Coastal sedimentation across North America doubled in the 20th century despite river dams

Abstract

The proliferation of dams since 1950 promoted sediment deposition in reservoirs, which is thought to be starving the coast of sediment and decreasing the resilience of communities to storms and sea-level rise. Diminished river loads measured upstream from the coast, however, should not be assumed to propagate seaward. Here, we show that century-long records of sediment mass accumulation rates (g cm −2 yr −1) and sediment accumulation rates (cm yr −1) more than doubled after 1950 in coastal depocenters around North America. Sediment sources downstream of dams compensate for the river-sediment lost to impoundments. Sediment is accumulating in coastal depocenters at a rate that matches or exceeds relative sea-level rise, apart from rapidly subsiding Texas and Louisiana where water depths are increasing and intertidal areas are disappearing. Assuming no feedbacks, accelerating global sea-level rise will eventually surpass current sediment accumulation rates, underscoring the need for including coastal-sediment management in habitat-restoration projects.

Watershed Suspended Sediment Supply and Potential Impacts of Dam Removals for an Estuary

“…Observations from river gauge stations confirm declining sediment supply with many large rivers draining to the US East Coast experiencing declines in suspended sediment concentration, potentially associated with dam construction (Weston 2014) in addition to factors such as land use changes and climate-driven discharge trends. However, impacts of reduced sediment supply from damming are not necessarily observed in coastal regions, with many estuarine depocenters continuing to accumulate sediment at rates in excess of sea level rise (Rodriguez et al 2020). The increasing interest in dam removals raises questions about potential impacts of the release of impounded sediment at watershed and estuary scales.…”

Accretion, retreat and transgression of coastal wetlands experiencing sea-level rise

“…The vulnerability of coastal wetlands to future sea-level rise (SLR) has been extensively studied in recent years, and models of coastal wetland evolution have been developed to assess and quantify the expected impacts (Alizad et al, 2016b;Belliard et al, 2016;Clough et al, 2016;D'Alpaos et al, 2011;Fagherazzi et al, 2012;Kirwan and Megonigal, 2013;Krauss et al, 2010;Lovelock et al, 2015b;Mogensen and Rogers, 2018;Rodriguez et al, 2017;Rogers et al, 2012; Published by Copernicus Publications on behalf of the European Geosciences Union.…”

“…However, the projections of coastal wetland resilience under high rates of SLR appear to be at odds with paleo-environmental reconstructions of wetland responses to rising seas during the early Holocene (Horton et al, 2018;Saintilan et al, 2020). One explanation for this discrepancy is that models fail to reproduce the flow attenuation caused by the friction induced by substrate cover and specific wetland features like inner channels, embankments and flow constrictions (Hunt et al, 2015) and its effects on sediment availability, which may result in overestimation of wetland accretion rates (Rodriguez et al, 2017). Bathtub models do not provide information on flow discharges or velocities, so they need an independent specification of sediment concentration.…”

“…Here, we investigate how accretion and migration processes affect wetland response to SLR using a computational framework that integrates detailed hydrodynamic and sediment transport mechanisms that affect vegetation and landscape dynamics and that is efficient enough to allow the simulation of long time periods. The framework consists of a fast-performance quasi-two-dimensional hydrodynamic model (Riccardi, 2000;Rodriguez et al, 2017) that we have extensively tested in wetlands (Rodriguez et al, 2017;Saco et al, 2019;Sandi et al, , 2019Sandi et al, , 2020a and a sediment advection transport model (Garcia et al, 2015) that we couple with vegetation formulations for preference to tidal conditions to obtain realistic predictions of wetland accretion and migration under SLR. Our framework incorporates two vegetation species, mangrove and saltmarsh, and accounts for the effects of manmade features like inner channels, embankments and flow constrictions due to culverts.…”

“…We simulate the dynamics of internal channels, which can provide insight into wetland studies with strong influence of natural channels Silvestri et al, 2005) or manmade drainage channels (Manda et al, 2014). We carry out simulations with embankments and culverts representing flood-sheltered environments, which can resemble intentional flood attenuation works for coastal protection (Van Loon-Steensma et al, 2015) or unintentional flood attenuation as a result of roads, tracks, pipes and other infrastructure typical of heavily human-occupied coasts (Kirwan and Megonigal, 2013;Rodriguez et al, 2017;Temmerman et al, 2003).…”