BANK EROSION, CHANNEL BATHYMETRY, AND FLOODPLAIN SEDIMENTATION DYNAMICS ALONG THE REGULATED LOWER ROANOKE RIVER, NORTH CAROLINA

The lower Roanoke River bottomland, on the North Carolina Coastal Plain, contains the largest intact, forested wetland in the Mid-Atlantic region. Streamflow on the lower river has been regulated for about 50 years by a series of upstream dams. Prior to dam closures, the bottomland experienced extensive sediment aggradation, arising from post-settlement agriculture (legacy sediment), particularly in the upper and middle reaches of the lower river (up to 4 m of vertical deposition since the early 1700's). Previous floodplain studies using tree-ring and clay-pad techniques suggest that sedimentation rates had been increasing up to the time of initial dam closures (circa 1950, were suppressed during the following decade (1951-60) and have recently increased along some middle and lower reaches. Bank erosion from both particle by particle removal and mass wasting is common along the upper and middle reaches. We established 60 bank and channel monitoring cross sections along a 100 km reach, which have been leveled and the banks instrumented with 4-10 erosion pins each. Banks along the middle reaches where erosion is greatest (preliminary results from the period October 2005 to June 2007) are retreating 80-160 mm/yr (normal to the bank profile); erosion rates are highest low on the bank slope and on cut banks. Mass wasting may remove several meters during a single failure and is most common along cut banks. Bank erosion is linked to channel-bed geometry, which is dynamic and may change, vertically, by 10 meters over a 6-month period. Deposition may occur along uncommon point-bar sections at a rate of about 50 mm/yr. Bank erosion is the likely source of downstream floodplain deposition where deposition rates may average 40 mm/yr. The system is, at once, affected by long- and short-term impacts that migrate downstream over decades to centuries creating variable complex non-equilibrium conditions.