Seasonal Patterns of Flood, Wind, and Wave Coherence Along the US West Coast: Implications for Sediment Dispersal and Deposition.

Small rivers with mountainous catchments account for over half of the terrigenous material delivered to the coastal ocean. Dispersal, deposition, and cycling of sediments and associated nutrients ultimately influence shelf carbon budgets. Recent work on collision margins revealed that these small rivers flood during energetic ocean conditions, creating conditions conducive to dispersal of sediments via multiple sediment transport mechanisms. Furthermore, the relative timing of the peak flood and peak wave conditions influences dispersal and depositional patterns. Our study sought to identify patterns of fluvial and oceanic coherence along the US west coast by analyzing 10 years (1996-2006) of NOAA buoy, Climate Prediction Center storm track, and USGS gauge data. Discharge, buoy data, and storm tracks for the Umpqua, Eel, Salinas, and Santa Clara rivers and their adjacent shelves revealed temporal and spatial coherence patterns of wave event and flood magnitude, frequency, and timing. The seasonal period of energetic waves for each river-shelf system began 1-2 months earlier than the flood season (November) and extended 1 month beyond the end of flood season (May). There was also a seasonal disconnect in the timing of peak wave energy (December) and peak floods (January to February). Peak flood activity occurred in January on the Umpqua and Eel Rivers, and in February for the Salinas and Santa Clara Rivers. Flooding and wave events were more frequent and greater in magnitude on the Eel and Umpqua river shelves. The Salinas and Santa Clara rivers displayed lower fluvial inputs, but Salinas River shelf wave events were greater in magnitude. The magnitude and timing of peak waves and peak discharges during floods varied characteristically for each river-shelf system. Peak discharge and peak wave magnitude during floods generally decreased from December to May, indicating that dispersal and depositional patterns vary over the season. Additionally, the timing between peak river discharges and peak waves during floods revealed that waves tend to be most energetic before the peak discharge on the Eel and Umpqua Rivers. Peak waves generally occurred after the peak discharge on the Salinas and Santa Clara Rivers. The timing of waves and floods for each river-shelf system was correlated with storm movement and characteristic wind patterns. For example, from January through May, storm tracks shifted northward of the Eel River shelf, resulting in a wind shift from south to north winds, and an increasing disconnect between peak wave energy and peak discharge.