River Landforms Flashcards
(3 cards)
1
Q
How is a waterfall formed?
A
- Alternating bands of resistant and lest resistant rock are subjected to different rates of vertical erosion in the upper course of the river
- Soft rock erodes rapidly as result of abrasion, solution and hydraulic action, producing a steepening effect which accelerates erosion
- Results in overhang of resistant cap rock which may eventually collapse due to lack of support
- Plunge pool is formed and deepened as result of abrasion and hydraulic action
- Continuous erosion of soft rock and subsequent collapse of cap rock causes waterfall to retreat upstream. Produces gorge.
2
Q
How is a meander formed
A
- Centrifugal forces + swing of flow around pools and riffles causes thalweg to be directed to outer banks to be eroded more quickly
- Outside of river=high velocity with water depth greater and friction lower. River competence is greater
- High energy of flow + position of thalweg= lateral erosion + undercutting of outer bank due to abrasion, solution and HA= River cliff created
- HELICOIDAL FLOW erodes deeper into the outer bend and transfers material to inside of meander bend
- Characterised by lower velocities due to higher level of frictional contact with river bed
- Less effective transportation of sediment as river capacity is reduced and deposition occurs in these low energy conditions
- Sediment aggradation= formation of point bar deposits and gentle slip off slope
3
Q
How are oxbow lakes formed
A
- Continuous shift of meander accomplished by erosion on outer bank
- Shallow low velocity flow on inside forms a distinctive slip off slope
- Abrasion + HA occurs on outside of the meander
- High current velocity may cause more rapid migration of the meander which may catch up with the loner loop
- Eventually river may break through neck of meander, shortening course of river, leaving meander loop abandoned as a “cut off”
- Deposition of alluvium eventually “seals off” former loop, producing a seasonal crescent shaped oxbow lake
