Linking salt marsh lateral change with vertical accretion: Insights from the Oregon coast, USA

Abstract:

The study of salt marsh morphodynamics has mainly focused on comparisons of vertical accretion to relative sea level (RSL) rise. Recently, greater emphasis has been placed on elucidating the drivers of salt marsh lateral change, but research has focused on modeling. To better inform models, more field-based, empirical research of the linkages between vertical and lateral salt marsh sediment accumulation is needed. Wind-wave attack is a primary driver of marsh edge erosion, which is increasing with sea level rise and intensification of storms. In sediment-limited systems, internal redistribution of sediment through edge erosion may provide a critical source of material for elevation growth of the marsh platform vulnerable to RSL rise. In sediment-rich systems, erosion may be offset on seaward edges by accumulation on mudflats leading to elevation gain and expansion of vegetated marsh. Using the Oregon coast as a natural laboratory, we ask: (1) how are patterns of salt marsh vertical accretion and lateral change related over short (~10 y) and long (~140 y) time scales?; (2) how are the histories of land-use change, fluvial discharge, and RSL change reflected in salt marsh morphological change? To answer these questions, we measured ~140 y of lateral salt marsh change in 5 Oregon estuaries with vertical sediment accretion data; varying estuarine morphologies as a proxy for wave action (drowned river mouth, bar built, & lagoon); and differing rates of RSL change and sediment supply. Preliminary results indicate that over the last century salt marshes displaying positive accretionary balances also exhibit net lateral growth even when RSL is falling. In addition to experiencing high mean annual sediment loads Oregon marshes are located in watersheds that experienced intensive logging and wildfire. Further, periods of salt marsh vertical accretion appear to correlate with periods of lateral expansion, and both are coincident with intense land use. Conversely, marshes that display a negative or neutral accretionary balance are those that are net eroding or have a stationary edge, likely as a result of low sediment loads. Thus, sediment supply, especially when elevated over natural loads, appears to drive both vertical and lateral salt marsh accumulation more so than RSL rise or estuarine morphology.