Competing effects of vegetation density on sedimentation in deltaic marshes

River deltas, which critical ecosystem services important to the world’s population, are undergoing rapid degradation due to anthropogenic and natural factors. Marsh vegetation, which defines the most important delta ecosystem, has a strong effect on sediment retention and land-building, controlling both how much sediment can be delivered to and how much is retained by the marsh. An understanding of how vegetation influences these processes would improve the restoration and management of deltaic marshes. We use a random displacement model to simulate sediment transport, deposition, and resuspension within a freshwater marsh. As vegetation density increases, velocity declines, which reduces the supply of sediment to the marsh, but also reduces resuspension, which enhances the retention of sediment within the marsh. The competing trends of supply and retention produce a nonlinear relationship between net deposition and vegetation density, such that an intermediate density yields the maximum net deposition The optimum density, yielding the maximum net deposition, depends on stem diameter and water surface slope. The findings agree with field observations of sedimentation response to seasonal variation in vegetation density observed in a marsh within the Mississippi River Delta.