Paper No. 6
Presentation Time: 2:15 PM
HISTORIC AND RECENT FLOODPLAIN SEDIMENT DEPOSITION PATTERNS ON THE LOWER ROANOKE RIVER, NC
The floodplain of the lower Roanoke River on the Coastal Plain of North Carolina supports the largest contiguously forested riparian wetland on the Atlantic Coast. This floodplain has been impacted by substantial aggradation in response to upstream colonial agriculture between the mid-18th and mid-19th centuries. Additionally, since the mid-20th century stream flow has been regulated by a series of high-dams. We used artificial markers (clay pads), tree-ring (dendrogeomorphic) techniques, and pollen analyses to document sedimentation rates/amounts over short-, intermediate-, and long-term temporal scales, respectively. These analyses occurred along 58 transects at 378 stations throughout the lower river floodplain from near the Fall Line to the Albemarle Sound. Present sediment deposition rates ranged from 0.5 to 3.4 mm/yr and 0.3 to 5.9 mm/yr from clay pad and dendrogeomorphic analyses, respectively. Deposition rates systematically increased from upstream (high banks and floodplain) to downstream (low banks) reaches, except the lowest reaches. Conversely, legacy sediment deposition (1725 to 1850 AD) ranged from 40 to about 5 mm/yr, upstream to downstream, respectively, and is apparently responsible for high banks upstream and large/wide levees along some of the middle stream reaches. Dam operations have selectively reduced levee deposition while facilitating continued backswamp deposition. A GIS-based model predicts 453,000 Mg of sediment is trapped annually on the floodplain and that little watershed-derived sediment reaches the Albemarle Sound. Nearly all sediment in transport and deposited is derived from the channel bed and banks. Legacy deposits (sources) and regulated discharges affect most aspects of present fluvial sedimentation dynamics. The lower river reflects complex relaxation conditions following both major human alterations, yet continues to provide the ecosystem service of sediment trapping.