friday, oct 16, 2009major downstream trendsriver systemsalluvial riverschannel typesSlide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Riffles, pools, and cascadesPool - riffle sequencea few river termsBankfull DischargeSlide 18straight channelsalternate barsmeanderingSlide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30braidingSlide 32Slide 33Slide 34broad patternsSinuosity: Gradient and substratelongitudinal profileSlide 38mississippinisqually -friday, oct 16, 20091. 5 cr cruise reports due today 2. Nisqually ft this weekend3. Cruise on Puget Sound next Wednesday4. on to rivers!water and nutrientshabitat to diverse flora and faunaroutes for commercerecreation, electricitywhy?major downstream trendsdischarge width depth velocity gradient grain size river systemscan be subdivided into major segments:1. bedrock/mountain2. alluvial3. deltaicalluvial riverserodible channel boundaries (alluvial banks and bed) transport capacity ≤ sediment supplyInput ≥ Outputsed. storage can be bigchannel forms/patterns reflect self-organization to deal with this…channel typesColluvial (sediment from hillslope)Alluvial (fluvial sediment)A. CascadeB. Step-poolC. Plane-bedD. Pool & riffleE. Dune – ripple (most common)ordered by decreasing gradientand degree to which everyday flow can carry sediments flow[Montgomery & Buffington 1997]Colluvial 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.poolsbars/rifflesRiffles, pools, and cascadesRiffles and pools alternate in somewhat predictable patternsassociated with optimal flow organization to move sedimentPool - riffle sequenceRiffle to riffle = 5 - 7 channel widthsbanka few river termsBankfull Dischargebankfull discharge typically equates to a roughly 2-year recurrence interval flow.Three basic map-pattern forms of streams:•Straight•Meandering•BraidedChannel Patternsstraight channelsrelatively rare•represent a relatively immature channel form•straight channels common where streams are confined by topography or follow geologic structures•often mountain streamswhy are straight channels uncommon?recall: input ≥ output river needs to manage sedimentinteraction between flow & eroding/depositing bed causes organized periodic bar depositionbar local width local velocity local erosion excess sediment new bar … rivers don’t want to be straight!alternate barsNaka R., Japanmeanderingwhat if the banks are erodible?higher velocity near bars erosionerosion deposition …flow & sedimentation patterns self-reinforcingmeandering leads to formation of:cut bankspoint barsnote velocity patterns:highest on outside of bendsmeanderingerosion on outside of bends (cut banks) where velocity is greatest deposition on the inner sides of bends where velocity is slowermeanders grow through timePoint BarsCut Banksmeanderingprogressive growth of meanders leads to formation of meander beltsmeanderingGrowing meanders can intersect each other and cut off a meander loop, forming an oxbow lakemeanderingmost characteristic of large, low-gradient, fine-grained riverscontinuous, gradual change in channel coursecreate floodplains wider than the channelvery fertile soilseasonal floodingHolden Crater, MarsThree basic map-pattern forms of streams:•Straight•Meandering•BraidedChannel Patternsmultiple converging & diverging channels separated by mid-channel barsbraidingbraidinghigh sediment load (relative to water Q)constantly changing coursefloodplain generally completely occupied by channelsmany small islands called mid-channel barsusually coarse sand and gravel (bedload grain sizes)braidingwhy not a single channel?amount of bedload grossly disproportionate to transport capacity of waterleads to frequent deposition of coarsest sediment – often in bars that locally divide the flowBraided channels typical in regions with:easily erodible banks and/or a high sediment loadcommon near mountains & glaciers (not exclusively)broad patternsfour dimensions:longitudinallateraltime (temporal)verticalSinuosity: Gradient and substrateSmall meandershigh gradientcoarse substratesBig meanderslow gradientfine substrateslongitudinal profiletypical mountain streamRakaia R. NZlong. channel pattern variationmississippichannel maturity sinuosity (degree of meandering)longitudinal & temporalnisqually - longitudinal variation (pattern, gs, gradient,
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