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

Fluvial 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. Plain Language Summary: Sediment carried by rivers creates water‐quality problems in estuaries downstream. One solution is to reduce erosion from upland hillslopes. Eroded sediment, however, is rarely transported directly to estuaries, but is often deposited in floodplains, a process that is rarely considered in watershed management. This study quantifies floodplain deposition and erosion in the mid‐Atlantic region caused by changing forest cover, urban development, and mill‐dam construction. The model is adjusted to accurately reproduce observed floodplain deposits from three time periods: prior to European colonization (before 1750), legacy (from 1750 to 1950), and modern (1950–2017). The calibration process, however, is only partly successful for the period before 1750 due to the limited number of older dated samples available. The model predicts that the rates of sediment transport and floodplain sedimentation and erosion have increased through time from before 1750 to the present. When deposited, sediment "awaits" on floodplains before being remobilized for tens, hundreds, and even thousands of years, indicating that long time periods will be needed before estuaries downstream can realize the full intended benefits of watershed restoration. Key Points: A model driven by land‐use changes and tuned to stratigraphic data predicts floodplain sediment budgetsAll sediment‐budget components have increased through time, and millennia are needed to reach equilibrium after disturbancesWell‐dated stratigraphic sections, currently unavailable, are needed to fully calibrate the model for conditions preceding 1750 [ABSTRACT FROM AUTHOR]