Fluvial and coastal sedimentary response to extreme fluctuations in rainfall, central California, USA

Understanding landscape response to changing climate is essential to protecting lives, property, and infrastructure. In some regions, climate warming is associated with enhanced fluctuations between wet and dry hydrologic extremes; such hydroclimatic ‘whiplash’ has been evident in California, USA, over the past decade. We studied sedimentary responses to these changes on the central California coast, focusing on the 357-km² San Lorenzo River watershed and its nearshore zone. This study used fluvial suspended-sediment data together with biannual topographic and bathymetric coastal surveys to quantify the magnitudes and time scales of extreme-event signals. We find that in two extreme wet years (2017 and 2023), fluvial sediment loads were an order of magnitude greater than in years with average rainfall and nearly three orders of magnitude greater than during extreme drought. Extreme wet years are associated with substantial accretion in the nearshore zone around the river mouth, totaling several hundred thousand tonnes of new sediment in 2017 and 2023. The signal of aggradation can persist along the coast for 3–4 years. In wet years with multiple river floods, the floods occurring later in the wet season supply disproportionately coarse suspended sediment to the coast (60–75% sand), indicating their outsized importance on littoral sediment budgets as beach-building events. The dominance of coarse fluvial sediment after unusually large seasonal rainfall is attributable to landslides supplying coarse material to stream channels. In contrast, river floods occurring earlier in the wet season or in non-extreme-wet years supply finer-grained suspended sediment (20–30% sand, attributed to less landslide activity under drier antecedent soil conditions) and thus are less likely to influence coastal morphology. Understanding these process changes will be important for effective management of fluvial and coastal systems under a future, warmer climate with greater risk of both drought and extreme rain.