Changing Channel Characteristics and Hydrological Cycle

Changing Channel Characteristics of Rivers and Water Cycle

IGCSE Geography

Learning Objectives

• Explain the hydrological characteristics and processes that operate within rivers:

  • Downstream changes

  • Energy in water

  • Channel shape

  • Channel roughness​

• Explain the water cycle

IGCSE Geography

Open Ended

Using the video, describe how rivers are formed.

Any body of water that flows in a channel and uses gravity to move downhill.

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Stream - includes large and small flows

River - the main stream of the entire river system​

River or Stream

Changing Channel Characteristics

• Downstream changes

• Energy in water

• Channel shape​

• Channel roughness​

Downstream Changes

Downstream - in the direction of a stream's current

Three Main Roles of Rivers

• erode the river channel

• transport materials

• create new erosional and depositional landforms

Ero​de - 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​

Erosion Zone

Transport Zone

Deposition Zone

Three River Zones

These zones are found in all parts of a river, although one process tends to be dominant. For example, more erosion in the upper part and less deposition in the lower course.

Downstream and upstream have different qualities.

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Velocity, discharge, and load increase downstream while gradient and size of load decrease downstream.

Energy in a River

Produced by gravity and the flow of water.

Kinetic Energy

Provided by the weight and elevation of the water

Potential Energy

Energy in a river

Potential - the energy of what can be (stationary, stored energy)

Kinetic - the energy of what is​ (energy in motion, active energy)

Three Gorges Dam in Yangtze River (China)

The rougher the channel the more energy is lost. In a smooth channel with little frictional loss, more energy is available for work.

Channel Shape

River's efficiency: a measure of the ability of a channel shape to transfer water downstream.

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Measured by its hydraulic radius: cross-sectional area divided by the wetted perimeter.

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The higher the ratio, the more efficient the stream and the smaller frictional loss.

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The ideal form is semi-circular.​

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​​The shape of the channel is also determined by the material forming the channel

Alluvium - deposit of clay, silt, sand, and gravel.

​There is a close relationship between velocity, discharge and the characteristics of the channel in which water is flowing.

Solid rock only allows slow changes whereas alluvium allows rapid changes.

Silt and clay produce steep, deep and narrow valleys. (cohesive and stable)

Sand and gravel promote wide, shallow channels.

Channel Roughness

Channel Roughness

Friction is caused by irregularities in the river bed, boulders, trees and vegetation, and contact between the water and the bed and bank.

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This causes friction which slows down the velocity of the water.​

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Channel Roughness - the measure of the amount of frictional resistance water experience when passing over land and channel features.​

Discharge

Discharge - the volume of water passing a given point over a set of time.​

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Expressed in m³ or cubic metres​ per second (cumecs). Found by multiplying the cross-sectional area and the mean velocity.

Large rivers with a higher width and depth (w/d) ratio are more efficient than smaller rivers with a lower w/d ratio since less energy is spent in overcoming friction.

Discharge

​Discharge normally increases downstream as does with, depth and velocity.

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In contrast, channel roughness decreases.​

Changing Channel Characteristics

• Downstream changes

• Energy in water

• Channel shape​

• Channel roughness​

How does water in the river begin its journey?

Draw

Label the illustration below.

Download the Rivers.pdf posted on MS Teams. Look for the illustration on the first page and trace the movement of water.

Evaporation

The evaporation of sea water forms clouds

These clouds blow towards land where they rise, causing precipitation like rain, snow or hail to fall on the ground below.

Rain water sinks into the ground (Vertical Transfer)

• Water collects on plant leaves by interception, drips off or moves by stem flow into the soil.

• It can then filter through the soil through spaces in the surface layers -- infiltration.

• The water can also move through saturated ground below the water table -- percolation.​

​Interception - water intercepted by the leaves, branches of plants and the forest floor.

Infiltration - the flow of water from aboveground into the subsurface.​

Percolation - the movement of water through the soil.

Water moves back to the sea (Horizontal Transfer)

• Surface run-off - when water flows overground to river, lakes or the sea.

• Channel flow - the flow of water in a stream, river or lake.

• Through flow - when infiltered water moves through soil to a river

• Groundwater flow - when percolated water moves below the water table to a river​

​Run off - occurs when there is more water than the land can absorb.​

Some water is stored on the surface

• Channel storage - happens in rives lakes (used for water supply)

• Ground water storage - found in underground rocks. A water-carrying rock is called an aquifer (chalk)

• Soil moisture storage - when water is store in the soil and is used by plants

• Short-term storage - occurs after interception on plan leaves, flowers, etc.​

Water eventually returns to the Athmosphere

• Evaporation happens when sea, lake or rive water is heated by the sun

• Transpiration is when plants lose moisture

• Evapotranspiration is both.​

EAL Activity

c/o Ms. Jona​