USE OF NATURALLY OCCURRING RADIONUCLIDES IN ASSESSMENT OF LONGITUDINAL AND LATERAL CONNECTIVITY OF THE RIVER CHANNEL WITH FREQUENTLY INUNDATED FLOODPLAINS

Rivers are the major source of particulate material in the oceans. During transport from uplands to the ocean, complex dynamics within river basins alter the residence time, quality and quantity of the materials delivered to the coastal zone. Floodplains are major sites for alluvial storage of sediments and organic matter in watersheds.

In this study we examined the connectivity between floodplains, main channel and the estuary in the Lower Roanoke River. The Roanoke River begins in the Blue Ridge Mountains of northwestern Virginia and empties in Albemarle Sound in eastern North Carolina. Anthropogenic disturbances associated with first settlements in the Roanoke River Basin caused a rapid increase in floodplain sedimentation rates, setting favorable conditions for organic matter burial. In the beginning of 1950’ a series of reservoirs were established on the mountain and piedmont reaches of the Roanoke. That in turn caused change in the flow regime, reduced the amount of sediments delivered to the floodplains and their storage/burial capacity. Floodplains in the Lower Roanoke River are subjected to frequent and persistent flooding. The extent and frequency of inundation events in the lower Roanoke are greatly influenced by seiching of the Albemarle Sound along with the magnitude of flow releases from the Roanoke Rapids Dam.

Over 200 samples representing suspended sediment, channel bed, banks and floodplains were collected between 2009 and 2011 and examined for radioisotopes Pb-210, Be-7, Cs-137, Ra-226 and K-40. Isotopic analyses reveal complex interactions between the floodplains and the river channel. Sediments deposited in the floodplains are re-introduced to the main channel during inundation events and become a major particle source.