Presentation on theme: "GE Sedimentary processes and products"— Presentation transcript:
1 GE0-3112 Sedimentary processes and products Lecture 6. RiversGeoff CornerDepartment of GeologyUniversity of Tromsø2006Literature:- Leeder Ch Rivers.
2 Contents 6.1 Introduction – importance of fluvial systems RiversGEOContents6.1 Introduction – importance of fluvial systems6.2 Fluvial channels6.3 Floodplains6.4 Fluvial architecture
3 Importance of fluvial systems RiversGEOImportance of fluvial systems1) 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 GEO1) Rivers are :erosive agentsconduits for sediment transport to lacustrine and marine basins.
5 RiversGEO2) Fluvial sediments are mostly transient but form thick deposits in several settings:coastal plainsintermontane basinstectonic forelandsModern and Holocene terraced fluvial deposits at Tana, N. Norway.
6 3) Fluvial deposits are sensitive palaeoenvironment indicators: RiversGEO3) Fluvial deposits are sensitive palaeoenvironment indicators:tectonic slope changessourceland geologyclimatesea-level changePostglacial fluvial terraces at Porsanger, N. Norway
7 Fluvial channels Size and gradient Shape (form) Processes RiversGEOFluvial channelsSize and gradientShape (form)ProcessesBedforms and internal structures
8 Bankfull width Channel size is measured as bankfull width. RiversGEOBankfull widthChannel size is measured as bankfull width.NormalChannel widthBankfull
9 Channel size Size varies by four orders of magnitude: RiversGEOChannel sizeSize varies by four orders of magnitude:<2 m (small streams)>20 km (Brahmaputra, Ganges).
10 Channel size vs. discharge RiversGEOChannel size vs. dischargeChannel widthChannel depthQ = whuDischargeMean flow velocityDischarge 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). RiversGEOWidth vs depthDepth (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): RiversGEOLong profileDownstream 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. RiversGEODownstream changes: Amazon R.
14 Tectonic disturbance of river profiles across the Himalayan front. RiversWGEOTectonic disturbance of river profiles across the Himalayan front.E
15 Channel shape Parameters for describing channel planform shape: RiversGEOChannel shapeParameters for describing channel planform shape:Sinuosity (P)BraidingAnastomosingChannel types illustrating characteristics of sinuosity, braiding and anastomosing (Galloway & Hobday 1996).
19 Controls on channel shape RiversGEOControls on channel shapeSediment loadStream powerBank stability
20 Braiding on sandy substrate RiversGEOBraiding on sandy substrateMeandering on clayey substrate
21 Channel variability Meandering Braided Gandak River, Nepal-India RiversGEOChannel variabilityMeanderingBraidedGandak River, Nepal-India
22 River confluences Deep scour at confluences. RiversGEORiver confluencesDeep scour at confluences.May be several times deeper than contributing tributaries.Mobile scour-and-fill units at the base of a succession.
43 Avulsion and channel belts RiversGEOAvulsion and channel beltsSudden 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. RiversGEOChannel beltsPalaeochannels of the Holocene Rhine-Meuse.Stacking patterns – fluvial architecture.
45 Incision – aggradation cycles RiversGEOIncision – aggradation cyclesRegional 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 RiversGEOFluvial incision and knickpointsFall in relative sea-level causes upstream knickpoint migration.
47 Depositional architecture and stacking patterns RiversGEODepositional architecture and stacking patterns
48 Depositional architecture and stacking patterns at Tana RiversGEODepositional architecture and stacking patterns at TanaMasjok, Tana