Identifier

etd-0404103-084700

Degree

Master of Science (MS)

Department

Oceanography and Coastal Sciences

Document Type

Thesis

Abstract

Louisiana's deltaic coast is a dynamic sedimentary environment made vulnerable by Mississippi River channelization, which restricts freshwater, sediment, and nutrient inputs to adjacent estuaries. Freshwater diversions now reconnect some estuaries with historic fluvial sources, like Breton Sound basin, which receives Mississippi River suspended loads through the Caernarvon freshwater diversion. The project objectives were to quantify short-term sediment accumulation using sediment traps, evaluate sediment trap effectiveness, and compare long-term deposition using feldspar horizon markers and 210Pb dating to the annual sediment trap deposition. Magnitude and variations of sediment deposition at 14 sites were compared during an 18-month study in the upper 415 km2 of Breton Sound estuary, Louisiana. Short-term dry deposition over sediment traps (t=15d) varied for hydrologic pulsing regimes, averaging 3.143 g/m2/d during non-pulsing periods (<183m3/s) and 4.740 g/m2/d during pulsing periods (¡Ý183m3/s). Deposition was greatest near the diversion and decreased with increasing distance from the diversion. Exterior marsh sites (n=5) received more total and allochthonous sediment than interior marsh sites (n=9). Fluvial pulses are significant sediment delivery mechanisms for interior marshes within close proximity (<6km) to the diversion. Exterior marsh deposition is influenced by both fluvial pulsing and resuspension events. Annual deposition over feldspar markers was highly variable but averaged 3.4 cm/yr. In contrast, long-term deposition measured using 210Pb and 137Cs geochronology revealed sedimentation rates within the last 75 years of about 0.12 cm/yr. Near vertical Pb and Cs activities occur within a distinct clay layer attributed to the 1927 flood, where 210Pb dating confirms its origin as circa 1927. Ephemeral short-term sediment deposition is driven by overland flow and highly variable due to prevailing winds and tides. Long-term deposition includes cumulative effects of internal sediment processes, such as compaction and organic matter decomposition, and is more representative of actual accretion rates (i.e., land-building). Estuarine sediment budgets indicate land-building processes are 66% deficient relative to the combined effects of sea level rise, subsidence, and erosion. A critical issue for managing coastal marshes and addressing land loss is elucidated here. Management efforts may be best directed at optimizing estuarine sediment loading based on peaks in river sediment discharge.

Date

2003

Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Jaye Cable

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