River-plume sedimentation modeling for sequence stratigraphy: application to the Eel margin, northern California

Sediment deposition in the coastal zone from riverine input is of key importance in understanding the construction of sediment sequences through time. A turbulent-jet model is used to simulate sediment input to the continental margin from a riverine source. Turbulent-jet models track the momentum, which is the first-order force carrying most of the sediment flowing into the ocean. The Froude number at the river mouth remains supercritical during floods of the Eel River, justifying the use of a jet model for the initial dispersion of the sediment-laden plume. Sediment is deposited from the plume prior to the centerline Froude number decreasing to its critical value. Empirically derived first-order removal-rate constants are used for each grain size in an advection–diffusion equation to predict the areal extent of sediment deposition. One strength of using a jet model is its ability to simulate deposits on a daily time-step over intermediate time scales (thousands of years) using a limited number of input parameters. A number of simulations of the Eel River plume are shown for conditions of the present day and 18,000 yr BP, during the last glacial maximum. The model results for modern conditions show that a majority of the deposition occurs within 20 km north of the river mouth and is typically confined to within 10 km of the coastline, corresponding well with the shelf sands on the Eel margin. However, the initial mud deposit from a 1995 flood was farther offshore and northward. Additional advection of the mud fraction must occur after it leaves the surficial plume and prior to its initial deposition. The model runs of 18,000 yr BP (with 100 m lower sea level, wetter and colder climate, and the river mouth assumed to be at the canyon) show that the plume is able to transport sand, silt and clay out of the canyon and onto the continental slope.