Elucidating patterns of channel erosion, sediment deposition, and vegetative regrowth on the restored Nisqually River Delta

Abstract

Tidally-influenced salt marshes are an ideal habitat for a variety of species including overwintering waterfowl and outmigrating juvenile salmon. Post-restoration salt marsh formation is dependent upon several factors such as frequency and duration of tidal inundation, sediment input from upstream sources, and local prevalence of halophilic plant species. Here we present five years of monitoring data to examine biophysical and vegetative changes on the restored Nisqually River Delta in Puget Sound, Washington, USA. We used these data to evaluate tidal channel morphology, marsh elevation, and vegetative community structure through time. Overall, major channel area increased 42% and major channel length increased 131% from pre- to post-restoration conditions across the Delta, with channel bed erosion occurring at rates of 6.5 cm/year on average. Furthermore, our analyses suggested that mudflat elevations in the restoration area were increasing at a rate of 0-3 cm/year, with Sloughs closer to the mouth of the Nisqually River experiencing more rapid sediment accretion rates as a result of higher initial elevations. These factors interacted to determine vegetative community composition on the Delta. Low marsh, saline-tolerant species were prevalent at newly colonizing restored sites, while high marsh halophilic and brackish species were dominant at two unaltered reference sites. Vegetation colonization occurred faster at sites with higher initial elevations, although species diversity still increased slightly (about 10%) through time at low elevation sites closer to open water. This integrative approach can be used as a tool for scientists and managers to evaluate the immediate effects of dike or levee removal. By incorporating multiple years of monitoring data into a spatially explicit model, we can provide simple projections of geomorphological changes following restoration