SOURCES AND SINKS OF SUSPENDED SEDIMENT IN THE LOWER ROANOKE RIVER IN NORTH CAROLINA, USING RADIOISOTOPE FINGERPRINTING METHOD

Rivers function as an active interface between land and the ocean delivering particles and dissolved matter from terrestrial environments to coastal zones. The Roanoke River connects the Blue Ridge Mountains of northwestern Virginia with Albemarle Sound in eastern North Carolina. Beginning in the 1950’ a series of reservoirs were established on the mountain and piedmont reaches of the Roanoke. Up to 99% of sediments get trapped in these reservoirs, thus the lower (coastal plain) section of the Roanoke River basin (below the dams) became a major source of particles reaching the coastal zone. There is a need to better understand sediment transport and accumulation in coastal ecosystems where sediment discharge (source and delivery) have been altered by human operated dams. The Lower Roanoke was examined to define the major sources of suspended sediment (runoff, bank erosion or river bed re-suspension) and to determine the correlation between sediment dynamics and flow conditions altered by the dam and weather events.

Water samples were collected in two week intervals over a one- year period, and flow rate data were collected from USGS sampling sites. Sediment samples were also collected from the banks, channel and the floodplain. Sediments were analyzed for gamma decay of isotopes ²¹⁰Pb, ⁷Be, ¹³⁷Cs and ⁴⁰K. Radioisotopes were used as tracers to define sources, sinks and residence time of particles within the lower river basin.

Isotopic analyses reveal complex interactions between the floodplain, river channel and water column, suggesting sediments accumulated in the floodplain and resuspended during high flow as a major particle source. That in turn suggests that dam regulation of the magnitude and duration of water discharge plays a significant role in sediment delivery to the estuary. Elevated water levels related to weather events also have a non-linear effect on the pathways and fates of particulate material.