A Large Sediment Accretion Wave Along a Northern California Littoral Cell

The northern California littoral cell of the Klamath River, which is a mixed rocky and sandy system with significant shoreline curvature, was investigated by examining similar to 40 yr of satellite-derived shoreline positions and historical records. We find that an accretion wave of sediment was initiated near the Klamath River mouth in the late 1980s and translated downcoast over the subsequent decades. The wave passed rapidly (similar to 2,500 m/yr) through a rocky coastal reach with more oblique wave directions and slowly through a sandy reach (similar to 200 m/yr) where wave crests approach at more normal angles. Within the sandy reach, the accretion wave extended over 200 m offshore, was similar to 10 km long, incorporated 20 +/- 6 million m3 of sediment, and averaged 1.3 +/- 0.4 million m3/yr of longshore sediment transport over a 20-yr interval. Diffusion of the accretion wave was observed, but the diffusivity coefficient (eobs similar to 0.01 m(2)/s) was lower than values predicted by theory, which we attribute to net sediment transport convergence in the study area caused by the curvature of the shoreline. Examining historical records, we find that increased sediment discharge in the Klamath River occurred during the 20th century from industrial-scale logging and climatic extremes. Thus, we hypothesize that increased river sediment discharge introduced new sediment to the littoral cell that initiated the observed accretion wave. These hypotheses can be tested with stratigraphic and mineralogic investigations of the broad study area beach that has formed during the past 150 years.