LINKING THE LAND TO THE SEA: TRACKING FLUVIAL BEDLOAD FROM COASTAL WATERSHEDS TO DELGADA SUBMARINE CANYON, NORTHERN CALIFORNIA

Delgada and Spanish submarine canyons offshore of northern California provide an ideal laboratory for investigating modern canyon connectivity to fluvial and littoral sources, because Delgada Canyon intercepts sediment from the Spanish Flat littoral cell while the head of Spanish Canyon lies offshore from littoral source areas. In the first phase of this project, the morphology of the uppermost 200 meters of both Delgada and Spanish canyons was surveyed in 2015 via high-resolution 200- and 400-kHz multibeam sonar. Comparison with a previous swath multibeam survey conducted in 2008 suggests that Delgada is characterized by: (1) removal of more than 50,000 m³ of sediment from the most landward channel and headwall region; (2) migration of 30-60 meter wavelength crescent-shaped sediment waves within the axial channel; and (3) the aggradation of channel-margin levees. Conversely, Spanish Canyon is devoid of any of these features, instead notable for its subdued topography and absence of bedforms or any significant changes through time. In the second phase of the study we characterized the grain size and composition of sediment in tributaries and the littoral zone onshore of Delgada Canyon between Big Flat and Buck creeks using conventional methods and photogrammetry. We take advantage of a unique geological circumstance in which durable quartzite clasts supplied to the beach from coastal tributaries are unique lithologically from sea cliff-generated mudstone clasts thus permitting the discrimination of coarse sediment sources of littoral sediment. Ongoing work will compare X-ray fluorescence-derived elemental concentrations of sediment from the beach above the Delgada headwall to the composition of coarse sediment from an 861 m water depth sediment core taken within the Delgada Canyon channel. Initial results suggest that material sourced from Big Flat and Shipman creeks is likely present above the active Delgada Canyon headwall, and is continually fed into the system via slumping and density driven currents generated at the headwall. The proximity of Delgada Canyon to the adjacent, high-relief King Range leads us to posit that a coarse sediment source along the coastline promotes canyon activity.