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

Abstract

New and previously published stratigraphic data define Holocene to present sediment storage time scales for Mid-Atlantic river corridors. Empirical distributions of deposit ages and thicknesses were randomly sampled to create synthetic age-depth records. Deposits predating European settlement 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 total accumulation. Sediments deposited from 1750 to 1950 (“legacy sediments”) accumulated 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 stratigraphic 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 predominance of stored legacy sediments and the influence of relatively few, much older early Holocene deposits. The present age distribution, if represented by an exponential 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 distances 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 distributions, evaluated through time, not only encapsulate the history of sediment storage, but they also provide data for calibrating watershed-scale sediment-routing models over geological time scales.

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

“…As a result, Pizzuto et al. (2022) assembled “synthetic” stratigraphic sections from probability distributions of the thicknesses of each unit and the distribution of basal radiocarbon ages. Pizzuto et al.…”

“…Pizzuto et al. (2022) describe their results as defining distributions of regional‐average deposit thicknesses and their potential maximum ages. The resulting sections do not reflect specific locations, and the analysis cannot reveal systematic variations in stratigraphy associated with geomorphic setting (distance from modern stream channels, e.g.,), drainage‐basin area, or basal age.…”

“…Piedmont river corridors consist of gravel‐bedded stream channels typically (but not exclusively) flowing adjacent to active floodplains (Bodek et al., 2021; Noe et al., 2022; Pizzuto et al., 2016, 2022). Piedmont river valleys are relatively narrow, ranging from 3.6 to 188 m in width (floodplain width data are presented in the Supporting Information of Noe et al., 2022; Table S1 in Supporting Information S1).…”

Channel head response to anthropogenic landscape modification: A case study from the North Carolina Piedmont, USA, with implications for water quality

“…Millponds behind these dams served as accumulation traps for the newly eroded hillslope sediment; the evidence of these reservoirs is observable in streambank sediment records throughout the Piedmont (e.g., Lyons et al, 2015; Starek et al, 2013; Walter & Merritts, 2008; Wegmann et al, 2012). These meters‐thick packages are just some of the many types of deposits referred to as “legacy sediment” as they record the legacy of anthropogenic disturbances to background sediment erosion, transport, and deposition rates in headwaters and trunk channels across the Piedmont of eastern North America (Costa, 1975; Jackson et al, 2005; James, 2013; Pizzuto et al, 2022; Reusser et al, 2015; Trimble, 1975; Walter & Merritts, 2008).…”

“…However, much of it is still awaiting export to receiving streams, estuaries, and ultimately the Atlantic Ocean. Notably, the slow re‐erosion and downstream transport of legacy sediments, compared to its emplacement rate (e.g., Pizzuto et al, 2022), continues to contribute to chronically turbid stream waters that will persist for centuries to perhaps millennia along the east coast of the United States (Jackson et al, 2005; Voli et al, 2013; Wohl & Merritts, 2007).…”