Patterns of River Influence and Connectivity Among Subbasins of Puget Sound, with Application to Bacterial and Nutrient Loading

Puget Sound is an estuarine inland sea fed by 14 major rivers and also strongly influenced by the nearby Fraser River. A comprehensive, particle-based reanalysis of an existing circulation model was used to map the area of influence of each of these rivers over a typical seasonal cycle. Each of the 131,000 particles released in the 15 rivers was associated with a freshwater volume, a nutrient load, and a fecal coliform load based on statistics from 10 years of Washington Department of Ecology monitoring data. Simple assumptions regarding mortality and nutrient utilization/export rates were used to estimate the decrease in bacterial and nutrient load as individual parcels of river water age. Reconstructions of basin-scale volume fluxes and salinities from the particle inventory provide consistency checks on the particle calculation, according to methods suitable for error analysis in a wide range of particle-based estuarine residence time studies. Results suggest that river contributions to total freshwater content in Puget Sound are highly nonlocal in spring and summer, with distant, large rivers (the Fraser and Skagit) accounting for a large fraction of total freshwater. However, bacterial mortality and nutrient export rates are relatively fast compared with transport timescales, and so significant loadings associated with major rivers are in most cases only seen close to river mouths. One notable exception is fecal coliform concentration in Bellingham Bay and Samish Bay, which lie north of Puget Sound proper; there, it appears that the Fraser River may rival local rivers (the Samish and Nooksack) as a pathogen source, with the much higher flow volume of the Fraser compensating for its remoteness.