Watershed and ocean controls of salt marsh extent and resilience

Abstract

The formation and evolution of tidal platforms are controlled by the feedbacks between hydrodynamics, geomorphology, vegetation, and sediment transport. Previous work mainly addresses dynamics at the scale of individual marsh platforms. Here, we develop a process‐based model to investigate salt marsh depositional/erosional dynamics and resilience to environmental change at the scale of tidal basins. We evaluate how inputs of water and sediment from river and ocean sources interact, how losses of sediment to the ocean depend on this interaction, and how erosional/depositional dynamics are coupled to these exchanges. Model experiments consider a wide range of watershed, basin, and oceanic characteristics, represented by river discharge and suspended sediment concentration, basin dimensions, tidal range, and ocean sediment concentration. In some scenarios, the vertical accretion of a tidal flat can be greater than the rate of sea level rise. Under these conditions, vertical depositional dynamics can lead to transitions between tidal flat and salt marsh equilibrium states. This type of transition occurs much more rapidly than transitions occurring through horizontal marsh expansion or retreat. In addition, our analyses reveal that river inputs can affect the existence and extent of marsh/tidal flat equilibria by both directly providing suspended sediment (favoring marshes) and by modulating water exchanges with the ocean, thereby indirectly affecting the ocean sediment input to the system (favoring either marshes or tidal flats depending on the ratio of the river and ocean water inputs and their sediment concentrations). The model proposed has the goal of clarifying the roles of the main dynamic processes at play, rather than of predicting the evolution of a particular tidal system. Our model results most directly reflect micro‐ and meso‐tidal environments but also have implications for macro‐tidal settings. The model‐based analyses presented extend our theoretical understanding of marsh dynamics to a greater range of intertidal environments. © 2020 John Wiley & Sons, Ltd.

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

“…The parameters w s and τ d were varied to reproduce similar levels of accretion observed in the wetlands where the original modelling framework was applied (Rodriguez et al, 2017). The values obtained were τ d = 0.02 Pa and w s = 2×10 −4 m s −1 , which are consistent with values reported by Larsen et al (2009) and Temmerman et al (2005). This model does not have an erosion term, which is not a bad simplification over vegetated surfaces that receive flows that are typically very slow.…”

“…On the other hand, more detailed description of hydrodynamic and sediment transport mechanisms can be incorporated into the computations of wetland dynamics using conventional two-or three-dimensional flow and sediment transport models (Ganju et al, 2015;Lalimi et al, 2020;Temmerman et al, 2005). A detailed description of flow and sediment transport processes can potentially result in a better estimation of wetland dynamics including accretion and migration processes, but implementation can be seriously limited by computational cost and data availability (Beudin et al, 2017).…”

How Old Are Marshes on the East Coast, USA? Complex Patterns in Wetland Age Within and Among Regions

“…Age distributions of marshes varied as a function of upland or riverine influence, as measured by RTR (ratio of annual river discharge to tidal estuary volume). Although there are many drivers of wetlands (e.g., vegetation feedbacks; Marani et al, 2010; Morris et al, 2002), sediment supply is a major control of marsh platforms (Bouma et al, 2016; D'Alpaos et al, 2012; Yousefi Lalimi et al, 2020). Cores from sites with a higher RTR formed earlier and varied less in age than sites with a lower RTR (Figure 2b).…”

“…As SLR decelerated and wetlands established along the coast (Saintilan et al, 2020), sediment loads would have varied with watershed characteristics including catchment area, slope, and drainage network structure, as well as bedrock geology and glacial and climatic history (Bell & Laine, 1985; Gordon, 1979; Johnson & Fecko, 2008; Leopold et al, 1964; Meade, 1982; L. F. Phillips & Schumm, 1987; Sella et al, 2007). Sediment supply also reflects the balance of sediments and water from marine and inland sources (Braswell & Heffernan, 2019; Ganju et al, 2013; Yousefi Lalimi et al, 2020). Near‐coastal storms can also impact landform stability and wetland dynamics (Cahoon, 2006; Davis, 1994; Leonardi et al, 2018).…”

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

“…On the other hand, more detailed description of hydrodynamic and sediment transport mechanisms can be incorporated into the computations of wetland dynamics using conventional two or three dimensional flow and sediment transport models (Ganju et al, 2015;Lalimi et al, 2020;Temmerman et al, 2005). A detailed description of flow and sediment transport processes can potentially result in a better estimation of wetland dynamics including accretion and migration processes, but 65 implementation can be seriously limited by computational cost and da ta a vaila bility (Beudin et al, 2017).…”