Pure Geography SA2 Overall Revision Notes
Chapter 4: Rivers and Coasts
The River System
2. Watershed: A stretch of land that surrounds drainage basin (a.k.a. boundary)
3. Drainage Basin: A river system that collects rainwater and comprises of streams and rivers
4. Tributaries: Streams that flow in the river which is mainly found in the upper course.
5. Distributaries: Branch out from the river which is mainly found in the river delta.
6. Confluence: Area where the stream meets the river
7. River Mouth: Place where the river meets the ocean
8. River Delta: Landform created at the mouth of the sea where the river flows into. It is formed by the deposition of the sediment carried by the river as it flows into the sea.
3 Courses of a River
Upper /
- Place where the river begins
- Many tributaries join up to form larger streams which in turn join to create a river
Middle /
- River starts to meander (curve)
- Many tributaries join the river
Lower /
- Meanders are common
- Many distributaries
- River flows towards the river mouth and enters the sea
- Delta (land formed by sediments) may form at river mouth
Pure Geography SA2 Overall Revision Notes
Chapter 4: Rivers and Coasts
Factors affecting River Speed and Volume
Definition / High Speed / Volume / Low Speed / Volume
Gradient / How steep a river is. / River is steeper (usually at upper course) / River is gentler (lower course)
Roughness of River Channel / How rough a river channel is (affected by obstacles in river) / Fewer obstacles àSmoother channel àFaster / More obstacles à More friction à Reduced speed
Wetted Perimeter / Area in contact with the river. Shape of river channel affects the wetted perimeter (Length and Breadth in contact with the river) / A smaller wetted perimeterà Lesser friction à Faster / A larger wetted perimeter àMore water à More friction à Slower
Factors affecting Volume of water in River
Factor / Definition / High Speed / Volume / Low Speed / Volume
Permeability of Rocks / Permeability is the ability of rocks being able to hold water / Permeable rocks (holds more water) à Lesser surface runoff / Less Permeable rocks àMore surface runoff (more volume of water)
Vegetation Cover / Vegetation: Plants at the side of the river / Sparse vegetation à Less rainwater infiltrate ground à More surface runoff à Large volume / More vegetation àMore rainwater infiltrate ground àLesser surface run off àSmaller Volume
Climate / How rainfall / temperature affects the river
Wet/dry weather à Water level fluctuates (drop/unstable) / Hot / Wet (High Evaporation) à High volume of water / Dry àLow volume of water throughout the year
Size of Drainage Basin / Drainage Basin: An area which collects water (comprises of rivers) / Bigger Drainage Basin àLarger Surface area à Higher volume of water / Smaller drainage basin àSmaller surface area àSmaller volume of water
Pure Geography SA2 Overall Revision Notes
Chapter 4: Rivers and Coasts
Erosion, Transportation and Depositional Processes
Pure Geography SA2 Overall Revision Notes
Chapter 4: Rivers and Coasts
River Landforms
1. Floodplains are found at the lower course of a river.
2. When river overflows due to heavy rain, flooding occurs and water spreads over a larger area, losing speed.
3. When water recedes, river deposits material carrying on the land, leaving behind sediments on river banks and beds.
4. Coarser sediments are deposited closer to river edges and fine particles are deposited away from river. The deposited materials build up on both sides of river due to flooding.
5. Over time, the accumulation of sediments at the flat plains called floodplains is formed. Coarser and heavier materials accumulated nearer to the river form raised banks called levees.
Valleys and Gorges (Example:
/ 1. Valleys and gorges are usually found in the upper course where vertical erosion is prominent.
2. In the upper course, the volume of water is small but the gradient is steep.
3. The swift flowing water exerts a powerful force on the river bed, forming a V-shaped valley through hydraulic action, abrasion and solution.
4. A gorge is an exceptionally deep and narrow valley formed when the river erodes vertically through resistant rocks leaving very steep valley walls.
Waterfalls (Example: Iguazu Falls, border of Argentina and Brazil)
1. River flows through across rocks of different resistance,
2. eroding the less resistant rocks more rapidly,
3. causing a change of gradient in river course.
