Week 9 fluvial processes.docx

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Geographical Biogeosciences
GEOB 103
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Week 9 Fluvial ProcessesFluvial GeomorphologyFluvial Geomorphology is the study of erosion transport and deposition ofsediment primarily under the influence of flowing waterIn comparison mass movements refer to the erosion transport and deposition of sediment primarily under the influence of gravityThings like debris flows that occur in streamchannels and involve aspects of both flowing water and gravity are right on the boundary between fluvial and mass movement processesDivision of a Drainage Basin Based on Dominant ProcessesWe can divide a stream channel network or watershed into 3 zones based on whether fluvial erosion transportation or deposition is the dominant processThe degree to which the stream channel network where most of the work of flowing water is done is coupled with mass movement processes changes systematicallyWe will explore the division of a watershed and the interactions between mass movements and fluvial processes using a local example Lynn Creek near SecheltZone 1 erosion is the dominant process In the upper reaches of the basincalled the headwater streams the stream channels are relatively steep and fastThe primary fluvial process here is erosion and immediate transport with little sediment redepositionHillslopesweathering and mass movements are common on the hillslopesadjacent to the headwaters supplying relatively large volumes of sediment to the streams which is also transported downstreamDebris flows are common in the stream channels in the erosion zone in these mountainous streamsLanduse ie road construction and cut blocks can frequently disturb thehillslopes and these disturbances are quickly transmitted to the headwater streamsZone 2 transport is the dominant processdownstream from the headwaters the stream valley becomes wider and the stream gradient is reducedSome of thesediment transported in the erosion zone is deposited here allowing depositional features like channel bars islands and meanders to formMass movements are partially intercepted by the flatter ground around these stream channelsSediment may be temporarily stored in these areas but ultimately will be reeroded by the stream and carried further downstreamMost of the sediment supplied to these parts of the stream channel networkcomes from the stream channel network upstreamAs the size of a system increases the relative importance of mass movements on the channel network decreasesZone 3 deposition is dominant close to the base level for a stream usually the mean sea level the system runs out of energy to transport the sediment eroded in Zone 1 and transported through Zone 2As a result sediment is deposited more or less permanentlyThese deposits are found at the mouths of rivers ie at lakes or oceansSediment deposited here may be transported up or down the shore line by waves or it may be buried compacted and turned into sedimentary rock eg the Gulf IslandsNOTE The 3 zones are typically arranged along a continuum from steep headwaters to nearly flat depositional fans or deltasThis idealization applies well to steep mountainous drainage basins but less well to stream channels in morecomplex topography where patterns of erosion transferstorage may be more complexErosion By Flowing WaterSheet Flow ErosionConsider a hillslope that is not dissected by stream channelsIf Hortonian overland flow or saturated overland flow occur on this surface the water will flow down hill as a very thin sheet of waterThis is referred to as sheet flow and may be capable of eroding sediment from the surface and transporting it downslope
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