PNNL

Abstract

The Skagit River is the largest river in the Puget Sound estuarine system. It discharges about 39% of total sediment and more than 20% of freshwater into Puget Sound. The Skagit River delta provides rich estuarine and freshwater habitats for salmon and many other species wildlife. Over the past 150 years, economic development in the Skagit Estuary delta has resulted in significant losses of wildlife habitat, particularly due to construction of dikes. This has resulted in disconnected and isolated the pathways of freshwater and natural sediment to the mudflats and tidal marsh area near the river delta. Diked portion of the delta is known as Fir Island where irrigation practices for agriculture land over last centaury has caused soil to be saturated progressively and resulted in land subsidence. This has also caused reduced efficiency of drainage network and impeded fish passages through the area. In this study, a three-dimensional tidal circulation model was developed for the Skagit River delta to assist estuarine restoration in the Fir Island area. The hydrodynamic model used in the study is the Finite Volume Coastal Ocean Model (FVCOM). The hydrodynamic model was calibrated using field data collected from the study area specifically for the model development. Wetting and drying process in the estuarine delta is simulated in the hydrodynamic model. The calibrated model was applied to simulate different restoration alternatives and provide guidance for estuarine restoration and management. Specifically, the model was used to help select and design configurations that would improve the supply of sediment and freshwater to the mudflats and tidal marsh areas outside of diked regions and then improve the estuarine habitats for salmon migration.