4. Over time, river plunges from great height to hit river bed below with tremendous force, forming a waterfall. / 5. Waterfall can also be formed by faulting, where rocks are uplifted, causing displacement of rocks where 1 layer is higher than the other. When the river flows across an area where faulting occurred, gradient drops suddenly.
6. Repeated pounding of river bed leaves a depression at base of waterfall.
7. Depression is further deepened by rocks and boulders swirling around, forming a plunge pool.
8. Over time, as the harder rock is undercut it produces an overhang which will eventually collapse.
9. As the rock collapses the waterfall retreats upstream, leaving a gorge.
Meanders and Oxbow Lakes
/ 1. Meanders can occur anywhere along the course of the river, however they are more prominent in the middle/ lower course.
2. As the river travels downstream, it tends to find the easiest route, thus it will bend.
3. Therefore, bends of the river at middle/lower course allows erosion to occur at the outer bank of a river.
4. Eroded materials are deposited at the inner bank where water moves in a spiral fashion. Along the outer bank, speed of river is high due to less friction and greater volume.
5. Over time, the concave bank gets undercut[1] and a steep-sided cliff known as a river cliff is formed.
6. As the bends become more obvious, a meander is formed.
Formation of Oxbow Lakes
7. Constant erosion at 2 consecutive concave banks will cause the 2 meanders to more closer, forming a loop. Over time, it becomes more distinct, separated by land.
8. As deposition and erosion continues, the 2 meanders eventually meet.
9. Sediments deposited at convex banks will dam up the river, forming an oxbow lake which will be separated from the main river. The main river will flow in a straight path.
10. Due to deposition the old meander bend is left isolated from the main channel as an ox-bow lake.
Formation of Deltas
/ 1. Deltas are found at the lower course of the river.
2. As a river approaches sea/lake, speed decreases, thus deposition occurs.
3. Although sediments can be removed by tides, if rate of deposition is faster than removal, sediments will build up at mouth.
4. Over time, more sediment is deposited and delta extends outwards into the sea. Sediments carried will block flow of main river.
5. As a result, smaller channels known as distributaries are formed to find their way out to reach the sea or lake.
6. Deltas only form under certain conditions
a. The river must be transporting a large amount of sediment
b. The sea must have a small tidal range and weak currents
c. The sea must be shallow at the river mouth
Pure Geography SA2 Overall Revision Notes
Chapter 4: Rivers and Coasts
River Management Strategies
1. Key Principles:
a. Aimed to improve the speed of flow (take note: This can increase the amount of erosion, reducing the ability of the river to hold water).
b. Increase the capacity of the river. (N.B. By increasing the volume of water, more water will flow downstream, causing flooding there)
River Channelization
- Processes such as resectioning is the widening or deepening of a river to let it carry more water, whereas
- Realignment deals with straightening the river channel to increase the speed of flow.
- Although this can solve the problem upstream, however it will increase the chances of flooding downstream.
- This is because when there is greater speed and volume of the water in the river, more erosion will occur and thus more sediment will be deposited at the river bed downstream, reducing the capacity of the river to hold water.
- When the huge volume of water reaches downstream, it will eventually cause flooding.
Pure Geography SA2 Overall Revision Notes
Chapter 4: Rivers and Coasts
Introduction
Pure Geography
Chapter 7: Coastal System and Processes
Coastal Processes
Pure Geography
Chapter 7: Coastal System and Processes
Coastal Landforms
2. Hydraulic action and abrasion may erode a crack on the rock surface.
3. The crack can enlarge to form a notch, which is further deepened to produce a cave. Further undercutting by the waves will cause the roof of the cave to collapse.
4. An overhanging cliff is formed, which will eventually collapse and its sediments deposited at the foot of the cliff.
5. Those sediments might be picked by crashing waves and thrown against the base of the cliff, causing further erosion. A cliff is formed.
6. Continuous erosion may cause the cliff to retreat further inland and over time, a wave-cut platform is formed at the foot of the cliff.
Headlands and Bays
/ 1. A headland is made of resistant rocks which extend outwards into the sea and formed due to differential erosion of coastal rocks.
2. Coasts with alternate strips of resistant hard rock and less resistant soft rock will cause the soft rock to be eroded faster than the hard rock.
3. The result is the formation of an indented coast with the remaining hard rocks forming headlands extending into the sea and soft rocks forming bays.
