Shelf-to-canyon sediment-transport processes on the Eel continental margin (northern California)

Abstract

To investigate the processes by which sediment is supplied to the head of a submarine canyon, an instrumented tripod and a mooring were deployed in the northern thalweg of the Eel Canyon during autumn and winter 1999–2000. This was done as part of the STRATAFORM program, and in combination with a long time-series benthic-tripod data collection on the Eel continental shelf. Sediment-resuspension events on the shelf were forced by waves, and near-bottom suspended-sediment concentrations (SSC) were enhanced during the Eel River flood season. Periodic SSC fluctuations in intermediate waters (corresponding to water depths equal to the shelf-break depth) were predominantly recorded at semidiurnal tidal frequencies, associated with decreases of water salinity and increases of temperature. Within the Eel Canyon, increases of water turbidity were not directly related to the Eel River discharge, but they were linked to the occurrence of storms. This relationship was evident in the bottom-boundary-layer measurements at 120 m depth in the canyon head, although farther down-thalweg (280 m depth), significant increases of near-bottom SSC associated with storm events were recorded also. The highest SSC measured within the canyon coincided with a highly energetic storm on 28 October 1999, in the absence of any river flood event, but associated with a down-canyon density-driven flow. On the shelf at 60 m depth, near-bottom SSC during this storm event reached extremely high concentrations (>10 g l −1), characteristic of fluid-mud suspensions. The across-shelf sediment transport near the bottom showed a persistent off-shelf direction through the entire recording period, while the along-shelf transport fluctuated in direction, but resulted in net transport toward the Eel Canyon head. Within the canyon, near-bottom sediment fluxes were continuously directed down-canyon, while the across-canyon flux was negligible. Sediment fluxes through intermediate slope waters (above the canyon rims) were directed toward the north, following the orientation of the adjacent shelf-break. Results from this field study have identified some of the major processes controlling the off-shelf sediment export in the Eel continental margin, and corroborate previous findings that a substantial portion of the Eel River sediment discharged on the shelf can be exported into the Eel submarine canyon.