Provenance and Wave Modeling Evidence for Littoral Convergence and Sediment Delivery to Mattole Submarine Canyon, Northern California

A significant proportion of the sediment eroded from continents is transferred from rivers to littoral cells, then through submarine canyons to deep ocean basins. Accumulation and periodic discharge of sediment from canyon heads is a likely driver for the sediment gravity flows believed to be responsible for much of this mass transport, but the precise mechanisms by which this occurs are not well understood. We investigated a littoral cell adjacent to Mattole submarine canyon in northern California to better understand the influence of bathymetry and coast shape on sediment trajectories. We documented the beach slope and grain size and provenance of beach sediment along 15 kilometers of coast adjacent to Mattole Canyon and conducted numerical wave modeling to calculate the dominant direction and magnitude of sediment transport, and estimated sediment mobility in the vicinity of the canyon head. Sediment provenance data shows distinct sediment compositions to the north and south of the Mattole canyon head that become mixed above the canyon head. The coarsest clasts occur nearest to sediment sources and areas with the steepest beach slopes. Wave modeling suggests the Mattole headwall region is subject to both sustained sheltering from waves due to canyon bathymetry and littoral convergence above the canyon head for much of a typical year. A simple mass balance of sediment flux from coastal streams, including the Mattole River, suggests that multiple head-clearing events could occur in a typical decade, consistent with the train of sediment waves apparent in the bathymetry in the Mattole Canyon thalweg.