STRATIGRAPHIC DATA CALIBRATES A MODEL OF HOLOCENE-PRESENT RIVER CORRIDOR SEDIMENT STORAGE FOR U.S. MID-ATLANTIC PIEDMONT RIVERS

Floodplain sediment storage strongly influences downstream sediment routing, but storage is difficult to include in watershed models due to its characteristic long timescales. In the mid-Atlantic U.S., 18^th and 19^th Century watershed sediment disturbance and mill damming dramatically increased the amount of sediment stored in river corridors; these deposits continue to influence contemporary watershed management. To quantify the influence of sediment storage on sediment delivery in this region over event and millennial timescales, a model of floodplain sedimentation is calibrated using stratigraphic data defining floodplain sediment accumulation over 3 time periods: pre-Settlement (before 1750), “legacy” (1750-1950), and modern. The model determines the magnitude of storm discharges using regional regression equations that account for drainage basin area, forest cover, impervious surfaces, and other watershed characteristics; other empirical regression equations estimate storm durations. Channel width, slope, and roughness (all assumed temporally invariant) are computed from regional hydraulic geometry equations. Water level estimates assume steady uniform flow, with a correction for nearby mill dams. Sediment concentrations are assessed using a rating curve, which is well-defined for modern conditions. The model computes floodplain deposition during overbank flows, while stored sediment is eroded based on age (e.g., elapsed time since deposition). Calibration of the model to modern floodplain sediment thicknesses provides an estimate of the effective sedimentation velocity, equivalent to the settling rate of fine silt. With the sedimentation velocity determined, model calibration determines suspended sediment concentrations for legacy and pre-Settlement time periods. Pre-Settlement suspended sediment concentrations were ~3% of those prevailing today. Surprising, sediment concentrations from 1750-1950 were almost identical to present values. Apparently, the dramatic watershed disturbances that lead to the accumulation of legacy sediment in the 18^th and 19^th Centuries were not markedly different from present processes, at least from the perspective of floodplain overbank deposition.