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 ∼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 (∼2,500 m/yr) through a rocky coastal reach with more oblique wave directions and slowly through a sandy reach (∼200 m/yr) where wave crests approach at more normal angles. Within the sandy reach, the accretion wave extended over 200 m offshore, was ∼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 (εobs ∼0.01 m2/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. Plain Language Summary: The existence of beaches along the world's coasts is dependent on the availability of sand or other sediment sources that build up these landforms. As sediment sources come and go over time, beaches may accrete or erode in response to these changes. These kinds of shoreline change patterns can be measured using new satellite‐based techniques that are especially valuable for coasts with limited ground‐based surveys. We applied these satellite‐based shoreline techniques to a portion of the northern California coast, which revealed that a massive body of sediment appeared on this coast in the 1980s, and that this sediment moved downcoast over the subsequent decades. The sediment stayed in a relatively continuous body—termed a coastal accretion wave—that extended 100's of meters offshore and several kilometers alongshore. The satellite measurements also allowed us to track the evolution of the shape and speed of this accretion wave and compare these results with physical conditions and sediment movement theory. Comparing with historical measurements, maps, and accounts, we hypothesize that sediment from the Klamath River has been a primary source of sediment to this coast, including the formation of the accretion wave and study area's broad beach. Key Points: An accretion wave of ∼20 million m3 of littoral sediment formed in the late 1980s and translated ∼15‐km downcoast over the next 30+ yearsAlongshore diffusion of the wave was less than predicted by theory, which is attributed to transport convergence from the shoreline curvatureIncreases in river sediment discharge from human impacts and climate extremes may have initiated the accretion wave [ABSTRACT FROM AUTHOR]