Meanders form through the process of helicoidal flow. At bends in the river, centripetal force throws water towards the outer bank, eroding and undercutting it. Slower flow on the inside of bends leads to deposition and the formation of point bars. Helicoidal flow causes eroded material to be deposited downstream in the next meander loop, furthering the meandering shape over time as the river seeks the most efficient path.

1. Meander Formation

4. Learning Outcomes You will be able to explain the sequence involved in the formation of a meander. You will be able to explain the sequence using the key terminology. KEY WORDS – helicoidal flow sinousity thalweg centripetal force

6. Meander Formation Riffles and Pools Riffle – a shallow part of the channel Pool – a deep part of the channel On average pools occur 5-6 times the width of the river bed

7. Meander Formation The riffle reduces the hydraulic radius of that section of the channel as the water flows inefficiently over it.

9. Asymettrical shape

10. The meandering Thalweg Thalweg – the line of fastest flow in a river

11. Centripetal Force At times of high flow the water is thrown around the riffles and is thrown to the edge by centripetal force eroding and undercutting it. (MAINLY ABRASION AND HYDRAULIC ACTION)

12. Deposition Slower flow on the inside of the bend leads to depostion there. It is slower because the reduction in the rivers efficiency being reduced due to the frictional contact with the river bed.

13. Helicoidal Flow The movement of water in a meander has been found to move from the outside to the inside. This helicoidal flow allows material eroded from the outer bank to be moved to the point bar on the inside of the next meander downstream.

14. Helicoidal Flow Corkscrew motion In bends, water forced to outside of bend hits bank with force Water piles-up on outside of bend Forced downward along river cliff, back towards point bar Transfers and reverses motion around next alternating bend

16. Exiting the meander As the water exits the meander the channel becomes shallower. This reduces the hydraulic radius thus increasing deposition. As such new riffles are created.

18. Sinuosity Actual channel length Straight Line Distance A sinuosity of 1.5 is the dividing line between a straight and meandering channel. = sinousity

19. Point Bar River Cliff Thalweg

20. Thalweg Deposition Point Bar Helicoidal Flow River Cliff Abrasion Hydraulic Action Riffle Pool Sinuosity Meander Centripetal Force

21. Explain the formation of a meander (8 marks)

24. Meander Formation Explain the formation of a meander using helicoidal flow sinousity thalweg centripetal force