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Abstract: Rates and the spatial extent of near-channel floodplain sedimentation on regulated and unregulated reaches in two upland rivers in central Vermont, U.S.A. are measured using the short-lived fallout radionuclide 210Pb. We find consistent profiles of 210Pb inventories across all sites; inventories are low immediately next to the channel, increase to a peak value as the inundation frequency decreases and then asymptotically diminish with distance from the channel to the equilibrium inventory associated with atmospheric deposition alone. We infer from our data that flow regulation has impacted sediment deposition to floodplains below the dam; total sediment deposition is less and it is constrained to a narrower band immediately along the active channel. Flow regulation does not appear, however, to impact the general form of the 210Pb inventory profile, suggesting a uniformity of process across regulated and unregulated floodplains. [Copyright &y& Elsevier]

Abstract: We explore the novel technique of using short-lived fallout radionuclides to date fluvial surfaces with the goal of determining lateral migration rates. Specifically, we use the fallout radionuclide 210Pb (half-life t 1/2 ~22.3years) to determine the lateral migration rates of meandering reaches on the Winooski River, VT, the upper Connecticut River, NH, and the Genesee River, NY. We find that, particularly near the channel, 210Pbex inventories are affected by the initial 210Pbex inventories of freshly deposited sediment (inheritance). Inheritance differs at each site, with lower inheritance occurring at sites with larger upstream drainage areas. After accounting for inheritance, 210Pbex inventories in surface sediments along a transect orthogonal to the channel yield migration rates of 0.7, 3.1, and 4.7m/y for the Winooski, Connecticut, and Genesee Rivers, respectively. These rates agree well with values derived independently from historical aerial photography. Variations in 210Pbex inventories along the transects also provide details of the channel migration histories not evident from the limited available aerial photographs, including evidence for both nearly constant linear and episodic channel migrations. Results suggest the broad applicability and added value of our approach for quantifying decadal-scale variations in fluvial surface ages. [Copyright &y& Elsevier]