Learning Objectives

• ​Explain how rivers erode, transport and deposit.

Recap: Hydrologic Cycle

Three Main Roles of Rivers

• erode the river channel

• transport materials

• create new erosional and depsitional landforms​

Erode - gradually wear away by natural agents (soil, rock, or land)

Channel - where a river flow. It has a bed and two banks.

Deposition - the laying down of material carried by rivers​

The geological process in which earthen materials are worn away and transported by natural forces such as wind or water.

Erosion

Main Types of Erosion

Abrasion (or corrasion) - wearing away of the bed and bank by the reload carried by a river​

Attrition - the wearing away of the load carried by river which creates smaller, rounder particles

Main Types of Erosion

Attrition - the wearing away of the load carried by river which creates smaller, rounder particles

Main Types of Erosion

Hydraulic action - which is the force of air and water on the sides of rivers and in cracks

Solution ( or corrosion) - the removal of chemical ion, especially calcium, which causes rocks to dissolve

Factors Affecting Erosion

Load - the heavier and sharper the load the greater the potential for erosion

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Velocity and discharge - the greater the velocity and discharge the greater the potential for erosion

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Gradient - increased gradient increases the rate of erosion​

Gradient - stream gradient refers to the slope of the stream's channel.

Factors Affecting Erosion

Geology - soft, unconsolidated rocks, such as sand a​nd gravel, are easily eroded

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pH - rates of solution are increased when the water is more acidic

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Human Impact - deforestation, dams and bridges interfere with the natural flow of a river and frequently end up increasing the rate of erosion​

pH - the unit expressing the acidity or alkalinity of a solution.

Occurs when a river slows down and it loses its energy. When the river loses energy, it drops any of the material it has been carrying.​

Deposition

Deposition

• Typically occurs as a river floods across a floodplain or enters the sea, or behind a dam.

• It is also more likely during low flow conditions (like drought) than during high flow (flood) conditions -- as long as the river is carrying sediment.​

• ​The larger, heavier particles are deposited first, the smaller, lighter ones later.

Floodplain - a flat area of land next to a river or stream.

Deposit - put or set down in a specific place.​

Features of Deposition

• Deltas

• Levées

• Slip-off slopes (point bars)

• Oxbow lakes

• Braided channels

• Floodplains​

Take or carry from one place to another.

Transport

Main Types of Transport

​Suspension - small particles are helped up by turbulent flow in the river

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Saltation - heavier particles are bounced or bumped along the bed of the river

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Solution - the chemical laod is dissolved in the water​

Main Types of Transport

Traction - the heaviest material is dragged or rolled along the bed of the river

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Flotation - leaves and twigs are carried on the surface of the river​

River Profiles

• Long profile - shows how the river changes over its course from its source (where it starts) to its mouth (where it meets the sea).

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• Cross profile - a view of the valley from one side to another.

Long Profile

Long Profile

Varies as it moves downstream

1. In the upper stage, the river's gradient is quite steep.

2. In the middle stage, it's more gentle.

3. In the lower stage, it's very gentle and almost flat.​

Long Profile

• Irregularities, or knick-points, may be due to:​

  • ​geological structure, hard rock erode slowly, which can result in the formation of waterfall and rapids.

  • variation in the load, for example when a tributary wit a coarse load my lead to a steepening of the gradient of the main valley

  • sea level changes - a relative fall in sea level will lead to renewed downcutting which enable the river to erode former floodplain and form new terraces and knick-points.

Knick point - a part of a river or channel where there is a sharp change in channel slop, such as waterfall or lake.

Cross-section Profile

Cross Profile

1. upper: close or near to the river's source the valley has a narrow floor and steep sides, ie. V-shaped.

2. middle: lower down the river, the floor is wider and the sides are more gently sloping.

3. lower: when the river is near to the sea it has a wide floor and gentle sides.

Three Stages

EAL Activity

c/o Ms. Jona​

Activity

Make a 3-5 minute presentation

1. Gorges and waterfalls

2. Meanders

3. Oxbow lakes

4. Floodplains

5. Levées

6. Braided channels

7. Delta​

Your pair's presentation should include the following:

• Definition of the concept

• How it is formed

• Examples

• Functions/Impact to human activities​