Sediment deposition in a modern submarine canyon: Eel Canyon, northern California

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Previous studies on the Eel margin have shown that a substantial amount of terrigenous sediment may be rapidly transported and deposited seaward of the shelf break during the Eel River flooding season. Eel Canyon, located ∼10 km seaward of the Eel River mouth, has been investigated to determine its role in seaward escape of sediment over seasonal timescales. In order to characterize seasonal deposition, box cores were collected in upper Eel Canyon (<900 m water depth) during periods of high and low sediment discharge from the Eel River (winter and summer, respectively). Cores were collected at the beginning and end of three typical flooding seasons (January 1998–April 2000), to assess the regularity of sediment escape to the Eel Canyon. Results showed that sediment deposition was temporally variable over the course of a year, with thick sediment deposits (up to 19 cm) being formed during the flooding season. These deposits were identified as having detectable 7Be activity, high percentage of clay and distinct physical sedimentary structures. Sediment signatures indicated sediment was composed primarily of recently derived fluvial sediment that was rapidly accreted on the seabed (on the order of days to months from when it was discharged from the river). The 3-year time series showed that sediment reached the canyon on an annual basis and extreme flooding events from the Eel River were not a necessary condition for sediment escape. Fluvial sediment was deposited in most areas of the upper canyon, following each winter flooding season, with the thickest deposits consistently observed in upper channel thalwegs of the canyon. Preferential deposition in thalwegs, along with preservation of physical structures, suggests that gravity-driven sediment flows may be important transport mechanisms in these areas. The spatial distribution of deposit thicknesses for cores collected in April 2000 was used in conjunction with a detailed swath map to construct a seasonal sediment budget for the upper Eel Canyon alone. It was found that 12.0%±3.5% of the Eel River sediment discharge can be accounted for in the upper Eel Canyon, suggesting that the canyon can act as a significant sink of Eel River sediment over seasonal timescales.