reading material “river deltas” from “the coast of puget sound” j.p. downing, puget sound...
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Reading Material
“River Deltas”from “The Coast of Puget Sound”J.P. Downing, Puget Sound Books
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Puget Sound Morphology
Glacial Originscour – flow under ice sheetformed depressionse.g., Main Basin, Hood Canal, Lake Washington
sedimentary deposits – also raised land surfaceglacial tills, outwash deposits, lake deposits
old glacial sediment now provides new input to PScliff erosionlandslidesland surface erosion
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Bathymetry (water depth)
Shallow entranceglacial origin – moraineoceanographic name – sillprimary sill is Admiralty Inlet
Several others divide PS into separate basins (>200 m)
Main Basin has 46% of water volume
Sinuous shape – result of originSouthern Basin has 29% of shorelines
Fluvial (river) sediment supplyfills PS from shorelineWhidbey Basin has 43% of tidelands
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Bathymetry (water depth)
Shallow entranceglacial origin – moraineoceanographic name – sillprimary sill is Admiralty Inlet
Several others divide PS into separate basins (>200 m)
Main Basin has 46% of water volume
Sinuous shape – result of originSouthern Basin has 29% of shorelines
Fluvial (river) sediment supplyfills PS from shorelineWhidbey Basin has 43% of tidelands
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Hydrography (water properties)
Salinity (amount of salt dissolved in water)river water has 0 ppt (parts per thousand)
ocean water has ~35 ppt – differs around world
brackish water at depth in PS – 20-30 ppt
Density (low salinity = low density)river plume flows over more dense brackish
water
Input of river water - varies with space and timenorthern PS rivers supply the most watersmall input during late summerlarge input during late autumn and winter rainslarge input during spring snowmelt
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Types of river-mouth environments
estuary – semi-enclosed setting river and salt water meet and mix
fjord – estuary with glacial origin deep, with shallow sill near mouth
delta – river mouth receiving much sediment
estuary filled with sediment shoreline growing seaward
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Puget Sound Sedimentation
Sources of sedimentshallow – shoreline erosion, landslides
deep – biological productivity, algal debris much carbon decomposes,
forming methane gas
all depths – river discharge deltas form near river mouths river plume carries sediment
deeper
near sill – inflow with deep ocean water
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Mechanisms associated with Sedimentation
plume transport – turbid surface waterriver momentum, tides, wind
flocculation – silt and clay particles form larger aggregates, which sink quickly
landward bottom flow – traps sediment near river
delta formation – thick deposits near river mouth
topset = tidal flats foreset = steep surface, rapid
accumulation bottomset = deep deposits, escape
seaward
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Deltaic Sedimentation
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Tidal wetlands on the topset of the Skagit delta
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Nisqually Nature Center at high tide
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Nisqually Nature Center at low tide
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from source…
…to sink
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