Slide 1Importance of alluvial riversSlide 3Bankfull DischargeSlide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Formation of MeandersSlide 20Pool - riffle sequenceRiffles, pools, and cascadesSlide 23Slide 24Slide 25Braided StreamsBraided ChannelsSlide 28Slide 29Slide 30Variability in river systemsVariation in time and spaceSinuosity: Gradient and substrateSlide 34Channel typeSlide 36Slide 37Slide 38Slide 39Slide 40Slide 41Slide 42Slide 43Slide 44Alluvial Rivers•Erodible channel boundaries (alluvial banks and bed) •Transport Capacity ≤ Sediment Supply•Storage can be quite high•Input ≥ OutputImportance of alluvial riversRivers:–Provide water and nutrients for agriculture–Provide habitat to diverse flora and fauna–Provide routes for commerce–Provide recreation–Provide electricitybankBankfull DischargeBankfull DischargeTypically bankfull discharge equates to a roughly 2-year recurrence interval flow.Three basic map-pattern forms of streams:•Straight•Meandering•BraidedChannel PatternsSTRAIGHT CHANNELSStraight channels are rare.•Straight channels form where streams are confined by topography or follow geologic structures.•Generally mountains streams.Streams generally erode on outer (cut) banks where velocity is greatest, and deposit on the inner sides of bends where velocity is slower.Meanders tend to grow as the flow erodes the banks, favoring development of meandering channels.Meandering ChannelsMeandering channelsLoops or meanders form as stream erodes its banks.Erosion takes place on the cut bank, which is theoutside loop of the meander.Deposition takes place on the point bar, which ison the inside loop of the meander.Change their channel course graduallyCreate floodplains wider than the channel–Very Fertile soil–Subjected to seasonal floodingMeandering channelsPoint BarsCut BanksMeandering channels•Meandering streams often characterized by large loopy bends across their floodplains.•Meanders occur most commonly in channels that lie in fine-grained stream sediments and have gentle gradients.Growing meanders can intersect each other and cut off a meander loop, forming an oxbow lake.Old channels abandoned as a river meanders across its floodplain form oxbows.Oxbow lakeSacramento River, CAOwens River, CAMeander “train” = belt of meanderingSacramento River, CAOwens River, CANote old meandersChannel migration zone = area across which the river is prone to move.Meander “train” = belt of meanderingMeander beltMeander beltFormation of MeandersPoint bar depositsPool - riffle sequenceRiffle to riffle = 5 - 7 channel widthsRiffles, pools, and cascadesRiffles and pools alternate in somewhat predictable patternsHolden Crater, MarsBraided ChannelsBRAIDED CHANNELSMany converging and diverging streams separatedby gravel bars (or sand bars).Braided Streams•High sediment load•Anastamosing channels•Constantly changing course•Floodplain completely occupied by channels•Many small islands called mid-channel bars•Usually coarse sand and gravel deposits.Braided Channels•If a stream is unable to move all the available load, it tends to deposit the coarsest sediment as a bar that locally divides the flow.•Braided channels tends to form in streams having highly variable discharge, easily erodible banks, and/or a high sediment load.•Glacial streams generally are braided because:–The discharge varies both daily and seasonally.–The glacier supplies the stream with large quantities of sediment.Braided Channels•Braided channels clog themselves with sediment, so channels always shifting•Generally in streams near mountain frontsBraided Streams•Four dimensions:–Longitudinal –Lateral–Vertical–TimeVariability in river systems The four dimensions of a stream systemThe shape, size and content of a river are constantly changing, forming a close and mutual interdependence between the river and the land it traverses.Variation in time and space•Small meanders–high gradient–coarse substratesSinuosity: Gradient and substrate•Big meanders–low gradient–fine substratesLongitudinal Profile of Mountain RiversChannel type–Bedrock–Colluvial–Alluvial •A. Cascade•B. Step-pool•C. Plane-bed•D. Pool & riffle•E. Dune – rippleColluvial ChannelsSmall headwater channels at the tips of the channel network where sediment transport is dominated by landslide processes.Cascade ChannelsThe steepest of mountain channels, characterized by tumbling flow around individual boulders; disorganized streambed structure.Step-Pool ChannelsChannels displaying full-width-spanning accumulations of coarse sediment that forms a sequence of steps.poolsstepsPlane-Bed ChannelsChannels lacking well-defined bedforms and instead displaying long reaches lacking pools.Pool-Riffle ChannelsThe most common mountain river morphology; characterized by alternating sequence of pools and bars.poolsbarsLongitudinal Profile of Mountain
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