POST-SETTLEMENT “LEGACY” FLOODPLAINS OF THE EASTERN U.S. MAY NOT BE TERRACES: EVIDENCE FROM MERCURY INVENTORIES, SOUTH RIVER, VIRGINIA

Accelerated vertical floodplain accretion began soon after European settlement of the eastern and midwestern U.S. Many geomorphologists have proposed that valley alluviation was completed by the early 20^th Century, creating post-settlement “legacy” alluvial terraces isolated from ongoing fluvial processes. This hypothesis, however, has not been evaluated by extensive measurements of overbank sedimentation rates in the eastern U.S. We estimate the thickness of floodplain sediment deposited along a 4.5 km reach of the South River from 1930-2007 using mercury inventories from 107 cores and our reconstruction of the river’s history of mercury contamination. A calibrated HEC-RAS model indicates that 53% of the 100-year floodplain is inundated every two years, while 83% is inundated every 5 years. Median centennial accumulation rates are 3.8 cm/100 yrs for the <0.3 yr floodplain, 1.37 cm/100 yrs for the 0.3-2 yr floodplain, 0.4 cm/ 100 yrs for the 2-5 yr floodplain, and 0.1 cm/100 yr for the 5-62 yr floodplain. These rates are consistent with independent estimates obtained using dendrochronology. The total mass of sediment stored in our short study reach since 1930 is 4.9 +/- 1.7 (95% confidence interval) x10⁷ kg, representing 8-12% of the annual suspended sediment load of the South River. These sedimentation rates are 1.5-3 orders of magnitude less than published values for post-settlement alluviation, but the South River’s floodplain should not be considered a terrace: it is frequently inundated by floodwaters and continues to store a significant fraction of the river’s annual suspended sediment load.