The structure of the Eel River plume during floods

Several large floods of the Eel River in northern California occurred during 1997 and 1998, with peak discharge ranging from 4000 to 12,000 m3 s−1. The flood conditions persisted for 1–3 days and were usually accompanied by strong winds from the southern quadrant. The structure of the river plume was strongly influenced by the wind-forcing conditions. During periods of strong southerly (downwelling favorable) winds, the plume was confined inside the 50-m isobath, within about 7 km of shore, with northward velocities of 0.5–1 m s−1. Occasional northerly (upwelling favorable) winds arrested the northward motion of the plume and caused it to spread across the shelf. Sediment transport by the plume was confined to the inner shelf (water depths less than 50 m), during both southerly and northerly wind conditions. During southerly wind periods, fine, unaggregated sediment was rapidly transported northward to at least 30 km from the river mouth, but flocculated sediment was deposited within 1–10 km of the river mouth. During northerly (upwelling-favorable) winds, most of the sediment fell out within 5 km of the mouth, and negligible sediment was carried offshore, even though the low-salinity plume extended beyond the 60-m isobath. Although sediment deposition from the plume is confined to the inner shelf, the stratigraphy indicates that the principal flood deposits on the adjacent continental shelf occur in a patch between the 60- and 90-m isobath. Thus, the deposition on the inner shelf is ephemeral, and some mechanism other than plume transport delivers the sediment from the inner shelf to the mid-shelf.