Shifting Sediment Dynamics in the Coos Bay Estuary in Response to 150 Years of Modification

Estuaries worldwide have experienced modifications including channel deepening and intertidal reclamation over several centuries, resulting in altered fine sediment routing. Estuaries respond differently based on preexisting geometries, freshwater and sediment supplies, and extents and types of modification. The Coos Bay Estuary in Oregon is a relatively small estuary with complex geometry that has been extensively modified since 1865. A sediment transport model calibrated to modern conditions is used to assess the corresponding changes in sediment dynamics. Over similar to 150 years, channel deepening (from similar to 6.7 to 11 m), a 12% increase in area, and a 21% increase in volume have led to greater tidal amplitudes, salinity intrusion, and estuarine exchange flow. These changes have reduced current magnitudes, reduced bed stresses, and increased stratification, especially during rainy periods. Historically, fluvially derived sediment was dispersed across broad, deltaic-style flats and through small tidal channels. Now, river water and sediments are diverted into a dredged navigation channel where an estuarine turbidity maximum (ETM) forms, with modeled concentrations >50 mg/L and measured concentrations >100 mg/L during discharge events. This new ETM supplies sediment to proximal embayments in the middle estuary and the shallow flats. Overall, sediment trapping during winter (and high river discharges) has increased more than two-fold, owing to increased accommodation space, altered pathways of supply, and altered bed stresses and tidal asymmetries. In contrast to funnel-shaped estuaries with simpler geometries and river-channel transitions, these results highlight the importance of channel routing together with dredging in enhancing sediment retention and shifting pathways of sediment delivery.

Automated summary

Estuaries worldwide have undergone modifications over centuries, impacting sediment dynamics. The Coos Bay Estuary in Oregon, a small estuary with complex geometry, has experienced significant changes since 1865, leading to alterations in tidal amplitudes, salinity intrusion, and estuarine flow. These changes have resulted in reduced current magnitudes, increased stratification, and the formation of an estuarine turbidity maximum supplying sediment to proximal embayments.