In Possession Sound, a salt wedge estuary in the Whidbey Basin in Northwest Washington State, interactions between the saltwater of Puget Sound and the freshwater of the Snohomish River create a dynamic environment. Students from the Ocean Research College Academy deployed two Sea-Bird Conductivity-Temperature-Depth (CTD) probes within the estuary to monitor water chemistry in this complex system. A grant from the National Science Foundation supported eleven students to analyze water chemistry data from summer 2018 to increase their understanding of biogeochemical processes in the estuary and communicate their knowledge to the general public. Data from the month of May were analyzed to investigate the river’s influence on the temperature and salinity. Lower levels of salinity correlated with peaks in freshwater input as indicated by river discharge data. Salinity also correlated with tide height. To further investigate the spatial influence of the Snohomish River on Possession Sound, this study endeavored to manufacture a Fast Oceanographic Automated Measurement (FOAM) sampler in collaboration with Gravity Consulting. This device uptakes surface water and pumps it over an EXO 2 Sonde to measure surface water chemistry while traveling on a research vessel, associating data with GPS points, to track the freshwater plume. Salinity was used to examine the relationship between the spatial extent of the plume, river discharge, and tidal patterns, and temperature and pH was analyzed to explore the influence of the river. Lower salinity levels were used as indicators of higher freshwater influence. During periods of high river discharge, average salinity values will be lower and the freshwater influence will cover a larger area. During high tides, higher average salinity values are expected and there will be smaller areas of freshwater influence. It is hypothesized that areas of higher river influence will be warmer and more acidic than areas of increased saltwater influence.

Located in the Whidbey Basin of the northern reaches of Puget Sound, Possession Sound contains the Snohomish River estuary, encompassing a river system that is the third largest contributor of freshwater to the Puget Sound. Myself, and several of my fellow students have had the opportunity to collaborate with the University of Washington, the Washington State Department of Ecology, and a local environmental consultant -- Gravity Marine. Utilizing data collected by permanently moored Sea-Bird CTD probes, during research cruises, and by the United States Geological Survey (USGS), my research partner and I have created a model to facilitate an investigation of how both tides and the discharge of freshwater from the Snohomish River influence the water quality of Possession Sound. To better understand the complex patterns of the water entering Possession Sound from the Snohomish River, we analyzed the relationship between a continuous stream of water quality data (temperature, salinity and turbidity) from a Sea-Bird CTD probe at the mouth of the Snohomish River and continuous discharge data gathered by the USGS at a station 12 miles up the river. The distance between these two sites results in a delay between when river discharge data is recorded up river and when it influences the water quality at the river mouth. From the analysis of these two locations and with guidance from ourcollaborators as well as outside professionals, we used the statistical analysis language R to create a model that predicts the travel time of water from the USGS stream gage to Possession Sound. This model can be applied when considering the effect on the estuary of important factors from the river, such as nutrient loading; influxes of cold water, which promotes upwelling; and the river’s contribution of heavy metals and other pollutants.

The physics behind wave-driven mixing of river and ocean waters and current-driven wave breaking are not well understood. The current body of work surrounding river-ocean interactions focuses on large rivers. However, small rivers, which are much more strongly influenced by waves, make up the majority of such systems, and contribute significantly to global riverine discharge. Examining the momentum balance of river flow in opposition to wave-driven forcing from the ocean is necessary to understand how waves influence the travel and mixing of river water. One way to measure this interaction is using instrumental drifting buoys that follow the path of the river water and take temporal measurements of water properties. These leave gaps in our knowledge, as such buoys do not provide a description of the entire system, only specific points. To fill in these gaps, Unmanned Aerial Vehicle (UAV) footage was used to understand broader wave-current interactions at the Quinault River mouth, a small river that feeds directly into the Pacific Ocean. The town of Taholah, WA, is on its banks, and faces challenges due to wave-driven flooding. The size of the surf zone, the nearshore region where waves break at high frequency, was mapped with UAV footage, and related back to local environmental conditions, such as tidal phase. At low water, the momentum from the river is maximized, and so is the cross-shore extent of the surf zone. This decreases salinity around the river mouth, as freshwater is trapped by the surf zone. At high tide, these conditions are reversed, and fresh water streams can be detected past the surf zone, suggesting the river water has escaped from this region of high turbulence. The conditions under which these escapes occur are to be understood by combining analyses of UAV footage with drifter and tidal data.

Suspended sediment in the bottom boundary layer impacts both ecosystems and geomorphology. High concentrations of suspended sediment affect light attenuation, harming benthic plants, and sediment resuspension and transport can affect the distribution and size of sediment on the seafloor. The purpose of this project was to determine the variations and controls on suspended sediment in the Elwha River nearshore region and to find relationships between bed shear velocity and suspended sediment concentration. The 2011 Elwha River dam removal released a large pulse of sediment, giving us the opportunity to study a coastal environment with fine sediment deposits, and varying hydrodynamic conditions. Data collection measured near-bed turbidity and wave conditions, and sediment grab samples were collected to characterize bed conditions. Harmonic tidal analysis was used to predict tidal current velocity. Over the sampling period, on the east side of the river mouth, currents ranging from ~0 to 100 cm/s and wave heights up to 1.0 m were sufficient to resuspend sediment. Suspended sediment concentrations generally ranged from 1.5 to 25 mg/L. In Freshwater Bay, currents ranging from ~0 to 31 cm/s and wave heights up to 0.86 m were not sufficient to resuspend sediment. Instead, fine sediment settled out of the water column, resulting in near-bed sediment concentrations generally ranging from 1 mg/L to 101 mg/L. These findings show how variable the processes controlling sediment in suspension can be in a tidal environment with complex morphology.

Methane reservoirs are commonly found throughout the world’s oceans and the release of methane from seafloor reservoirs is thought to make up 5 to 10% of the global atmospheric methane. In fact, the greatest deep-sea mass extinction in the last 97 Myr during the Paleocene-Eocene Thermal Maximum (PETM) may have been caused by methane release from seep sites along the upper continental slope margin. Recently, methane reservoirs along this margin have been gaining attention due to their potential to accelerate current global warming. Changes in seafloor pressure and temperature could destabilize these seafloor deposits and cause methane bubble plume release into the ocean. At SHR, an extensively studied active seep site located ~ 90 km offshore Oregon, discontinuity in methane plume release was observed, but still not well understood. Hence, using Acoustic Doppler Current Profiler (ADCP) and pressure data archived by the Ocean Observatories Initiative (OOI) Cabled Array, we are investigating the potential correlation between tides and the presence of methane plume at SHR. Our study detects methane plume structures based on the proxies of echo contrast caused by acoustic-bubble interaction. By analyzing the derived plume structures and their correlation with 226 tidal cycles, we expect a trend of plume release triggered by low tides. Our study provides the first high-temporal-resolution analysis on the methane plume release at SHR using OOI acoustic data.