Work of Rivers
In the rainy season water flowing down a hill cuts a channel in the soil. Many such channels join to form a stream and then a river. The river wears away soil on the way, carries it and drops it further ahead.
Every external agent does three works: erosion, transport and deposition. A river erodes near its source, transports in the middle and deposits on plains and near the sea.
Key idea 1 : Factors affecting the work — The work of a river depends on the slope of the land, type of rock, volume of water, length of flow and amount of sediment.
Key idea 2 : Erosional work — A river starts at great height where its speed is high, so its power to erode is great. Fast water, sand, pebbles and tributaries wear away the bed and banks. This makes gorges (canyons), V-shaped valleys and waterfalls.
Key idea 3 : Transport and deposition — At the foothills the slope changes and coarse sediment is dropped in a triangular shape called an alluvial fan. When the slope lessens the river flows slowly and bends even for small obstacles; these are meanders. In floods natural levees and flat flood plains form on the banks and a delta forms near the sea.
- Easy : Near the source is the river’s speed high or low? — High.
- Medium : Triangular deposit at the foothills? — Alluvial fan.
- Exam-type : Formed by silt near the sea? — Delta.
In the figure: Landforms along a river’s path.
Source = erosion; middle = transport; end = deposition.
A delta is not made by erosion; it is made by deposition of silt.
Memory clue: Top to bottom: erosion → transport → deposition. Memory clue: "Ero-Tra-Dep".
Spot the difference
| Part | Work | Landforms |
|---|---|---|
| Source | Erosion | Gorge, V-valley, waterfall |
| Middle | Transport and deposition | Alluvial fan, meanders |
| Mouth | Deposition | Levees, flood plain, delta |
Practice questions
- 1Which three works do external agents do?
- 2State factors affecting the work of a river.
- 3Why does a river erode more near its source?
- 4State landforms made by erosional work.
- 5How is an alluvial fan formed?
- 6Why does a river form meanders?
- 7Konkan has creeks but no deltas; state a likely reason.
- 8Explain the link between levees and flood plains.
Show answers (try them yourself first)
- 1Erosion, transport and deposition.
- 2Slope, rock type, water, length, sediment.
- 3Because slope and speed are high there.
- 4Gorges, V-valleys, waterfalls.
- 5At the foothills slope drops and coarse sediment is dropped in a triangle.
- 6Because of the gentle slope she avoids small obstacles.
- 7Konkan rivers are short and fast so silt goes into the sea.
- 8In floods silt piles up as levees with a flood plain beyond.
Work of Glaciers
When ice keeps piling up in a freezer the lower ice is pressed hard. In snow regions too layers of snow pile up and by their weight the ice slowly creeps down the slope.
A huge mass of ice slowly moving down a slope is a glacier. Like a river, a glacier also erodes, transports and deposits.
Key idea 1 : How a glacier forms — In regions where temperature stays below freezing, precipitation is snow. The weight of upper layers and friction melt the ice at the base and the glacier starts to move. Its speed is 1 cm to 1 m a day.
Key idea 2 : Erosional work — Though slow, the mass of ice is huge so a glacier erodes its bed and banks greatly. This makes cirques, sharp ridges (arêtes), horns, U-shaped valleys, hanging valleys and sheepback rocks (roches moutonnées).
Key idea 3 : Transport and deposition — The sediment a glacier carries is called moraine. By position there are four types: ground, lateral, medial and terminal. Deposition makes drumlins (upturned-boat-shaped hills) and eskers (long winding ridges).
- Easy : If a glacier moves 30 cm a day, how much in 10 days? — 3 m.
- Medium : Which moraine is at the very end? — Terminal moraine.
- Exam-type : Valley made by a glacier is shaped like? — U.
In the figure: Erosion and deposition by glaciers.
The Jakobshavn glacier of Greenland is one of the fastest in the world, moving about 46 m a day. (As per the textbook; speed changes with time)
Ice is heavy so erosion is big; moraine = carried sediment.
A glacier is not still; it moves daily though not as fast as a river.
Memory clue: U valley = glacier; V valley = river. Memory clue: "U for Ice, V for flowing water".
Spot the difference
| Item | River | Glacier |
|---|---|---|
| Valley shape | V shape | U shape |
| Speed | High | Very low |
| Sediment | Levees, delta | Moraine, drumlin |
Practice questions
- 1What is a glacier?
- 2How does a glacier start to move?
- 3State landforms made by glacier erosion.
- 4What is moraine? State its types.
- 5State the difference between a glacier valley and a river valley.
- 6Why does a glacier erode a lot though slow?
- 7Where are glaciers found in India?
- 8A glacier moves 40 cm a day; how far in 15 days?
Show answers (try them yourself first)
- 1A huge mass of ice moving down a slope.
- 2Ice at the base melts due to weight and friction.
- 3Cirque, arête, horn, U-valley, hanging valley.
- 4Carried sediment; ground, lateral, medial, terminal.
- 5Glacier valley is U-shaped; river valley is V-shaped.
- 6Because the mass of ice is huge.
- 7In the high Himalayas.
