Submarine canyon sediment transport and accumulation during sea level highstand: Interactive seasonal regimes in the head of Astoria Canyon, WA

The majority of submarine canyons on Earth today do not directly intersect littoral or fluvial sediment sources, yet these systems are rarely studied. The shelf-incised head of Astoria Canyon receives sediment from the nearby Columbia River and is subject to energetic forcing from shelf and slope processes, making it an ideal site to evaluate the modern activity of canyons in high-stand sea level conditions. This study uses in-situ data from Astoria Canyon to identify the active sediment transport processes and patterns of accumulation in temperate canyon systems that are decoupled from their sediment sources during sea level highstand. Hydrodynamic data from a benthic tripod deployment in the head of Astoria Canyon shows that sediment resuspension and transport during summer is driven by internal tides and plume-associated nonlinear internal waves. Observations of shoreward-directed currents and low shear stresses (<0.14 Pa) along with sediment trap data suggest that seasonal loading of the canyon head occurs during summer. Nearby long-term wave data show that winter storm significant wave height often exceeds 10 m, driving shear stress capable of resuspending all grain sizes present within the canyon head. Swell events are generally concurrent with downwelling flows, providing a mechanism for episodic downcanyon sediment flux. Century-scale accumulation rates evaluated from sediment cores show slow accumulation in the upper canyon head, but rates progressively increase with depth to at least 300 m. The depositional environment in Astoria Canyon continues to respond to fluvial and oceanic forcing over an annual cycle. This study indicates that canyon heads can continue to function as sites of sediment winnowing and bottom boundary layer export even with a detached, shelf-depth canyon head.