Findings can inform scenarios of sea level rise
After surveying and analyzing centuries of evidence in the floodplain of the lower Roanoke River, USGS researchers, along with colleagues from the universities of Wisconsin and North Carolina, have developed a highly accurate estimate of sediment deposition amounts along the course of the river over three timescales — annual, decadal, and centennial.
The investigators used a range of techniques, including evidence from clay pads, tree-rings, and pollen analyses, at numerous locations (58 transects, 378 stations) and employed GIS technology to model sediment deposition rates and characteristics to gain insight into the sediment dynamics of the Roanoke, one of the largest river flood plains on the mid-Atlantic coast.
The scientists found that sediment deposition rates from AD 1725 to 1850 were an order of magnitude higher than present deposition rates and still affect the sediment dynamics of today. These high rates have been attributed to land clearance and poor agricultural practices during and after the colonial period. This legacy sediment deposition formed high banks upstream and the large, wide levees found along the middle reaches of the river.
Furthermore, dam operations, most notably the Kerr Dam completed in 1953, have reduced deposition on natural levees but facilitated backswamp deposition. A GIS-model of current river dynamics indicates that little sediment presently reaches Albemarle Sound because it is trapped on the floodplain, generally benefitting lower floodplain ecosystems and mitigating the transport of excess nutrients to coastal marine systems.
The study findings highlight the important role played by landscape alteration, including post-Colonial forest clearance and dam emplacement, in controlling modern sediment dynamics. The use of multiple techniques to determine sediment deposition rates should improve capabilities of developing accurate sediment budgets along different reaches of the river. In turn, this will aid predictions of the response of the river and associated habitats to changing sea level.
The research was recently published in the journal Geomorphology.
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