A Model for Mid‐Holocene—Present U.S. Mid‐Atlantic Piedmont River Corridor Sediment Budgets Fit to Stratigraphic Data

Abstract

AbstractFluvial sediment transport and deposition shapes river, floodplain, and estuarine systems but also can create water quality problems, which in some cases motivates efforts to reduce hillslope erosion. However, watershed management efforts rarely consider the fate of eroded sediments and the time scale of transport to estuaries. This study presents a modeling approach to estimate regionally averaged floodplain deposition, erosion, and sediment flux for the mid‐Atlantic U.S. caused by changing forest cover, urban development, and milldam construction. Regional regression equations estimate discharge at 3‐month intervals, and (temporally invariant) channel width and slope. Sediment concentrations are determined from a rating curve (defined for modern conditions by gaging‐station data). Tuning the model to floodplain stratigraphic data suggests that‐sediment concentrations prior to European colonization (i.e., before ∼1750) were 5%–8% of those prevailing today, while legacy (1750–1950) sediment concentrations were 25%–35% of present values. The calibration process, however, is only partly successful for the period before 1750 due to the limited number of older dated samples available. Nonetheless, the model accurately reproduces the observed age distribution of floodplain deposits. Computed sediment‐budget components increase monotonically from before 1750 to the present, and the ratio of budget components remains similar from one time period to the next. The model also predicts that millennial timescales are needed for mid‐Atlantic floodplains to equilibrate following a change in sediment regime, a finding with important implications for river corridor and watershed restoration planning.