### Revision Helper for Rivers

Dear Geographers,
Below is a summary of the major themes and concepts for the topic of "Rivers". I've provided some illustrations,photos, animations and videos but they are not exhaustive. You MUST refer to my notes for detailed explanations and sketch diagrams. I will not be uploading the notes onto this website.

Love, Mrs Koh

" The path of least resistance makes all rivers and some men crooked"
Napolean Hill

Illustrate, describe and explain the hydrological cycle.
1. Visit the following website  http://techalive.mtu.edu/meec/module01/HydrologicCycleQuiz.htm for an interactive quiz on the key elements of the hydrological cycle. Click http://polaris.umuc.edu/cvu/envm/hydro/hydro.html for an animation of the hydrological cycle.
2.
What is STEMFLOW and THROUGHFALL?

Stemflow (where water trickles along twigs and branches and finally down the trunk) - C
Throughfall (water falls through the gaps in the vegetation or drops from leaves, twigs or stems) - GUESS??

PERMEABILITY

Identify the features of a drainage basin

• View this animation showing the function of a drainage basin - drains rivers and its tributaries.
•

The three courses of a river

Long profile of a river

Characteristics of the river in the upper, middle and lower course

Describe and explain factors that affect the energy of a river: velocity and volume.

• Velocity
1. Wetted perimeter

3. Roughness of a channel
Rocks that protrude out from the bank can slow the pace of the water as friction slows it down as it passes the obstacles.

4. Gradient (steepness) of the river

5. Channel shape

Symmetrical River Channel

Asymmetrical River Channel

Asymmetrical River Channel Diagrammatically
• Volume
1. Climatic conditions
2. Permeability of rock

Permeability of rocks is determined by the size of the pores between the rock particles. If the rock has small pores, water cannot easily pass into the rock and this means that the rock is impermeable. However, if the rock has large pores, water can easily pass through and thus the rock is permeable.

1. Presence of vegetation
2. Size of drainage basin
3. Amount and duration of rainfall

Click here for an interactive activity where you can learn how rainfall and type of vegetation affect the volume of water in a river.

Describe the two directions of river erosion

• Describe and explain with the help of diagrams 4 fluvial erosional and 4 fluvial transportational processes of a river.
•
4 erosional processes

1. View this animation showing the 4 major river erosional processes.
• Corrasion (abrasion)
• Attrition
• Solution (Corrosion)
• Hydraulic Action

4 transportational processes

1. View this animation showing the 4 major river transportational processes
Suspension

Saltation

Traction

Suspension

• Explain the conditions that result in river deposition.
•
1. View animation showing how river deposition takes place

Describe and explain using the Hjulstrom Curve, the relationship between particle size and velocity and how the particles are subjected to various river processes.

Describe and explain the formation of erosional landforms such as waterfalls,plunge pools, gorges, v-shaped valleys, inter-locking spurs and potholes with the help of well-labelled/well-annotated diagrams.

1. View an animation showing the retreat of a waterfall and formation of a gorge. It also shows headward erosion. Another similar animation.
2. View an animation showing the formation of a waterfall due to alternating bands of resistant and less resistant rocks. Also view the formation of a gorge.

1. The powerpoint slides below include a video explaining how a waterfall is formed.

Formation of a waterfall by rocks of different resistance

Formation of waterfall,plunge pool, gorge and rapids.

The Victoria Falls, an example of a waterfall that is formed by faulting. It is one of the 7 natural wonders of the world.

The Niagara Falls, an example of a waterfall that is formed due to alternating layers of resistant and less resistant rocks
It is made up of THREE waterfalls - namely the American Falls, Bridal Veil Falls and the Canadian Horseshoe Falls.

Geology of the Falls (cross sectional view)

A paranomic view of the Falls

Katherine Gorge, Australia

Rapids

V-shaped valley

Ashes Hollow

Interlocking Spurs

Formation of Pot Holes

•
Potholes on the river bed

• Explain the formation of pools & riffles, river cliffs, slip-off slopes and depositional landforms like ox-bow lakes, meander scars, floodplains and deltas with the help of well-labelled/well-annotated diagrams.
•
Pools & Riffles, Cross sections of a meander

Formation of a meander to a scar
1. View an animation showing the sequential formation of meanders, river cliffs, slip off slopes, cutoff, oxbow lake and meander scar.

