CARA'S TIPSDenudation: rivers, karst, wind, waves & glaciers; the hydrosphere
JAMBWAECNECO
What the syllabus asks
JAMB: Physical Geography B(iv), D, E: karst topography; erosion, transportation and deposition by running water, wind, waves and glaciers; landforms; oceans and seas (distribution, salinity, uses), ocean currents (causes, types, effects), lakes and lagoons (types, uses)
WAEC: Denudation: action of running water, underground water (karst), wind, waves and ice; resulting landforms; the hydrosphere: oceans and seas, ocean currents, lakes
NECO: Denudation: action of running water, underground water (karst), wind, waves and ice; resulting landforms; the hydrosphere: oceans and seas, ocean currents, lakes
What you will learn
- Explain the processes of erosion, transportation and deposition.
- Describe and explain the landforms of the upper, middle and lower courses of a river.
- Explain river capture, drainage patterns and the uses of rivers.
- Describe karst landforms and the work of underground water.
- Explain the work of wind in deserts and its landforms.
- Explain the work of waves, longshore drift and coastal landforms, with Nigerian examples.
- Describe glacial erosion and deposition landforms.
- Describe the distribution, salinity and uses of oceans and seas.
- Explain the causes, types and effects of ocean currents.
- Classify lakes by origin and state their uses.
Key points
- Denudation = weathering + mass movement + erosion; deposition occurs when an agent loses energy.
- River erosion: hydraulic action, abrasion, attrition, solution; transport: traction, saltation, suspension, solution.
- Upper course: V-valleys, interlocking spurs, waterfalls, gorges, rapids, potholes.
- Middle/lower course: meanders, ox-bow lakes, floodplains, levees, deltas, estuaries.
- Drainage patterns: dendritic, trellis, radial, centripetal, rectangular.
- Karst: clints, grikes, swallow holes, caves, stalactites, stalagmites, pillars.
- Wind: deflation and abrasion make mushroom rocks, zeugen, yardangs; deposits barchans, seifs, loess.
- Waves: cave → arch → stack → stump; cliffs and wave-cut platforms.
- Longshore drift builds beaches, spits, bars, tombolos and lagoons (Lagos Lagoon).
- Glaciers: cirques, arêtes, pyramidal peaks, U-valleys, hanging valleys; moraines, drumlins, eskers.
- Oceans cover ~71%; average salinity ~35‰; currents warm or cold, caused mainly by planetary winds.
- Lakes: tectonic, volcanic, glacial, erosional, depositional, inland drainage, man-made.
Start here: Denudation: erosion, transport and deposition
Stand by a river after a heavy storm and the water is brown. That colour is soil and rock being carried away. Slowly, over thousands of years, this lowers whole landscapes. Geographers call this denudation.
The four processes of river erosion
| Process | What happens |
|---|---|
| Hydraulic action | The force of water pushes into cracks and dislodges rock; air compressed in cracks bursts them open. |
| Abrasion (corrasion) | Pebbles and sand carried by the river scrape and grind the bed and banks, like sandpaper. |
| Attrition | The load particles knock against each other and become smaller and rounder. |
| Solution (corrosion) | Water dissolves soluble rocks such as limestone. |
The four ways a river transports its load
Why rivers deposit
- The speed falls: on reaching gentle slopes, a lake or the sea.
- The volume falls: in the dry season or through evaporation and seepage.
- The load increases suddenly: after a landslide.
- An obstacle slows the water: vegetation, a dam.
Distinguish between abrasion and attrition.
- Abrasion wears away the river’s bed and banks using the load as a tool.
- Attrition wears down the load itself as particles collide.
Show solution
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Practice questions
- upper course of a river
- lower course
- delta
- estuary
Show answer
A: upper course of a river
Vertical erosion dominates in the upper course.
- the river meets the sea
- the river is braided
- the gradient is very gentle
- a band of hard rock lies over softer rock
Show answer
D: a band of hard rock lies over softer rock
The soft rock is undercut, leaving a lip of hard rock.
- by wind erosion
- at the source of a river
- when a meander is cut off
- by a glacier
Show answer
C: when a meander is cut off
It is a feature of the middle and lower course.
- a glacier melts
- a river deposits sediment faster than waves and currents remove it
- a river cuts through hard rock
- waves erode a headland
Show answer
B: a river deposits sediment faster than waves and currents remove it
The Niger Delta is an example.
- one river erodes headwards and takes the headwaters of another
- a river floods
- a river dries up
- a dam is built
Show answer
A: one river erodes headwards and takes the headwaters of another
The captured stream leaves an elbow of capture.
Key words
- Denudation
- The general wearing down and lowering of the land surface.
- Erosion
- The wearing away and removal of rock by moving agents (water, wind, ice, waves).
- Deposition
- The laying down of material carried by an agent.
- Hydraulic action
- Erosion by the sheer force of moving water.
- Abrasion (corrasion)
- Wearing away of the bed and banks by the load scraping against them.
- Attrition
- The wearing down of the load itself as particles knock together.
- Solution (corrosion)
- Dissolving of soluble rocks by water.
- Load
- The material carried by a river, wind, glacier or waves.
- Interlocking spurs
- Ridges of high land that project alternately into a V-shaped valley.
- Meander
- A loop-like bend in a river.
- Ox-bow lake
- A horseshoe-shaped lake formed when a meander is cut off.
- Levee
- A raised bank of deposited material along a river channel.
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