Rivers Why study rivers Water and nutrients Habitat to diverse flora and fauna Routes for commerce Electricity Recreation They re beautiful major downstream trends discharge width depth velocity gradient grain size river systems can be subdivided into major segments 1 bedrock mountain 2 alluvial 3 deltaic Alluvial Rivers erodible channel boundaries alluvial banks and bed transport capacity sediment supply Input Output sed storage can be big channel forms patterns reflect selforganization to deal with this Channel Types flow Colluvial sediment from hillslope Alluvial fluvial sediment A Cascade B Step pool C Plane bed D Pool riffle E Dune ripple most common ordered by decreasing gradient and degree to which everyday flow can carry Montgomery Buffington 1997 Colluvial Channels Small headwater channels at the tips of the channel network where sediment transport is dominated by landslide processes Cascade Channels The steepest of mountain channels characterized by tumbling flow around individual boulders disorganized streambed structure Step Pool Channels Channels displaying full width spanning accumulations of coarse sediment that forms a sequence of steps pool s step s Plane Bed Channels Channels lacking well defined bedforms and instead displaying long reaches lacking pools Pool Riffle Channels The most common mountain river morphology characterized by alternating sequence of pools and bars pool s bars riffles Riffles Pools and Cascades Riffles and pools alternate in somewhat predictable patterns Associated with optimal flow organization to move sediment Pool Riffle Sequence Riffle to riffle 5 7 channel widths A Few River Terms bank bank Bankfull Discharge Bankfull discharge typically equates to a roughly 2 year recurrence interval flow Channel Patterns Three basic mappattern forms of streams Straight Meandering Braided Straight Channels Relatively rare represent a relatively immature channel form straight channels common where streams are confined by topography or follow geologic structures often mountain streams Alternate Bars Why are straight channels uncommon interaction between flow eroding depositing bed causes organized periodic bar deposition bar local width local velocity local erosion excess sediment new bar rivers don t want to be straight Naka R Japan Meandering What if the banks are erodible higher velocity near bars erosion erosion deposition Flow sedimentation patterns selfreinforcing Meandering leads to formation of cut banks point bars Note velocity patterns highest on outside of Meandering Erosion on outside of bends cut banks where velocity is greatest Deposition on the inner sides of bends where velocity is slower Meanders grow through time Meandering Cut Banks Point Bars Meandering Progressive growth of meanders leads to formation of meander belts Growing meanders can intersect each other and cut off a meander loop forming an oxbow lake Meandering most characteristic of large lowgradient fine grained rivers continuous gradual change in channel course create floodplains wider than the channel very fertile soil seasonal flooding Holden Crater Mars Channel Patterns Three basic mappattern forms of streams Straight Meandering Braided Braidin g multiple converging diverging channels separated by mid channel bars Braided Channels high sediment load relative to water Q constantly changing course floodplain generally completely occupied by channels many small islands called mid channel bars usually coarse sand and gravel bedload grain Braided Channels Why not a single channel amount of bedload grossly disproportionate to transport capacity of water leads to frequent deposition of coarsest sediment in bars that locally divide the flow Braided channels typical in regions with easily erodible banks and or a high sediment load common near mountains glaciers not exclusively Broad Patterns four dimensions longitudinal lateral time temporal vertical Sinuosity Gradient and Substrate Big meanders low gradient fine substrates Small meanders high gradient coarse substrates Longitudinal Profile Typical mountain stream Rakaia R NZ long channel pattern variation mississippi channel maturity sinuosity degree of meandering longitudinal temporal Nisqually longitudinal variation pattern gs gradient Q
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