Beach / 1. A beach is a zone of deposition along the coast. It is formed when materials carried by waves and currents is deposited on gentle slopes and constructive waves which allows materials to be deposited by the strong swash.
2. They are formed at protected bays along an indented coast due to wave refraction, where waves approach the shallow sea in front of the headland first.
3. As wave energy tends to concentrate at the headlands, erosion takes place there.
4. Along the bays, waves are diverged, thus their energy is spread out and weakened. Deposition occurs, and over time beaches are formed.
Spit and Tombolo / 1. Spits are beaches joined to the coast at just one end.
2. Formed by longshore drift.
3. When the direction of the coastline changes, it continues to deposit materials in the original direction which accumulate in the sea forming a spit.
4. The spit continues to grow as materials are continuously deposited, joining a nearby island to the mainland, forming a tombolo.
Pure Geography
Chapter 7: Coastal System and Processes
Coastal Protection Measures
1. The rationale and principle for coastal protection usually rest on the economic values of these coastal areas.
2. Often, the implementation of these measures led to more problems rather than solving the threats from coastal processes.
3. The 2 underlying principles are to reduce the energy of waves and/or retain coastal materials.
4. Types of Approaches:
- Hard engineering: Construction of physical structures to defend against the erosive power of waves.
- Soft engineering: Focuses on planning, management and changing individual attitudes towards coastal protection.
Measure / Advantages / Disadvantages
Seawalls (e.g. Kallang River in Singapore) / They are built along the coast which absorbs the energy of waves before they can erode away loose materials. / 1. However, they do not prevent the powerful backsplash of refracted waves from washing away the beach materials between the walls.
2. The backsplashes will eventually undermine the base of the seawall and eventually it collapses.
3. They are costly to build and maintain as repairs have to be made to prevent their collapse.
Breakwaters / 1. They help protect the coast and harbour by reducing the force of high energy waves before they reach the waves.
2. They create a zone of calm water behind them. / However materials deposited in the zones behind the breakwater are protected but the zones located away from the breakwater are not, and will be eroded away.
Groynes / 1. They are built at right angles to the shore to prevent longshore drift.
2. They absorb/reduce the energy of the waves and cause materials to be deposited on the side of the groyne facing the longshore drift. / 3. As no fresh materials are deposited on the downdrift side, that part will gradually be eroded.
4. To prevent this, the tips of groynes are sometimes angled about 5 to 10°, but if failed, erosion can still take place on unprotected areas.
Gabions / 1. They are wire cages filled with crushed rocks that are piled up along the shore to reduce coastal erosion by weakening wave energy. / 2. They need to be maintained as they are easily corroded.
3. If not properly maintained, the wire baskets become unsightly and pose a threat.
Beach Nourishment / Refers to the constant replenishment of large quantities of sand to the beach system, which leads to the improvement of beach quality and storm protection. / However it is expensive to constantly transport sand to fill up the beach as it is continuously eroded. Coral reefs may be in danger as the sand covers the corals. It can also led to muddy and polluted water.
Relocation of Property / 1. Coastal planners are trying to protect man-made structures by relocating them and let nature reclaim the beach slowly.
2. No building of new structures is allowed in coastal areas vulnerable to coastal erosion. / This approach is likely to be opposed by people who want to invest in the coastal areas.
Planting of Mangroves / 1. Mangroves can help to trap sediments and reduce coastal erosion. / 2. However some areas with violent destructive waves may not support mangroves.
3. The depth of the coast may also become shallower, affecting coastal transportation and port activities.
Coastal Dunes / 1. Coastal dunes can be found along the shore where lots of sand and strong winds exist. The winds carry and deposit the land on the coast further inland, forming coastal dunes gradually which help defend the coast from the sea. / 2. However they are very fragile and thus access points to the beach are controlled and designated.
4. Shrubs and trees are planted to stabilise the dunes to anchor the sand.
Growth of Coral Reefs / 1. They can weaken wave energy.
2. Artificial reefs are created along the coast to help enhance fishing opportunities, serve as undersea barriers and replace damaged coral reefs. / 3. Tourists can damage these coral reefs.
[1]The process in which material at the bottom of the cliff or river bank is eroded away. This may cause the upper portion of the cliff river bank to collapse.