RiversMain processes of river erosion in the upper course.Erosion is the wearing away of the land. There are different types of erosion:(i) Hydraulic action: This process involves

the force of water against the bed and banks

-Hydraulic power is high below waterfall and rapids where it will cause rocks to fragments especially when joints and bedding planes, and lines of weakness are present.

-Outside bend of river, Hydraulic action lead to undercutting and collapse of river banks to form cliffs

(ii) Abrasion/Corrasion: This is the process by which the bed and banks are worn down by the rivers load. The river throws these particles against the bed and banks, sometimes at high velocity

When active: River at bankfull/floodHow it looks like: The river appears brown, charged with sand and silt.(iii) Attrition : Material (the load) carried by

the river bump into each other and so are

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smoothed and broken down into smaller particles.

When river flows over bedrock the erosion of bedrock most effective by corrasion and result in pot holes.

When a river flows over alluvial channel erosion is effective by hydraulic action.

(iv) Corrosion This is the chemical action of river water. The acids in the water slowly dissolve the bed and the banks.

-Most active on rocks that contain carbonates such as limestone and chalk and silicate such as quartz.

Erosion of river channel:1. Lateral erosion :

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It occurs when a river does not follow a straight path but meanders, swinging from side to side.

The strongest current flows around the outside of the bend and hydraulic action and corrosion cause the riverbanks to be undermined and collapse.

It is most active where a river transports a large load or during short-lived flood under desert conditions.

Valley widening is due more to weathering and slope transport than lateral erosion by rivers.

2. Vertical erosion: Characteristic of fast-flowing rivers that

are transporting a large bedload of coarse, hard particles.

The particles abrade and pothole causing rapid lowering of river bed.

Such rivers tend to flow in deep, narrow gorges, as resistant rocks of the valley slopes restrict weathering of slopes either side.

3. Headward erosion:

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This process occur at the source of the river or at point where the long profile of the river is locally steep, e.g. at a waterfall, where the gradual retreat of the waterfall takes place.

In the upper course,

o The river channel is small, narrow and rough and usually shallow.

o The stones and rocks increase wetted perimeter.

o The volume of water is low as there are very few tributaries flowing into it.

What Landforms occur in the upper course of a river?1. V-shaped valleys and interlocking spurs2. Rapids3. Waterfalls

How are these landforms created?

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Why do V-Shaped valleys occur?1. In upper course of a river vertical

erosion dominates as the stream cuts downwards.

2. Weathering and erosion on the valley sides removes material cause valley sides to retreat forming a V-shaped valley.

3. This material moves downslope and it may eventually enter the stream channel where over time river will erode and transport it downstream.

4. If the river removes the material transported downslope (by slope processes) more quickly than material is provided then a steeper valley is produced.

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The steepness of the valley sides depends upon several factors:(i) Climate -Valley is steeper when there is

sufficient rainfall; for mass movement to occur, create enough discharge to transport bedload and erode vertically or river cross desert areas to wash down valley sides e.g. Grand Canyon.

(ii) Rock structure - Resistant, permeable rocks such as limestone produce vertical sides in contrast impermeable rocks such

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as clay are likely to produce more gentle slopes.

(iii) Vegetation - Vegetation help to bind soil together and thus keep hillslope more stable.

Interlocking spur:Look at the two photograph and answer the questions given below:

Watkins Glen, New York State, USA.

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Tributary valley dissecting the Long Mynd

Qn:1. (a) Compare the photographs shown

in Figures 2 and 3.

(b) Suggest reasons for the differences you observe.

Interlocking Spurs

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In the upper course the river does not have a huge amount of energy to erode as it does not have a high discharge and it has to transport large pieces of sediment.

When the river meets areas of harder rock that are difficult to erode it winds around them. A series of hills form on either side of the river called spurs. As the river flows around these hills they become interlocked. So, a series of interlocking spurs are often found in the upper course of a river valley.

Waterfalls and rapids:1. Waterfalls and rapids occur where

the long profile of a river is steep.2. This is usually the result of an

outcrop of more resistant rock, often called the cap rock, overlying a softer rock.

3. Erosion, especially by Hydraulic power of the water, is concentrated in the plunge pool at the base of the waterfall.

4. The waterfall becomes undercut and the hard cap rock above periodically

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collapses resulting in headward erosion of the waterfall and the formation of a gorge of recession.

Case study-Iguaçu Falls, Brazil.The Iguaçu River, a tributary of the Parana, forms part of the border between Brazil and Argentina. At one point along its course the Iguaçu plunges 80m over a 3 km wide crescent-shaped precipice. The Iguaçu Falls occur where the river leaves the resistant basaltic lava which forms the southern edge of the Brazilian plateau and flows onto less resistant rock, wile their crescent shape results from the retreat of the falls upstream.

By the end of the rainy season (January/February) up to 4 million litres of water a day can pour over the individual cascades-numbering up to 275-which combine to form the falls. The main attraction is the Devil’s Throat where 14 separate falls unite to create a deafening noise, volumes o spray, foaming water and a large rainbow. In contrast, by the end of the dry season (June/July) river levels may be very low-indeed, for one month in 1978 it actually dried up.

POTHOLES:1. Active corrosion along a stream bed produces potholes especially

in fast-flowing rivers with strong eddying.

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2. Potholes are cylindrical holes drilled into the bedrock by turbulent high velocity flow.

3. The eddying creates a shallow bowl that may become occupied by small stones and pebbles.

4. The constant swirling of the pebbles deepens the depression into pothole in a process called pothole drilling.

5. Adjacent potholes join together creating sudden and deepening of channels such as at the Strid on the River Wharfe, in North Yorhshire.

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Key Ideas(i) Erosion is the main process operating in the upper course of a river

(ii)The direction of erosion is vertical

(ii)There are four main types of erosion-hydraulic action, attrition, abrasion and corrosion.

(iv)Valleys are v-shaped with interlocking spurs.

(v)Waterfalls are formed where a river meets a band of less resistant rock. Plunge pools and gorges are features associated with the formation of waterfalls.

(vi)Rapids are smaller scale features formed where finer bands of varying resistance of rock are found.

(vii)Potholes are cylindrical holes drilled into the bedrock by turbulent high velocity flow by active corrosion along a stream bed, especially in fast-flowing rivers with strong eddying. Adjacent potholes join together creating sudden and deepening of channels

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