Spatially averaged stratigraphic data to inform watershed sediment routing: An example from the Mid-Atlantic United States

New and previously published strati -graphic data define Holocene to present sedi-ment storage time scales for Mid-Atlantic river corridors. Empirical distributions of deposit ages and thicknesses were randomly sampled to create synthetic age-depth re-cords. Deposits predating European settle-ment accumulated at a (median) rate of 0.06 cm yr-1, range from -18,000 to 225 yr old, and represent 39% (median) of the to-tal accumulation. Sediments deposited from 1750 to 1950 (legacy sediments) accumu-lated at a (median) rate of 0.39 cm yr-1 and comprise 47% (median) of the total, while modern sediments (1950-present) represent 11% of the total and accumulated at a (median) rate of 0.25 cm yr-1. Synthetic strati -graphic sequences, recast as age distributions for the presettlement period, in 1900 A.D., and at present, reflect rapid postsettlement alluviation, with enhanced preservation of younger sediments related to postsettlement watershed disturbance. An averaged present age distribution for vertically accreted sediment has modal, median, and mean ages of 190, 230, and 630 yr, reflecting the pre-dominance of stored legacy sediments and the influence of relatively few, much older early Holocene deposits. The present age distribution, if represented by an exponen-tial approximation (mean age -300 yr), and naively assumed to represent steady-state conditions, implies median sediment travel times on the order of centuries for travel dis-tances greater than -100 km. The percentage of sediment reaching the watershed outlet in 30 yr (a reasonable time horizon to achieve watershed restoration efficacy) is -60% for a distance of 50 km, but this decreases to <20% for distances greater than 200 km. Age dis-tributions, evaluated through time, not only encapsulate the history of sediment storage, but they also provide data for calibrating wa-tershed-scale sediment-routing models over geological time scales.

Automated summary

This study uses stratigraphic data to investigate sediment storage time scales in Mid-Atlantic river corridors, finding that pre-European settlement deposits and legacy sediments make up the majority of accumulated sediments, while modern sediments are less abundant. The age distributions evaluated over time reflect rapid post-settlement alluviation and enhanced preservation of younger sediments. These findings are important for the study of watershed-scale sediment-routing models.