river? 1 Energy of a river Larger amount of channel water A river can carry (more / less) load More energy for transportation OR A river can carry (heavier / lighter) load Higher speed of water flow

In what ways does a river carry its load? What factors control the amount of load in a river? 2 Rate of erosion The erosion rate of the river basin controls the supply of load. ( Dense / Sparse ) vegetation helps hold the soil and prevents it from being eroded.

In what ways does a river carry its load? What factors control the amount of load in a river? 3 Presence of lakes Lakes along a river channel help trap the load. This ( increases / decreases ) the amount of load that will reach the river. (Credit: Tsui Chung Pong David)

What makes a river lose its energy? Deposition occurs when a river loses its energy. The deposited materials are called ‘sediment’. Amount of channel water decreases (More / Less) energy for transportation The load deposits OR Speed of water flow decreases (Credit: Tsui Chung Pong David)

What makes a river lose its energy? How does a river drop its load? Please click below for footage The smallest and lightest sediment deposits last. What is the sequence of deposition? The largest and heaviest sediment deposits first.

What is the sequence of deposition of these four types of load? What makes a river lose its energy? How does a river drop its load? What is the sequence of deposition of these four types of load? The heaviest load (i.e. __________) will deposit first and form the bottommost layer. pebble silt The finest one (i.e. _______) forming the top layer.

What makes a river lose its energy? How does a river drop its load? The load forms layers at different distances from the mouth of the river. River Pebbles and sand Silt Clay Sea

Skill box How can we show the variation of river discharge over time? Let us see how we can interpret a hydrograph. By a hydrograph.

How can we show the variation of river discharge over time by a hydrograph? Rainfall Discharge Base flow This is a hydrograph.

How can we show the variation of river discharge over time? River discharge varies after rain. Discharge is low before rain starts. When rain starts, discharge begins to rise. After reaching the peak, the discharge starts to drop.

It is the maximum amount of water a river can hold. How can we show the capacity of the channel? It is the maximum amount of water a river can hold. Bankfull discharge If the discharge is below the bankfull discharge, it means that the river is able to hold the rainwater.

How can we show the capacity of the channel? The channel cannot hold the flood water Bankfull discharge If the discharge is above the bankfull discharge, _________ occurs. flooding

What is the cut-off of a river? Rainfall decreases or stops Base flow runs out Period of cut-off It rains again River discharge falls