Rivers GEO3112 2006 GE 0 3112 Sedimentary processes

Rivers GEO-3112 2006 GE 0 -3112 Sedimentary processes and products Lecture 6. Rivers Geoff Corner Department of Geology University of Tromsø 2006 Literature: - Leeder 1999. Ch. 17. Rivers.

Rivers GEO-3112 2006 Contents ► 6. 1 Introduction – importance of fluvial systems ► 6. 2 Fluvial channels ► 6. 3 Floodplains ► 6. 4 Fluvial architecture

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.

Rivers 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.

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

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

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

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

Rivers GEO-3112 2006 Channel size vs. discharge Channel width Discharge Q = whu Channel depth Mean flow velocity ► Discharge increases with increasing width, depth and velocity. ► Discharge, width, depth and velocity all increase downstream.

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

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 longprofile.

Rivers Downstream changes: Amazon R. GEO-3112 2006

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

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

Rivers ► Sinuosity GEO-3112 2006

Rivers ► Braiding GEO-3112 2006

Rivers ► Anastomosing GEO-3112 2006

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

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

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

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. ►

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

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

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

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

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

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

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

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

Rivers GEO-3112 2006

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

Rivers GEO-3112 2006

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

Rivers GEO-3112 2006 Anastomosing channels ► Vertical accretion dominates

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.

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

1945 Rivers GEO-3112 2006 1977 Changes 1945 - 1977

Rivers GEO-3112 2006 Crevasse splays Galloway & Hobday 1996

Rivers GEO-3112 2006

Rivers GEO-3112 2006

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.

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

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.

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

Rivers Depositional architecture and stacking patterns GEO-3112 2006

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

Rivers GEO-3112 2006 Fluvial architecture

Rivers GEO-3112 2006 Ancient fluvial deposits

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