The interaction between incoming salt water from the ocean, which is driven by tides, and exiting freshwater from rivers drives circulation through estuarine environments. As a result of the interaction between the incoming water and the exiting water, nutrients and sediment are moved around the estuary. This study focuses on the interaction between tides and the Snohomish River as it enters the Possession Sound estuary, located in Everett, Washington. To acquire data, I deployed a boat-mounted acoustic Doppler current profiler (ADCP), which uses sound to measure water current direction and velocity. I collected these data at varying tide stages at three different sites between the Snohomish River’s southern output to nearby Mount Baker Terminal, located 3.6 miles southwest of the river output. I have collected 8 samples over the course of 7 months at both ebb and flood tide stages. Each transect survey lasted 3-6 minutes and collected data from the first 20 meters of the water column. To get accurate analysis of current velocity and direction I split the data into categories based on depth. Preliminary analysis shows a southward current at many sites during all tide stages and depths. This raises questions about the scope of the river influence and the potential for southward currents regardless of tidal stage. However, further analysis of current velocity and river discharge are needed. Due to the complexity of the currents in the area, understanding how the river and the tides are interacting can provide a greater understanding of how these currents are impacting the dispersal of sediment and nutrients throughout the estuary.

Phytoplankton production depends on a number of factors including nutrient availability, water chemistry variables, and light penetration. Previous studies have shown light penetration to be important for submerged aquatic vegetation, the primary producers that support the marine food web and the ecosystem. Possession Sound is a productive sub-basin of the Salish Sea with complex influences on local water chemistry and primary productivity. Given the significance of primary production in a salt water estuary, this study looks at the seasonal relationships between water chemistry and light penetration, measured by photosynthetically active radiation (PAR) in the Possession Sound estuary across three sites. I collected Seabird CTD and YSI EXO profile data as well as PAR sensor results, in Possession Sound from July 2022 through March 2023. Accompanied with historical data collected by past Ocean Research College Academy researchers, I analyzed site dependent relationships as well as the seasonal relationships. Preliminary analyses showed PAR decreasing with higher salinity and turbidity, but increasing with temperature. Limited connection was observed with dissolved oxygen. Studying the relationships among light penetration and water chemistry allows us to better understand the complex relationships among the key factors determining seasonal primary production.