The use of Fallout 210-Pb to Determine Floodplain Inundation Pattern

Understanding the effects of flow regulation on floodplain inundation pattern is critical for the successful maintenance of a floodplain's ecological function and integrity. We explore the potential for using the fallout radionuclide 210-Pb to date floodplain surface ages along the regulated West River, where two flood control dams were built in 1961, and the unregulated White River in eastern Vermont. At five locations along the West River and at the control location along the White River, bulk core soil samples were collected laterally from the river up to terrace surfaces normally inundated in the pre-dam era to quantify excess 210-Pb inventories. At select locations complete vertical profiles of excess 210-Pb activity were also collected. Initial results show excess 210-Pb inventory increases with terrace elevation, indicating older floodplain surfaces lacking inundation from the regulated flow regime. At some sites radionuclide surface ages are well correlated with the ages of cored trees further suggesting floodplain stabilization. However, in most cases the radionuclide ages are older than expected, possibly suggesting some inheritance of excess 210-Pb with freshly deposited sediments. High activity peaks were observed at depth in the soil profiles, which may indicate either a buried paleosurface associated with channel migration or variable inheritance of deposited sediments over time.