Sediment transport and trapping in marsh systems: Implications of tidal flux studies

Abstract

Although the concept that marshes act as major sediment sinks may be accurate when viewing them over the last few millennia, tidal transport studies suggest considerable variability with most marshes presently exporting material on an annual basis. High-salinity marshes along the mid-Atlantic coast of the United States appear to be losing 1–2 kg m −2 yr −1 while submerged upland marshes on the Delmarva Peninsula are eroding at rates of up to 14 kg m −2 yr −1. By comparison, at least one deltaic marsh on the Dutch coast along with several estuarine marshes appear to be accumulating sediment. In order to assess the trapping ability of marshes in large estuaries, we constructed a sediment budget for Chesapeake Bay which included a variety of wetland types. Calculations indicate that estuarine marshes trap 5–11% of the annual Chesapeake Bay sediment input, or about one half that previously estimated. It appears that most sedimentation in estuaries, and perhaps other coastal systems, occurs in subtidal flats below the limit of emergent marsh vegetation. As mud flats become vegetated and estuarine infilling proceeds, there may be a tendency for tidal currents to become ebb-dominated which promotes a net export of particulates. The extent to which major storm events are capable of returning enough material to balance the long-term accretionary budget of tidal marshes and keep them abreast of rising sea level is open to question. Differences in tidal dynamics, seasonal changes in sea levels and higher temperatures may help explain why, in the U.S., southern marshes are more susceptible to export and eventual erosion than northern marshes. We hypothesize that another factor, the recent reductions of terrigenous sediment inputs from the southern river systems of the U.S., may also be critical. Sediment starvation may have led to undernourishment of wetland systems of the coastal zone over the last half century which may be reflected in the net export measured in the tidal marshes in this region. Furthermore, we postulate that changes in sediment inputs are more important than eustatic sea level rise in causing the past losses of marshes which are now undergoing mass erosion. Thus, future wetland survival will depend as much as particulate inputs to the coastal zone as on the prospects of a global rise in sea level, and more efforts should be made to quantify the sediment budgets of tidal marshes.