Effects of a Catastrophic Debris‐Flow on a Coastal Salt Marsh

ABSTRACT Extreme events are increasing in frequency and intensity as the climate changes. Compound extreme events—when multiple drivers or hazards occur together—are common, but the interactive effects of multiple events are rarely studied. Debris‐flows are caused by the compound events of wildfire followed by precipitation, with substantial effects on impacted systems, including unburned areas downslope. Debris‐flow responses are mostly documented in freshwater streams and rivers, but these events can also have downstream effects on estuarine systems when they occur in coastal mountain ranges. Here, we document the physical and biological effects of a woody debris‐flow in a tidal wetland in Southern California. In December 2017, the Thomas Fire burned 1140 km 2 of the coastal Santa Ynez Mountains and a subsequent atmospheric river in January 2018 caused a third extreme event—a catastrophic debris‐flow into Carpinteria Salt Marsh. Digital elevation models developed before and after the event showed that immediately afterwards, tidal channel elevations increased by up to 1 m compared to pre‐debris‐flow conditions. Elevations remained 0.26–0.68 m above the pre‐event conditions 1 year after the debris‐flow. Annual monitoring of invertebrate communities from 2012 to 2024 showed a slight decline in invertebrate density, and a 32% decline in invertebrate richness immediately following the debris‐flow (2018). Although these responses returned to the normal range of variation by the next year (2019), invertebrate community composition remained changed for the following 6 years, driven by a relative decline in oligochaetes and bivalves and a relative increase in amphipods. The debris‐flow also added novel hard substrate to previously soft‐sediment wetland habitat, facilitating colonization of a non‐native mussel and the development of extensive mussel beds. Together, these findings illustrate that debris‐flows in coastal mountain ranges can substantially alter the physical and biological attributes of downstream estuarine ecosystems. Our work also highlights the value of long‐term monitoring data in assessing extreme event impacts.