The Fourth Time Dimension of Sediments: Incorporating Geologic Data Into Watershed Sediment Routing Models To Predict Fine-Grained Legacy Sediment Delivery Timescales

Transport of fine-grained silt and clay-sized sediment through watersheds is often interrupted when sediments are stored in alluvial deposits. Once deposited, particles may be stored for durations of less than a year to many millennia, potentially greatly increasing travel times for sediment through watersheds, and creating a "legacy effect" where sediment delivered downstream today may have initially been eroded from upland sources, tens, hundreds, thousands, or even millions of years ago. While long sediment delivery timescales caused by sediment storage are often associated with human disturbances, geologists recognize that they are a characteristic feature of any river corridor with well-developed floodplains and other long-lived alluvial deposits regardless of anthropogenic influence. Here I to describe how these long timescales can be incorporated into watershed sediment routing models, using "legacy" deposits associated with European settlement of the mid-Atlantic region as an example. A meta-analysis of published stratigraphic descriptions of well-dated mid-Atlantic valley fills is used to create synthetic age-depth models of deposits likely to be exposed in eroding banks. Deposits are categorized as pre-European Settlement (PS), "legacy" (LS) sediments deposited after European Settlement but before 1900, and modern sediment (MS) deposited since 1950. Results demonstrate that 40% of the valley fills consist of PS deposits, while LS and MS deposits comprise 36% and 11%, respectively. Sedimentation rates of LS and MS deposits are statistically indistinguishable (0.32 cm/yr vs 0.26 cm/yr), demonstrating the continuity of ongoing sediment accumulation in these river valleys. Synthetic age-depth relationships can be recast as distributions of sediment waiting times, thereby quantifying the timescales associated with a single episode of deposition and remobilization. Waiting time distributions are heavy-tailed, with medians on the order of several decades and tails that extend back to the Pleistocene. When combined with sediment budget estimates of the probability of deposition, the full distribution of sediment travel times can be assessed. These sediment delivery timescales provide essential information for watershed sediment management.