Interannual variability of nearbed sediment flux on the Eel River shelf, northern California

Abstract

A 5-year data record of currents, sea level variations, and suspended-sediment concentration is analyzed to evaluate the interannual variability of currents and suspended-sediment transport on the Eel River continental shelf of northern California. Data were collected within 2 m of the seabed in 60-m water depth at a site located 25 km north of the Eel River mouth. This site is near the landward edge of the Eel River mid-shelf mud deposit that rests between 50 and 110 m depth and extends for approximately 50 km northward along this shelf. Results of this analysis show that sediment resuspension is forced primarily by nearbed oscillatory flows generated during winter months, and the nearbed sediment transport occurs both as suspended load and as fluid mud. In the alongshelf direction, the net annual sediment flux experiences significant northerly and southerly excursions over individual years but exhibits almost no net transport past the study site over the 5-year period. In the across-shelf direction, net annual sediment-flux magnitude varies from year to year but is always directed offshore. Frequency analysis indicates that net sediment flux in the alongshelf direction is primarily driven by winds and very low-frequency oscillations thought to be associated with mesoscale eddies formed on the shelf. Annual variations in the frequency and possibly the spatial scale of this eddy structure correlate to net northerly or southerly alongshelf sediment flux. In the across-shelf direction, sediment flux is driven by annual variations in the across-shelf currents. Mesoscale circulation features are suggested to impact the transport and redistribution of the Eel River mid-shelf sediment deposit on this mid-latitude continental shelf.