There are, in fact, many different types of meanders

Formation of river cliff and slip off slope
•
•
A steep river cliff caused by river erosion

A gentle slip off slope formed by river deposition. Photo taken in Kachanaburi, Thailand near the Death Railway.
I was there for a short holiday with my parents over CNY this year :)

With my daddy on one section of the railway track

River Cliff and slip off slope

The process of helicoidal flow (a secondary flow that causes deposition on the convex bank and erosion at the concave bank)

Diagram showing formation of ox bow lake

A cut-off

Meander and Ox bow lakes

Meanders & meander scars

Another photograph showing meanders, abandoned loops, meander scars (center of the photograph - light green patches)

The town of Shrewsbury, UK, placed inside a huge meander bend
Shrewsbury is a county town lying on the river Severn in the West Midlands region and is home to around 71 000 inhabitants. It is placed inside a meander, and was originally a medieval market town. The meander was very helpful for imports and exports as well as travel in the medieval ages.
Since 1990, Shrewsbury has experienced severe flooding problems becuse of its location - in 2005 the defense systems were increased dramatically to hold back the potential flooding. The land is very fertile due to all the layers of sediment and silt that have been forming over many years. Therefore, farming is a possibility on flood plains, around meanders. These crops must be resilient against high discharge in times of flooding.

Formation of a floodplain

Floodplains

1.  The powerpoint slides below include a sequential diagram and an animation of the formation of floodplains and levees.
2. Visit the following website http://www.ngfl-cymru.org.uk/vtc/meanders_floodplain/eng/Introduction/Plenary.htm for an interactive quiz on the key features of a floodplain and a meander.

Formation of a delta

The Nile Delta (arcuate delta) and the Bird's Foot Delta (Mississippi Delta)

River Channel Management

Dredging

• mainly to deepen/widen the river channel so as to increase its capacity to prevent flooding.
• it also helps to increase the flow of a river by reducing its wetted capacity

v      Limitations:

Ø      It can be very expensive

Ø      There may be a lack of a suitable site to dispose the dredged material. There may also be a lack of a suitable handling site nearby that is large enough to handle the dredging process.

Dredging at Changi

River Straightening

• Straightening rivers allows water to flow more rapidly downstream
• involves the removal of meanders there by reducing the length of the river channel
• The faster flow allows water to flow away from the area more quickly and wash away sediments which have accumulated on the river bed
•  This thus deepens the channel, allowing it to hold more water and minimize flooding.
Limitations

• expensive
• result in flooding downstream
• accumulation of sediments
• loss of wetlands
River Straightening in Chicago which began in 1926 and completed in 1930

Chicago River Today

Bank Protection

• artifical levees to increase the capacity of the river
• to reinforce the banks of the river to minimise erosion
• to increase the speed of river flow
Limitations

• expensive to maintain
• encourage flooding downstream especially in unprotected parts of the river
• can be breeched especially during prolonged and heavy rainfall or hurricane. Cause major flooding and worsen the situation when the broken concrete, swept by floodwaters hit/kill people and damages buildings
• give people living behind the levee a false sense of security
Artificial levees along the Mississippi

Breeched section of the levee in the 1993 Mississippi Flood

Breeched levees @ New Orleans due to the onset of Hurricane Katrina in 2005

Gabions

• less costly
• stackable along the banks of the river
• help divert the flow of water from the river banks to the centre of the channel
• protect the banks from being eroded by the force of running water, thereby reducing the amount of sediments that flow into the river
Limitations
• not as effective as artifical levees
• wire mesh corrode more easily, destablising the structure
• allows sediment to accumulate behind the structure
Gabions along the banks of a river

Close-up of a Gabion

Prolonged erosion by running water destablised a section of the gabion

Vegetation Planting

• make use of the roots of vegetation to hold the soil at the banks of a river together to minimise bank erosion
• to prevent the addition of eroded sediment into the river which slows down the speed of a river
• minimise the occurance of floods
• more environmentally friendly, aesthetically pleasing, cheap option
Limitations
• only in areas where climate allows the growth of vegetation
• roots of vegetation may not be strong enough to withstand rivers with high energy or when there is a sudden onset of flood caused by hurricane or prolonged heavy rain for e.g.
• excessive shade can also kill marine plants and aquatic life as it prevents sunlight from entering the water
• additional weight of vegetation can in fact add additional stress to the river bank, thereby resulting in bank failure

Vegetation Removal

• On the other hand, there are occasions which call for the removal of all vegetation along the banks or river bed. Vegetation clearance is often conducted to improve the speed of flow and stabiles the river channel.
Limitations
• The weight of vegetation, especially trees could add stress to the banks and cause them to collapse.
• More over, the vegetation may contribute woody debris to the river which may deflect the flow of water and accelerate bank erosion.
• Accumulation of such debris may also slow down the flow of water and encourage flooding.