1. GE0-3112 Sedimentary processes and products
Lecture 6. Rivers
Geoff Corner
Department of Geology
University of Tromsø
2006
Literature:
- Leeder Ch Rivers.

2. Contents 6.1 Introduction – importance of fluvial systems
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Contents
6.1 Introduction – importance of fluvial systems
6.2 Fluvial channels
6.3 Floodplains
6.4 Fluvial architecture

3. Importance of fluvial systems
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Importance of fluvial systems
1) Rivers are major erosive and sediment transport agents.
Fluvial sediments are mostly transient but may form thick deposits in several settings.
Fluvial deposits are sensitive palaeoenvironment indicators.

4. 1) Rivers are : erosive agents
GEO
1) Rivers are :
erosive agents
conduits for sediment transport to lacustrine and marine basins.

5. Rivers
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2) Fluvial sediments are mostly transient but form thick deposits in several settings:
coastal plains
intermontane basins
tectonic forelands
Modern and Holocene terraced fluvial deposits at Tana, N. Norway.

6. 3) Fluvial deposits are sensitive palaeoenvironment indicators:
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3) Fluvial deposits are sensitive palaeoenvironment indicators:
tectonic slope changes
sourceland geology
climate
sea-level change
Postglacial fluvial terraces at Porsanger, N. Norway

7. Fluvial channels Size and gradient Shape (form) Processes
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Fluvial channels
Size and gradient
Shape (form)
Processes
Bedforms and internal structures

8. Bankfull width Channel size is measured as bankfull width.
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Bankfull width
Channel size is measured as bankfull width.
Normal
Channel width
Bankfull

9. Channel size Size varies by four orders of magnitude:
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Channel size
Size varies by four orders of magnitude:
<2 m (small streams)
>20 km (Brahmaputra, Ganges).

10. Channel size vs. discharge
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Channel size vs. discharge
Channel width
Channel depth
Q = whu
Discharge
Mean flow velocity
Discharge increases with increasing width, depth and velocity.
Discharge, width, depth and velocity all increase downstream.

11. Width vs depth Depth (h) increases with increasing width (w).
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Width vs depth
Depth (h) increases with increasing width (w).
W/h ratios are higher in low-sinuosity rivers.
High sinuosity
(low w/h ratios)
Low sinuosity
(high w/h ratios)
NB: Symbols erroneously reversed

12. Long profile Downstream changes (in effluent streams):
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Long profile
Downstream changes (in effluent streams):
Discharge increases.
Gradient decreases (the flow is more efficient; with increased discarge the gradient must decrease to maintain equilibrium).
Graded river: concave long-profile.

13. Downstream changes: Amazon R.
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Downstream changes: Amazon R.

14. Tectonic disturbance of river profiles across the Himalayan front.
Rivers W
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Tectonic disturbance of river profiles across the Himalayan front. E

15. Channel shape Parameters for describing channel planform shape:
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Channel shape
Parameters for describing channel planform shape:
Sinuosity (P)
Braiding
Anastomosing
Channel types illustrating characteristics of sinuosity, braiding and anastomosing (Galloway & Hobday 1996).

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Sinuosity

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Braiding

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Anastomosing

19. Controls on channel shape
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Controls on channel shape
Sediment load
Stream power
Bank stability

20. Braiding on sandy substrate
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Braiding on sandy substrate
Meandering on clayey substrate

21. Channel variability Meandering Braided Gandak River, Nepal-India
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Channel variability
Meandering
Braided
Gandak River, Nepal-India

22. River confluences Deep scour at confluences.
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River confluences
Deep scour at confluences.
May be several times deeper than contributing tributaries.
Mobile scour-and-fill units at the base of a succession.

23. Jamuna-Ganges confluence, Bangladesh,
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Scour pool
Jamuna-Ganges confluence, Bangladesh,
Recent scours
Infilled scour

24. Depositional architecture and stacking patterns
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Depositional architecture and stacking patterns
Masjok, Tana

25. Channel sediment transport and bedforms
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Channel sediment transport and bedforms
bars (macrofoms)
dunes
ripples
Point bar
Mid-channel bar
Side bar
Dunes

26. Point bars and meanders
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Point bars and meanders
Helical flow around a meander bend

27. Point bar deposits Scroll bars
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Point bar deposits
Scroll bars
Lateral accretion (epsilon cross-stratification)

28. Fluvial point-bar depoits in the Spanish Pyrenees
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Fluvial point-bar depoits in the Spanish Pyrenees

29. Fining-upward point-bar successions
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Fining-upward point-bar successions

30. Channel bars Diffluence and confluence Downstream accretion Rivers
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Channel bars
Diffluence and confluence
Downstream accretion

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32. Planar cross-bedded unit with sigmoidal foresets (dune bedding), Tana
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Linguoid dunes, Tana
Planar cross-bedded unit with sigmoidal foresets (dune bedding), Tana

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34. Braided river succession, Masjok, Tana
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Braided river succession, Masjok, Tana

35. Anastomosing channels
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Anastomosing channels
Vertical accretion dominates

36. Floodplain Important processes: Overbank flooding
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Floodplain
Important processes:
Overbank flooding
Intermittent avulsion

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Floodplain deposits
Vertical accretion of overbank muds and organic sediment.
Lateral accretion on levees and crevasse splays.

38. Avulsion Avulsion site, c. 1870
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Avulsion
Avulsion site, c. 1870
Cumberland Marshes avulsion, Saskatchewan R.

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1945
1977
Changes

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Crevasse splays
Galloway & Hobday 1996

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42. Rivers
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43. Avulsion and channel belts
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Avulsion and channel belts
Sudden shift in channel reach (bend cutoff) or whole channel belt.
Controlled by internal (autocyclicity) or external factors (base-level, climate, tectonics).
Diversion more likely during extreme flood events or fault movement.

44. Channel belts Palaeochannels of the Holocene Rhine-Meuse.
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Channel belts
Palaeochannels of the Holocene Rhine-Meuse.
Stacking patterns – fluvial architecture.

45. Incision – aggradation cycles
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Incision – aggradation cycles
Regional cycles of incision and aggradation may occur on the scale of decades or more.
Causes may be ’intrinsic’ or extrinsic, e.g:
water and sediment discharge variations controlled by climate and catchment characteristics (e.g. ENSO).
eustatic sea level changes.
tectonics.

46. Fluvial incision and knickpoints
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Fluvial incision and knickpoints
Fall in relative sea-level causes upstream knickpoint migration.

47. Depositional architecture and stacking patterns
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Depositional architecture and stacking patterns

48. Depositional architecture and stacking patterns at Tana
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Depositional architecture and stacking patterns at Tana
Masjok, Tana

49. Rivers
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Fluvial architecture

50. Ancient fluvial deposits
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Ancient fluvial deposits

51. Further reading Cf. Colloquim literature on fluvial deposits. Rivers
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Further reading
Cf. Colloquim literature on fluvial deposits.