GE0-3112 Sedimentary processes and products Lecture 6. Rivers Geoff Corner Department of Geology University of Tromsø 2006 Literature: - Leeder 1999. Ch. 17. Rivers.

Contents 6.1 Introduction – importance of fluvial systems Rivers GEO-3112 2006 Contents 6.1 Introduction – importance of fluvial systems 6.2 Fluvial channels 6.3 Floodplains 6.4 Fluvial architecture

Importance of fluvial systems Rivers GEO-3112 2006 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.

1) Rivers are : erosive agents GEO-3112 2006 1) Rivers are : erosive agents conduits for sediment transport to lacustrine and marine basins.

Rivers GEO-3112 2006 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.

3) Fluvial deposits are sensitive palaeoenvironment indicators: Rivers GEO-3112 2006 3) Fluvial deposits are sensitive palaeoenvironment indicators: tectonic slope changes sourceland geology climate sea-level change Postglacial fluvial terraces at Porsanger, N. Norway

Fluvial channels Size and gradient Shape (form) Processes Rivers GEO-3112 2006 Fluvial channels Size and gradient Shape (form) Processes Bedforms and internal structures

Bankfull width Channel size is measured as bankfull width. Rivers GEO-3112 2006 Bankfull width Channel size is measured as bankfull width. Normal Channel width Bankfull

Channel size Size varies by four orders of magnitude: Rivers GEO-3112 2006 Channel size Size varies by four orders of magnitude: <2 m (small streams) >20 km (Brahmaputra, Ganges).

Channel size vs. discharge Rivers GEO-3112 2006 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.

Width vs depth Depth (h) increases with increasing width (w). Rivers GEO-3112 2006 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

Long profile Downstream changes (in effluent streams): Rivers GEO-3112 2006 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.

Downstream changes: Amazon R. Rivers GEO-3112 2006 Downstream changes: Amazon R.

Tectonic disturbance of river profiles across the Himalayan front. Rivers W GEO-3112 2006 Tectonic disturbance of river profiles across the Himalayan front. E

Channel shape Parameters for describing channel planform shape: Rivers GEO-3112 2006 Channel shape Parameters for describing channel planform shape: Sinuosity (P) Braiding Anastomosing Channel types illustrating characteristics of sinuosity, braiding and anastomosing (Galloway & Hobday 1996).

Rivers GEO-3112 2006 Sinuosity

Rivers GEO-3112 2006 Braiding

Rivers GEO-3112 2006 Anastomosing

Controls on channel shape Rivers GEO-3112 2006 Controls on channel shape Sediment load Stream power Bank stability

Braiding on sandy substrate Rivers GEO-3112 2006 Braiding on sandy substrate Meandering on clayey substrate

Channel variability Meandering Braided Gandak River, Nepal-India Rivers GEO-3112 2006 Channel variability Meandering Braided Gandak River, Nepal-India

River confluences Deep scour at confluences. Rivers GEO-3112 2006 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.

Jamuna-Ganges confluence, Bangladesh, Rivers GEO-3112 2006 Scour pool Jamuna-Ganges confluence, Bangladesh, 1993-96. Recent scours Infilled scour

Depositional architecture and stacking patterns Rivers GEO-3112 2006 Depositional architecture and stacking patterns Masjok, Tana

Channel sediment transport and bedforms Rivers GEO-3112 2006 Channel sediment transport and bedforms bars (macrofoms) dunes ripples Point bar Mid-channel bar Side bar Dunes

Point bars and meanders Rivers GEO-3112 2006 Point bars and meanders Helical flow around a meander bend

Point bar deposits Scroll bars Rivers GEO-3112 2006 Point bar deposits Scroll bars Lateral accretion (epsilon cross-stratification)

Fluvial point-bar depoits in the Spanish Pyrenees Rivers GEO-3112 2006 Fluvial point-bar depoits in the Spanish Pyrenees

Fining-upward point-bar successions Rivers GEO-3112 2006 Fining-upward point-bar successions

Channel bars Diffluence and confluence Downstream accretion Rivers GEO-3112 2006 Channel bars Diffluence and confluence Downstream accretion

Rivers GEO-3112 2006

Planar cross-bedded unit with sigmoidal foresets (dune bedding), Tana Rivers GEO-3112 2006 Linguoid dunes, Tana Planar cross-bedded unit with sigmoidal foresets (dune bedding), Tana

Rivers GEO-3112 2006

Braided river succession, Masjok, Tana GEO-3112 2006 Braided river succession, Masjok, Tana

Anastomosing channels Rivers GEO-3112 2006 Anastomosing channels Vertical accretion dominates

Floodplain Important processes: Overbank flooding Rivers GEO-3112 2006 Floodplain Important processes: Overbank flooding Intermittent avulsion

Rivers GEO-3112 2006 Floodplain deposits Vertical accretion of overbank muds and organic sediment. Lateral accretion on levees and crevasse splays.

Avulsion Avulsion site, c. 1870 Rivers GEO-3112 2006 Avulsion Avulsion site, c. 1870 Cumberland Marshes avulsion, Saskatchewan R.

Rivers GEO-3112 2006 1945 1977 Changes 1945 - 1977

Rivers GEO-3112 2006 Crevasse splays Galloway & Hobday 1996

Rivers GEO-3112 2006

Rivers GEO-3112 2006

Avulsion and channel belts Rivers GEO-3112 2006 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.

Channel belts Palaeochannels of the Holocene Rhine-Meuse. Rivers GEO-3112 2006 Channel belts Palaeochannels of the Holocene Rhine-Meuse. Stacking patterns – fluvial architecture.

Incision – aggradation cycles Rivers GEO-3112 2006 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.

Fluvial incision and knickpoints Rivers GEO-3112 2006 Fluvial incision and knickpoints Fall in relative sea-level causes upstream knickpoint migration.

Depositional architecture and stacking patterns Rivers GEO-3112 2006 Depositional architecture and stacking patterns

Depositional architecture and stacking patterns at Tana Rivers GEO-3112 2006 Depositional architecture and stacking patterns at Tana Masjok, Tana

Rivers GEO-3112 2006 Fluvial architecture

Ancient fluvial deposits Rivers GEO-3112 2006 Ancient fluvial deposits

Further reading Cf. Colloquim literature on fluvial deposits. Rivers GEO-3112 2006 Further reading Cf. Colloquim literature on fluvial deposits.