Rivers And Valleys

Question 1

Explain the formation of a V - Shaped valley?

Answer 1

• The river starts high up in the valley, where it flows quickly eroding the landscape vertically. The river cuts a deep notch into the landscape by hydraulic action, abrasion and attrition.
• As the river eroded vertically the valley sides become exposed to weathering like freeze-thaw, causing the rocks to become loose and fall which steepens the valley sides.
• Rocks that fall into the river help the process of abrasion, causing further erosion and steepening the valley sides
• As rocks continue to fall into the river they are transported down stream by the transportation processes, causing the channel to become deeper and wider. This creates a V - Shaped valley between interlocking spurs.

Question 2

Explain the formation of an ox-bow lake?

Answer 2

• An oxbow lake begins with a meander when the river is eroding laterally (side to side). The river velocity is faster on the outside bend, meaning that erosion occurs here through hydraulic action and abrasion.
• However on the outside bend river velocity is slower, meaning that deposition occurs here
• Over time continued erosion on the outer bends causes the neck of the meanders to narrow
• During a time of floods the river has more energy, meaning that it can cut through the narrow neck of the meanders. As the river is fastest in the middle of the deep channels and slower at the outer banks, it deposits any rocks that it was carrying. This causes the old meander to be cut off, forming a crescent shaped ox-bow lake

Question 3

Explain the formation of a waterfall?

Answer 3

• Usually forms in the upper course of the river
• The river flows over bonds of more and less resistant rock where the less resistant rock eroded faster.
• A plunge pool forms in an over deepened area caused by hydraulic action on the less resistant rock
• The river undercuts the harder rock leaving behind an overhang. River then forms a plunge pool below the waterfall.
• The overhang is unsupported and collapses into the plunge pool as it is pulled down by gravity. The collapsed rocks erode the plunge pool further by abrasion.
• The waterfall retreats upstream leaving behind a gorge

Question 4

What is the formation of a levee?

Answer 4

• The river is on its floodplain with a very low gradient.
• The river is mainly depositing (silt and sand).
• When the river floods a lot of silt is deposited on the river banks and much less on the floodplain.
• Frequent flooding continues this build up.
• When the river is flowing normally it deposits in the river bed.
• This raises the river above the floodplain.
• Usually the built-up levees protect the plain from flooding