Rivers: Profiles & Landforms

1. Rivers: Profiles & Landforms .......................

2. Source Upper Middle Lower of river Course Course Course River Profile

3. River speed Fastest?? Fastest?? Fastest??

4. Upper Course Lower Course

5. Contrasting river landforms from source to mouth Channel features Valley features Long profile

6. Upper Course - Channel features • River channel is rocky. • Covered with various shapes and sizes of boulder. • Discharge is low. • Under flood conditions rivers energy is expended on vertical erosion with hydraulic action and corrosion processes at work. • Potholes may form.

7. Vertical erosion

8. Pothole formation

9. V shaped valley Zig-zag bends (interlocking spurs) Steep sides Upper Course - Valley features • Valley sides are steep and form a ‘V’ shaped cross section. • Interlocking spurs.

10. V – shaped valley Form due to a combination of the following processes: Vertical erosion by the river itself. Physical weathering (eg: frost action) which provides debris to move down slope. Mass movement (inc: soil creep & landslides) to move debris down slope.

11. Interlockingspurs River flows around interlocking spurs

12. Upper Course – Long Profile • Generally the gradient is steep and the profile is uneven, particularly where waterfalls and rapids form.

13. Waterfall formation Soft rock is easy to erode, but the hard rock is resistant. So over time a ledge develops. Example Hard Rock – Lava Soft Rock – Sandstone or Conglomerates

14. Waterfall formation The water rushes over the ledge and erodes a plunge pool by abrasion and hydraulic action. Example Hard Rock – Lava Soft Rock – Sandstone or Conglomerates

15. Waterfall formation The ledge collapses into the plunge pool, where the debris helps to speed up the erosion. Example Hard Rock – Lava Soft Rock – Sandstone or Conglomerates

16. Waterfall formation The process is repeated and the waterfall gradually retreats upstream, carving out a gorge. Example Hard Rock – Lava Soft Rock – Sandstone or Conglomerates

18. WATERFALL RETREATS . . OVERHANG PLUNGE POOL UNDERCUTTING OF SOFT ROCK UPSTREAM . .

19. Resistant rock Less Resistant Rock Formation of rapids

20. Middle Course - Channel features • Channel is now wider and has smoother banks and bed compared to the upper course.

21. Middle Course - Valley features • River erosional energy is now increasingly expended horizontally rather than vertically. • Lateral erosion by the river’s meanders broadens the valley floor into a narrow flood plain. • Meanders gradually shift their course downstream.

22. Middle Course 1

23. Middle Course 2

24. Water flows faster on the outer curve of the bend (more energy), and slowest on the inner curve (less energy). So the outer bank gets eroded while material is deposited at the inner bank. Over time the outer bank gets worn away (river cliff) and the inner one builds up (river beach). The bend grows into a meander. Meanders A meander starts as a slight bend:

25. Meanders (refer to previous notes and diagrams) • Alternating series of irregularities develop • Pools – deeper stretches of slow moving water • Riffles – shallower section of faster flow, flowing above coarser material • River develops a winding or sinuous course • Faster flow on outer bend results in erosion and formation of River Cliff • Slower flow on inside of bend results in deposition and formation of Slip-off Slope

26. Meanders • Meanders develop and migrate laterally and downstream • Helicoidal flow further assists meander formation and transports sediment from river cliff to the slip-off slope on the inside of the next bend.

27. MEANDERS Most erosion on the outside of the bend .. Fastest flow Flood plain Possible break through point Meanders Possible ox-bow lake

28. Meanders

29. Middle Course - Profile

30. Middle Course - Profile • Gradient is now less steep. • Overall the profile is smooth – but an outcrop of resistant rock could still cause a waterfall.

31. Lower Course - Channel features • The channel is now at its broadest and deepest. • Bedload is carried entirely in suspension and is solution. • Deposition now dominates – particularly during floods. • Erosion also occurs – in the formation of meanders

32. Lower Course – Valley features • Thanks to lateral erosion the valley sides may now be several kilometres away. • Typically it may also contain the following features: • Floodplain & natural levées • Braided channels • Meanders • Oxbow lakes • Estuaries and deltas

33. Lower Course

34. Natural levées • As the river floods, sediment is dropped over all the flooded areas but most falls along the river channel itself. • This sediment raises the height of the banks is flooding occurs regularly • Levees themselves do not prevent flooding because as the banks are raised, more sediment is dropped on the river bed, raising the water level.

35. River is actually flowing above The floodplain !! Gentle valley sides Levees is often artificially strengthened Flat floodplain Coarse material Forms natural levees Layers of silt Deposited during floods

36. Braided channels • Formed by the choking of the main channel by the deposition of a considerable amounts of the river load. • The channel splits into several smaller channels which flow around fresh ‘islands’ of deposited material before rejoining.

37. Braided channels

38. Braided channels

39. Ox-bow lakes Downstream migration of meanders produce pronounced meander loops which may form ox-bow lakes during flood conditions 1 Lateral erosion on outside of bends of pronounced meander 2 Narrow neck of meander gradually becomes narrower 3 Neck is cut through by river during floods and river forms new straighter channel 4 Cut-off is sealed by deposition 5 Ox-bow lake begins to silt up (Core Higher, P 65, Fig 3.36)

40. An Oxbow lake starts as a meander. During a flood the river cuts across the meander, forming a new channel. Sediment is deposited along the sides of the new channel. The loops gets sealed off and an oxbow lake forms. The water in it becomes stagnant. The lake will remain sealed of until either the river floods into it or it dries out. Oxbow lakes

41. Oxbow lakes

43. NARROW MEANDER NECK FUTURE OX-BOW LAKE

44. Estuaries and deltas • An estuary is where a river meets the sea (it is tidal). • Deltas are essentially the seaward extension of the floodplain and form where tides are too weak to remove deposit sediment.

45. ……………. Thank YOU