Suspended Sediment Dynamics
of the Columbia River Estuary
David A. Jay
Department of Environmental Science and
Engineering, Oregon Graduate Institute
1
Salinity and/or SPM Data Available:
• Meager historic data: 1933, 1957, 1960s, 1970s,
mostly USGS and Corps of Engineers
• CREDDP/NOS data sets 1980-81 -- most extensive
salinity, velocity data set
• NSF Land-Margin Ecosystem Research Program,
1990-99 best joint salinity, velocity SPM data set for
process studies
• CORIE (ONR, NMFS) 1997-date monitoring of
velocity, acoustic SPM, some salinity
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Fine Sediment Supply -• 1946-99 supply is ~8 x106 mt tons yr-1 (CR+WR,
excludes sand)
• SPM supply ~ QRn, n = 2.5, so SPM supply is higher in
high-flow years
• Winter freshets supply much more fine sediment, with
SPM supply ~ QRn, n = 3.5 -4.5, but not more sand
• Quality of SPM varies seasonally:
– organics generally 1-5%
– winter SPM has higher mineral and woody content
• ETM productivity throughout the year based on SPM
supplied during the spring freshet
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Overview of Estuarine SPM Processes -• Two “reservoirs” of fine sediment in the estuary:
– Estuarine turbidity maximum (ETM), in sand-bedded channels
where fines are transient on bed
– Peripheral areas (fines permanent part of bed)
• Much storage in bays is also transient:
– Channels are re-supplied on spring tides after freshet
– ETM material shows signs of having been in an anoxic bed
• 70% of SPM is exported, 30% retained in peripheral areas
(USGS, based on Hanford nucleotides)
• Microbial processing of organic detritus from river forms
basis of ETM food web
• Salmonids may be more dependent on macro-detritus
4
ETM locations and CORIE Stations --
• Tansy, Am169 and Am012 are in the ETM
• Am169 and Am012 duplicate 1980 stations
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Conceptual View of ETM --
• ETM occurs near upstream limits of salinity intrusion
due to particle trapping by tides and mean flow
• Two ETM -- in North and South Channels
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Conceptual View of ETM --
• ETM food web based on epibenthic grazing on particles
and bacteria
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Retention of SPM in the Estuary -• Depends on:
– salinity intrusion length -- ETM occurs “near” head
of salinity intrusion
– strength of estuarine circulation -- two layer flow and
tides tends to trap SPM
– strength of tides -- neap tides trap SPM on bed,
spring tides put SPM into water column and export it
– topography -- governs position of salinity intrusion
and strength of two-layer flow
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Factors Governing Salinity Intrusion:
• For a channel of simple form -– Two-layer estuarine flow and salinity intrusion vary ~H3/Km, (H
is controlling depth, Km is vertical mixing coeff.)
– Salinity stratification varies as ~H5/Km2
– bars, holes and constrictions modify this somewhat
• Spring tides cause strong mixing, increasing Km
• During low-flow neaps, Km decreases greatly, increasing
salinity intrusion/stratification (neap-spring transition).
• Increasing H may cause neap-spring transitions to occur
at higher flows
• Salinity intrusion varies with river flow as QRm, 1/7<m<1
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Neap-Spring
Changes in
Salinity
Intrusion --
• Maximum salinity intrusion occurs on neap for most flows
• for low-flow years seasonal and neap-spring changes are similar
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Changes in
Salinity
Intrusion
with QR --
• High and low-flow neap-tide salinity intrusion
• 4x change in flow causes ~2x change in intrusion length
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SPM Residence
Time TR, May to
December 1997 -(SPM determined from
CORIE ADCP backscatter;
Fain et al., submitted 2000.
TR is the the SPM inventory
divided by the supply rate )
• SPM expelled on every spring tide during freshet (<d 170)
• TR 30-60 d in the South Channel, up to 120 d in the North Channel
• Implies that ETM re-supplied from peripheral areas
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SPM Reservoirs (Summer) -• SPM in transport in river -- 60-120 x 103 mt
tons, mostly in water column
• SPM in ETM area -- 120-250 103 x mt tons, on
bed or in water column
• Mobile material in peripheral bays -- ?? mt,
temporarily on bed, but can be resuspended and
supplied to ETM
• Reservoirs are large relative to amount of fines
to be dredged
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1997 SPM Transport Patterns -• During Freshet, SPM is
exported at all stations on
spring tides
• After freshet, SPM is
exported from South
Channel, but moves
landward in the north
channel
(red arrows are near-bed
transport, green near-surface;
from Fain et al., submitted 2000)
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Estuarine Trapping Efficiency E -• Trapping Efficiency E
= SPMmax/SPMriver
• CR is moderately
effective at particle
trapping
• Maximum E occurs at
intermediate QR in both
channels
• E greater in North
Channel, reflects
greater TR
QR
(from Jay et al., submitted
2001)
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Summary of Possible Project
Impacts on SPM and Toxics -• Changing ETM position through changes in
salinity intrusion and salinity stratification
• Changing residence time TR for SPM and toxics
• Changing timing of toxic inputs to ETM
• Changes to ETM food web, related to changes in
ETM position and TR
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Recommended Monitoring/Studies -• Monitor salinity intrusion and stratification, and
SPM concentrations at strategic locations
• Analyze historic salinity and SPM to data to
detect changes related to changing channel depth
• Understand toxics in relation to land-use,
sediment supply and sediment transport
processes
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