Rapid deposition of fluvial sediment in the Eel Canyon, northern California

The head of the Eel Canyon was investigated to understand the nature of sedimentation associated with winter floods from the Eel River, located about 10 km to the east. Efforts were focused on the winter of 1997/1998 with cores collected in January and March, which was a period with several typical winter flood events from the river (<3400 m3/s), relatively intense wave resuspension, and seaward transport by currents over the shelf. Cores collected from the head of the canyon in January showed little evidence of recent flood sediment (e.g., no 7Be). In contrast, March cores had clear signatures indicating flood layers (high 7Be activities, large 7Be inventories, low 210Pb activities and high clay content) that extended 6–8 cm in the seabed and were characterized by physical stratification. The flood signatures show a gradual intensification upward within the layer. These signatures may reflect a mixed supply of recent (<52 days) flood-derived Eel River sediment and older (>200 days) shelf sediment resuspended and transported by strong waves and currents on the adjacent shelf, with the Eel flood sediment becoming more dominant through the winter. The emplacement history of canyon flood layers is different than that for flood deposits formed on the shelf, and produces less abrupt changes in flood signatures. Observations below the surface region of Eel Canyon cores show an older record of multiple layers demonstrating flood signatures (low 210Pb activities, high clay content), which probably indicate preservation of past flood events within the last century. Deposition of thick flood layers suggests that the canyon is an important part of the Eel River dispersal system, although observations in deeper portions of the canyon are needed for a better understanding of the canyonʹs role in the fluvial sediment budget.