ASSESSMENT OF MODERN SEDIMENT STORAGE IN THE FLOODPLAIN OF THE LOWER TAR RIVER, NORTH CAROLINA

Rivers transport water, sediment, and other constituents from the continent to the sea, but in route material can often become stored temporarily or permanently. Along the Atlantic Coast of the United States, coastal plain rivers such as the Tar River are characterized as low-gradient meandering systems that develop wide floodplains which are subjected to frequent and prolonged flooding. As a result, these rivers are believed to experience storage of sediment, particularly near their estuarine mouths. The lower portion of rivers and their attached estuaries are also environmentally and economically important serving as critical habitat (e.g., nurseries for fish), recreational areas, and transportation pathways. Excess sediment is often considered a significant pollutant and can have adverse effects on biota. Suspended sediment also can supply excess nutrients and trace metals from anthropogenic activity.

Previous work in North Carolina suggests that alluvial storage can make up the majority (>50%) of the total sediment delivered to rivers. This study more closely examines the nature of lower floodplain sediment storage and more specifically focuses on calculating sediment accumulation along the Tar River. Cores were collected from three sites along seven different transects perpendicular to the main channel. Analysis of Pb-210 and Cs-137 were employed to calculate sediment accumulation rates, and grain-size data were made to inform radionuclide and sedimentation interpretations. Preliminary evaluation suggests that rates range from 0.4 to 0.8 cm/yr. However, several sites appear to have non-steady-state deposition possibly due to major overbank flood events. Grain-size data indicate a mixture of sand and mud at all sites with some variability in the nature of sediment accumulating. Using core observations and LiDAR topographic data, storage across the system is estimated to be significant.