Suspended Sediment Dynamics of the Columbia River Estuary

1. Suspended Sediment Dynamics of the Columbia River Estuary David A. Jay Department of Environmental Science and Engineering, Oregon Graduate Institute

2. 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

3. 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

4. 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

5. ETM locations and CORIE Stations -- • Tansy, Am169 and Am012 are in the ETM • Am169 and Am012 duplicate 1980 stations

6. 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

7. Conceptual View of ETM -- • ETM food web based on epibenthic grazing on particles and bacteria

8. 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

9. 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

10. 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

11. Changes in Salinity Intrusion with QR -- • High and low-flow neap-tide salinity intrusion • 4x change in flow causes ~2x change in intrusion length

12. 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

13. 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

14. 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)

15. 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 (from Jay et al., submitted 2001) QR

16. 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

17. 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