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Pure Geography - Rivers

1a. What are waterfalls?

• Waterfalls are sudden steep, vertical flows of fast-flowing water falling from great heights.

b. With well-labelled (annotated) diagrams, explain how waterfalls and plunge pools are formed.

• River flows from a band of resistant rock to less resistant rock.

• River erodes the less resistant rock more rapidly and causes a change in the gradient of the river course.

• Over time, the sudden steepening of the river bed from the resistant rock to less resistant rock causes the water to plunge down almost vertically, forming a waterfall.

• Repeat pounding of the river bed (hydraulic action) may leave a depression at the base of the waterfall.

• This depression, which is called a plunge pool, is deepened and enlarged by swirling rocks and boulders (abrasion).

• Examples of waterfalls – Niagara Falls, USA; Kota Tinggi Falls, P’Malaysia; Victoria Falls, Africa

• Waterfalls can also be formed by faulting.

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• One block of land can be uplifted relative to the adjacent block

• Displacement causes difference in height and results in water cascading down the fault scarp.

2. With well-labelled diagrams, explain how gorges are formed.

• The river flows across resistant rocks, it will erode its channel vertically, resulting in the formation of a narrow, steep-sided valley called a gorge.

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• Gorges can also be formed when a waterfall retreats upstream.

• At the base of the waterfall, the river undercuts the cliff face of the less resistant rocks.

• This results in an overhanging portion of rock losing support at the base and eventually collapses.

• As the waterfall retreats upstream, a long, narrow, deep valley called a gorge is formed.

• Examples of gorges – Rhine Gorge, Germany; Three Gorges, China

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3. Describe the shape of valley as the river moves from the upper course to the lower course.

� At the upper course of river, the gradient of the land is very steep. River tends to cut deep into the channel – vertical erosion, thus producing V-shaped, narrow and steep-sided valleys.

� At the middle course of the river, the gradient of the land is not very steep, compared to the upper course. Thus, the river erodes the channel at its sides – lateral erosion and producing a wider valley than the upper course.

� At the lower course of the river, the gradient is very gentle and thus lateral erosion rather than vertical erosion occurs. Valleys are thus broad and flat-bottomed.

� Examples of valleys: Rhine Valley, Germany; Ganges Valley, India

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4. With a well-labelled diagram, explain how floodplains and levees are formed.

• When there’s heavy rain, a river may overflow its banks and causes flooding.

• Once out of the river channel, the floodwater from the river spreads out over a larger wetted perimeter.

• The increased friction reduces the speed of the water, resulting in deposition of sediments.

• The larger-sized and coarser material is deposited first at the river bank, because of heavier weight.

• The smaller-sized and finer material is carried further away because they are lighter.

• Repeated flooding will cause the alluvial deposits to accumulate

• The lighter and finer material forms the floodplain, the wide and low-lying plain found on either side of the river.

• The heavier and coarser material forms a raised natural embankment, called levees.

• Examples – Mississippi, USA; Ganges, India

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5. With a well-labelled diagram, explain how slip-off slopes and river cliffs are formed.

• At the outer bank (concave bank), the speed of river is high due to lesser friction

• Thus the outer bank gets eroded. Undercutting of the bank results in an overhanging portion, which collapses to form a steep river cliff.

• The sediments removed from the outer bank are carried and deposited at the inner bank (convex bank), due to more friction at the bank.

• The continuous deposition of sediments at the inner bank makes the water shallower and encourages more deposition.

• The deposits build up and a gently-sloping slip-off slope is formed.

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6. With well-labelled diagrams, explain how ox-bow lakes are formed.

• The continuous erosion on the concave bank and deposition on the convex bank cause the formation of a very pronounced meander with two neighbouring concave banks getting closer, forming a loop.

• Over time, the narrow neck of land between the two neighbouring concave banks is finally cut through.

• A new straighter river channel and a cut-off are formed.

• When deposition finally seals off the cut-off from the river channel, a crescent-shaped lake called an ox-bow is formed.

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7a. Explain how deltas and distributaries are formed.

• As river approaches the sea, its speed decreases

• Thus it deposits its sediments are the river mouth. The load deposited exceeds the amount being swept away by tides, currents and waves.

• Over time, as layers and layers of alluvial material are deposited, a platform of alluvium is built up and it finally rises above the water, forming a flat alluvial platform called a delta.

• When this happens, river channel branches out to form many smaller channels known as distributaries.

• Examples: Yangtze Delta, China; Mekong Delta, Vietnam b. Explain the conditions favourable for the formation of a delta.

• There is active erosion along the course of the river.

• The river flows over less resistant rocks so that the weathered material and eroded rock particles can add to the load.

• There are tributaries which help to increase the load.

• There are no/few large lakes along the river to trap the load.

• There is gentle offshore gradient with shallow water, to encourage deposition of material

• The coastal waters are quiet and there are few or no strong currents and waves to remove the deposited sediment.

• The climate favours the fast growth of vegetation which binds and stabilizes the sediment.

- by Long LN (13/8/07)