Marine Sciences Department, The University of North Carolina at Chapel Hill. Rivers function ... with the last dam located 220 km upstream in Roanoke Rapids.
Fingerprinting of Suspended Sediment Sources and in The Lower Roanoke River in North Carolina. Anna Jalowska, Brent A. McKee Marine Sciences Department, The University of North Carolina at Chapel Hill Radionuclide analyses of 150 samples collected along the Lower Rivers function as an active interface between the 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’s, a series of reservoirs were established in the mountain and piedmont reaches of the Roanoke, with the last dam located 220 km upstream in Roanoke Rapids. These reservoirs trap up to 99% of sediments. As a result the lower (coastal plain) section of the Roanoke River basin (below the dams) has become a major source of particles reaching the Roanoke Delta. There is a need to better understand sediment transport and accumulation in coastal ecosystems where sediment flux (source and delivery) have been altered by dams.
Roanoke River indicate: Water samples were collected at two-week intervals over a one- year period. River bed samples were collected during two field trips and do not represent seasonal changes. One liter water samples were filtered through a 0.2μm filter to determine total suspended mater (TSM). Suspended sediment was separated from ~70 L samples through continuous flow centrifugation; then lyophilized and prepared for gamma analysis. Sediment samples were also collected from the banks, channel and floodplain. Samples were analyzed for gamma decay of 210Pb, 7Be, 137Cs and 40K isotopes. Flow rate data were obtained from USGS. River bed samples were processed for grain size analysis.
• In Reach 1, bank erosion contributes mainly to river bed sediment; • In Reach 1 suspended sediment indicate a mixture of bank erosion and sediment exchange
with a floodplain during
inundation events; • Our data indicates that floodplain exchange and river bed
210Pb, 7Be, 137Cs bind
strongly to particles, which makes them useful as sediment transport tracers. is produced in the upper atmosphere by cosmic ray spallation of nitrogen and oxygen and has a half-life of 53.4 days. It is useful as a sediment tracer on monthly scales (up to 8 months). High 7Be values indicate recent mobilization or deposition. • 210 Pb is a naturally produced radionuclide as a decay product of 238U. It is supplied through wet and dry fallout. Its half-life of 22.3 years, allows us to reconstruct sediment history up to a century. High 210 Pb values indicate recent exposure to atmospheric deposition, while lower values correspond with the age of sediments. • 137Cs is present in the environment as a result of fallout from testing of thermonuclear weapons, primarily from 1954–1968. High values would refer to eroding of sediments deposited in the 1960’s. Lower values indicate surface soils and mixing. Absence of 137Cs indicates that sediments are older than 1954. • 40K has a half-life of 1.3 billion years. High values indicate lithogenic sources from rock weathering, while low values would be associated with high organic matter content. •
7Be
resuspension become more dominant in downriver direction; • Suspended sediment from Reaches 2, 3 and 4 show no signature of bank material eroded in Reach 1. • In Reaches 2, 3 and 4 suspended sediment is deposited in the channel and on the inundated parts of floodplain. • Reaches 2, 3 and 4 reflect a strong influence from APES seiche. Suspended sediment signatures suggest active and complex exchange of sediments with the floodplain, the river bed and the estuary.
Figure 1. Lower Roanoke River basin and sampling locations.
The Lower Roanoke was examined to determine the major sources of suspended sediment (runoff, bank erosion or river bed resuspension). Isotopic analyses reveal complex interactions between the floodplain, river channel and water column, suggesting floodplain sediments resuspended during high flow events are a major particle source. That, in turn, suggests that dam regulation of 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 materials.
•Lunar tides are negligible in Albemarle Sound and water level changes at the mouth of the Roanoke River are a result of a seiche from the Albemarle–Pamlico Estuarine System (APES);
Figure 4. Conceptual model sediment dynamics below and above transition zone
Bonniwell, E. (1999). Determining the times and distances of particle transit in a mountain stream using fallout radionuclides. Geomorphology, 27(1-2), 7592.
Figure 2. Discharge from the dam compared with water level at sampling stations.
•Depending on the magnitude and duration of water releases from the dam, energy generated by the seiche can be carried as up to Williamston, 70 km up the river, affecting sediment distribution within Roanoke Delta; • High sedimentation rates within the Roanoke delta: 8-10 cm/year; •A mean bank erosion rate of 52 cm/year was documented by Hupp et al. (2009) and given as a major source of suspended sediment; •High sedimentation rates of muds in the delta cannot be explained by bank erosion upstream only, which initiated an investigation of other possible sources.
Hupp, C.R., et al. (2009). Bank erosion along the dam-regulated lower Roanoke River, North Carolina. Management, 2451(06), 97-108. Hupp, C. R., et al. (2009). Floodplain Geomorphic Processes and Environmental Impacts of Human Alteration Along Coastal Plain Rivers, USA. Wetlands, 29(2), 413-429.
Figure 3. Elevation map of the Lower Roanoke River with discrete extent of the estuary’s influence. River basin was divided in to 4 reaches to exam relationship between samples.
Luettich, R.A., Jr., et al. (2000). McNinch, “Semidiurnal seiching in a shallow, micro-tidal lagoonal estuary”, Continental Shelf Research, 22:1669-1681.
