River Erosion

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River Erosion

River erosion is the gradual removal of rock material from the river banks and bed.

A river erodes in the following ways:

Corrasion (abrasion): -

	The river uses its load to grind against the bed and sides
	The action would dislodge the materials and carry them away
	This process operates in 2 ways (verticle--> depth; lateral--> width)
	Potholes are the product of corrasion

Attrition:

	The loosened materials that are being carried away collide against the river sides and bed and against one another
	Over time, they would become smaller and eventually reduced to fine particles called silt

Solution (corrosion): -

The solvent action of water dissolves soluble materials and carry them away in solution

Hydraulic action: -

	The breaking down of rocks
	Removing and dragging rocks from the bed and banks of the river by the force of the running water (like a water jet)
	Usually there are lines of weakness like joints and cracks in the river
	The work of hydraulic action forms plunge pools (small lakes)

Potholes:

	Circular depressions on the river bed
	Formed by corrosion (abrasion)
	Most effective in flood conditions
	Pebbles which are trapped in hollows on the river bed are swirled about in turbulent/ fast flowing water

Plunge pools:

	A large depression at the foot of a waterfall
	This depression is deepened by hydraulic action of the plunging water
	Condition: likely to form when the water plunges into less resistant rocks
	It may be further enlarged by rock debris swirling about by turbulent water at the base

Rapids:

	A series of very short and fast falls
	Condition: an area of alternating bands of resistant and less resistant rocks

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Waterfalls: -

Vertical flows of fast moving water flowing from great heights

Formed by 2 ways: - Due to unequal resistance of rocks or faulting

	Unequal resistance- less resistant rocks are eroded more rapidly than resistant rocks
	As a result, there is a change in gradient
	By faulting- displacement of rocks results in a difference in height between 2 rocks, water plunges downwards

wpeB.jpg (6607 bytes) <Niagra Falls>

Gorges:

Valleys which are steep sided, deep and narrow

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The churning water at the base of the falls eats away the soft underlying rock layers of the gorge. The unsupported hard top rock then breaks off.

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