The Formation of Meanders, Oxbox Lakes and Waterfalls By Faye Ellames, Tom Wales and Dasha Plotnikova
Contents Waterfalls By Dasha Plotnikova Meanders By Faye Ellames Oxbow Lakes By Tom Wales Glossary
Waterfalls By Dasha Plotnikova
What is a waterfall? • A waterfall is a steep descent of water from a height. • They are beautiful water features found in the upper-course of a river. • In Venezuela, you can find the highest waterfall in the world: Angel.
How are waterfalls formed? • Found in the upper-course of a river, waterfalls begin to form when a river flows over a layer of soft rock with a stratum of harder rock in between. • As the river flows over the soft rock, it begins to erode it – deepening the river bed at (B). • Here, rapids may be found as the river flows very quickly over rocks and boulders.
Soon, a very steep back-wall is will form as the soft rock is eroded further. • Now, the waterfall begins to take it’s shape.
The water will ware away the soft rock further and form a plunge pool at the bottom (D). • Some of the hard rock is undercut and forms an overhang which eventually falls into the plunge pool. Due to hydraulic action, corrosion and corrasion combined, the hard rock erodes the bottom of the pool making it deeper.
As this activity continues; the rocks and water eroding the back-wall and hard rock collapsing into the plunge pool, the waterfall will retreat leaving a gorge. • Niagara Falls is retreating at the rate of one metre per year.
Questions • In which part of the river are waterfalls usually found? • Upper-course • Middle-course • Lower-course • What is the name given to the wall that is found behind the waterfall? • Spine-wall • Foot-wall • Back-wall
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Meanders By Faye Ellames
What is a meander? • Rivers flow through valleys and flood plains. • In time they start to curve and bend round the valleys to get downstream and to the sea faster.
When doing this they wear away land, leave soil behind and make bends in the river, known as meanders. • Meanders are normally found in the middle or lower course of a river. • In the next few slides I will explain how a meander is made.
Rivers are eroding all the time. • They erode downwards so the river is deepened. • The deeper the river, the fastest it flows. • The fastest flow is on the outside of the bend. • The water erodes the rocks on the outside of the river.
Over time this will make river cliffs. • It then carries these rocks down the river, making erosion faster.
On the inside of the river, the complete opposite is happening. • The flow is slower, so the rocks aren’t being eroded off as much. • All the bits of rock that have been eroded off are deposited (left) on the bottom of the river where it’s the shallowest (which then forms a river beach).
Questions • In which section of the river are meanders found? • Upper-course and middle-course • Middle-course and lower-course • Lower-course and upper-course • What are meanders? • Well developed river bends • A canal designed for carrying water for drinking purposes • An underwater ridge of sand and gravel
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Oxbow Lakes By Tom Wales
What is an Oxbow Lake? • An oxbow is a crescent-shaped lake lying alongside a winding river. The oxbow lake is created over time as erosion and deposits of soil change the river's course.
Stage 1 • The water in a river flows fastest at the outside of a river bend (A) and erode the outside bank (B). Where the water follows slower on the inside of the river band, it deposits material! (mud, sand and shingle) (C) The river bank on the outside is eroded into a river cliff at (B) and the Area of deposition at (C) is called a slip-off slope!
Stage 2 • Erosion continues on the outside of each bend (D), (E) and (F). Deposition continues on the inside of the river bend (G). the meander continues to change slowly over time.
Stage 3 • Erosion continues on the outside of the river bends until only a narrow neck of land is left at (H).
Stage 4 • The meander and river channel move across the flood plain by the combined effects of erosion on the outside of the meander and deposition on the inside creating new land at (I).
Stage 5 • The narrow neck of land is broken through (often during flooding) at (J). An island (L) is created by the old meander. The main flow of the river now continues straight on at (J) so the old river bend has been "cut off".
Stage 6 • Deposition now continues at (K).
Stage 7 • The old "cut off" meander now forms an ox-bow lake at (O).
Stage 7 • Deposition at (N) has completely cut off the water in the ox-bow from the main channel.
Stage 8 • Usually deposition, plants matter and the gradual collapse of the banks will fill up the ox-bow which may only be seen in times of flood in the future.
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Glossary Corrasion Fine material rubs against the river bank. The bank is worn away by a sand-papering action called abrasion, and collapses. Click here to return to the slide Corrosion Rocks forming the banks and bed of a river are dissolved by acids in the water. Click here to return to the slide Hydraulic Action The sheer force of water hitting the banks of a river Click here to return to the slide Stratum A layer of rock Click here to return to the slide