- 86 m.
Work of Wind and Sea Waves
In places like a desert a strong wind blows sand everywhere. On a sea coast waves hit and gnaw rocks and leave sand behind. In both places nature works like a sculptor.
Wind work is prominent in deserts and semi-arid regions. In coastal areas sea waves do erosion, transport and deposition.
Key idea 1 : Erosion by wind — The wind carries sand and small pebbles which rub and wear rocks on the way. This forms mushroom rocks, deflation hollows and yardangs.
Key idea 2 : Deposition by wind — Particles carried by wind are of different sizes. Fine ones go far and heavy ones drop near. Sand dunes, barchans, seifs, ripple marks and loess plains are landforms of wind deposition.
Key idea 3 : Work of sea waves — Waves carry stones, pebbles and sand and strike rocks; this forms sea cliffs, sea caves, sea arches and wave-cut platforms. The eroded material is brought to the coast and deposited where waves are weak, forming beaches, sand bars and lagoons.
- Easy : A mushroom-like rock worn by sand? — Mushroom rock.
- Medium : Do fine particles go far or drop near? — Go far.
- Exam-type : Which landforms do waves build by deposition? — Beach, sand bar, lagoon.
In the figure: Erosion-deposition by wind and waves.
Cliff-cave-arch = erosion; beach-bar = deposition.
Wind work is not equal everywhere; it is prominent in deserts and semi-arid regions.
Memory clue: Erosion = carved; deposition = piled. Memory clue: "Carved is erosion, piled is deposition".
Spot the difference
| Agent | Erosional landforms | Depositional landforms |
|---|---|---|
| Wind | Mushroom rock, yardang | Dunes, barchan |
| Sea waves | Sea cliff, cave, arch | Beach, sand bar, lagoon |
Practice questions
- 1In which regions is wind work prominent?
- 2State landforms formed by wind erosion.
- 3State landforms formed by wind deposition.
- 4State landforms formed by wave erosion.
- 5Where and why does a beach form?
- 6Explain the sequence of sea cliff, cave and arch.
- 7Why is wind work seen more in deserts?
- 8Find one landform made by waves in a coastal area near you.
Show answers (try them yourself first)
- 1In deserts and semi-arid regions.
- 2Mushroom rocks, deflation hollows, yardangs.
- 3Dunes, barchans, seifs, ripple marks, loess.
- 4Sea cliff, cave, arch, platform.
- 5Where waves are weak, sediment is deposited.
- 6Waves hit the cliff, first making a cave, then an arch.
- 7Because mechanical weathering is high and obstacles few.
- 8E.g. a sea cave or beach on the Konkan coast.
Work of Groundwater
Rain water seeps into the ground and gathers in the well. In summer the well water goes deeper and in the rainy season it rises. This work of water under the ground never stops.
Rain water seeps through cracks or porous rocks and collects on a non-porous layer; this is groundwater. Its upper surface is the water table.
Key idea 1 : Erosion and deposition — Groundwater dissolves soluble minerals of rock and carries them; this is its erosion. When water evaporates or dissolved matter becomes too much, deposition starts. This forms sinkholes, limestone caves and hanging and rising lime pillars (stalactites and stalagmites).
Key idea 2 : Water table — The water table changes with season, porosity of rock and amount of rain. It is near the ground in the rainy season and deeper in summer.
Key idea 3 : Limestone region — In limestone regions groundwater work is more effective because limestone dissolves easily. Landforms of such regions are called karst landforms.
- Easy : The water table in the rainy season? — Near the ground.
- Medium : A pillar hanging from a cave roof? — Stalactite.
- Exam-type : A pillar rising from the floor? — Stalagmite.
In the figure: How groundwater shapes landforms.
Hanging from above = stalactite; rising from below = stalagmite.
Groundwater does not work the same in all rocks; it is more in soluble rocks like limestone.
Memory clue: C = Ceiling = Stalactite; G = Ground = Stalagmite. Memory clue: "Tites hold tight to the ceiling".
Spot the difference
| Landform | Work | Place |
|---|---|---|
| Sinkhole | Erosion | Limestone region |
| Stalactite | Deposition | Cave roof |
| Stalagmite | Deposition | Cave floor |
Practice questions
- 1What is groundwater?
- 2What is the water table?
- 3How does the water table change with season?
- 4Which landforms are formed by groundwater erosion?
- 5State the difference between stalactite and stalagmite.
- 6Why is groundwater work more in limestone regions?
- 7Which season is suitable to dig a well? Give a reason.
- 8Give one landform each for river, glacier, wind, waves and groundwater.
Show answers (try them yourself first)
- 1Seeped water collected on a non-porous layer.
- 2The upper surface of groundwater.
- 3Near in rainy season, deep in summer.
- 4Sinkholes and limestone caves.
- 5Stalactite hangs from the roof; stalagmite rises from the floor.
- 6Because limestone dissolves easily in water.
- 7End of summer, as water table is lowest then and water lasts into the rains.
- 8Alluvial fan/delta; U-valley; barchan; beach; stalactite.