LB 20-178 - Quantifying geomorphic and vegetative change in restored tidal wetlands of the Nisqually River Delta

In the Pacific Northwest, significant advances toward restoration of estuarine habitat in Puget Sound, Washington, were made with the restoration of over 900 acres to tidal influence. Through a partnership between the U.S. Fish and Wildlife Service Billy Frank Jr. Nisqually National Wildlife Refuge (Refuge) and Nisqually Indian Tribe (Tribe), restoration efforts culminated in 2009 with the removal of nearly five miles of dike to restore over 750 acres of estuary. This complemented three earlier projects completed by the Nisqually Tribe between 1996 and 2006. Restoration projects were implemented with the aim of restoring and enhancing habitat for a variety of wildlife, particularly the federally threatened Nisqually fall chinook salmon stock. In particular, the area of focus for the 2009 restoration had subsided approximately 1m in some places, resulting in intertidal mudflats and sparsely vegetated areas following over a century of agricultural use and separation from tides. Since 2009, the U.S. Geological Survey, Western Ecological Research Center, Refuge, and Tribe have collaborated to conduct monitoring to assess the effectiveness of these projects to restore estuarine habitat, processes, and function. Following restoration, marsh recovery is reliant on inorganic and organic accretion of surface elevations that determine where vascular vegetation can establish. Subsequent hydrogeomorphic processes are also necessary to maintain marsh surface elevation in relation to sea level. Our study leveraged the availability of long-term field survey and remote sensing datasets to conduct an analysis of fine-scale spatiotemporal habitat change in tidal wetland restoration parcels 13-22 years following tidal reintroduction. To determine elevation and geomorphic change across the estuary, we conducted a change detection analysis with 2011 and 2020 aerial light detection and ranging (lidar) datasets. We conducted RTK GPS ground surveys to validate the imagery and incorporated vegetation surveys and multispectral aerial imagery to relate patterns of vegetation colonization to elevational change. Preliminary findings indicate that vegetation colonization continues to progress slowly in the area restored in 2009 and is predominantly occurring on the eastern side of the site. To assess vegetative succession, we analyzed community composition along resampled transects and compared trajectories between restored and reference parcels. This study will contribute information on the trajectory and time frame of vegetation recovery to restoration practitioners and may inform the site selection and design of future restoration projects.