Shifts in sediment routing and deposition associated with 150 years of estuary modification in Coos Bay, Oregon

Estuaries serve as important conduits and sinks for fluvial sediment, but during the past 100-150 years many estuaries have experienced channel deepening and loss of intertidal areas due to development activities. These changes inevitably alter hydrodynamics and sediment routing, though the full impacts are frequently unknown due to long histories of alteration that pre-date scientific observations of estuarine dynamics. In this study we evaluated bathymetry changes and modeled sediment dynamics in the Coos Bay Estuary (Oregon), the largest port facility between San Francisco and the Columbia River entrance, using a finite-volume hydrodynamic model. Model scenarios were run using interpolated 1860s bathymetry derived from historic charts, as well as modern bathymetry including a proposed dredging plan within 13 km of the estuary entrance. In the ~150 years since major developments began, the estuarine area has decreased by ~12%, due largely to reclamation of shallow intertidal areas. Deepening of the primary navigation channel from ~6.7 to 11 m has led to a 21% increase in volume. Re-routing of the primary fluvial sediment source has caused a slight coarsening (from ~114 to 125 microns) of a broad region of intertidal flats, and more efficient routing of fine-grained sediment to the primary navigation channel. Deepening of the primary channel has facilitated increased fine-grained sediment deposition in the upper 2-3 km section adjacent to the intertidal flats, necessitating ongoing maintenance dredging. These models highlight the fundamental shifts in sediment routing, bed textures, and depocenters that can occur in response to combined channel deepening and channel redirection.