LANDSCAPE RESPONSE TO HYDROCLIMATIC EXTREMES: A REGIME SHIFT IN SEDIMENT EXPORT FROM A COASTAL WATERSHED DURING A RECORD WET WINTER, CALIFORNIA

Small, steep watersheds are prolific sediment sources from which sediment flux is highly sensitive to climatic changes. Storm intensity and frequency are widely expected to increase during the 21^st century, and so assessing the response of small, steep watersheds to extreme rainfall is essential to understanding landscape response to climate change. During record winter rainfall in 2016–17, the San Lorenzo River, coastal California, had nine flow peaks representing 2- to 10-year flood magnitudes. By the third flood, fluvial suspended sediment showed a regime shift to greater and coarser sediment supply, coincident with numerous landslides in the watershed. Even with no singular catastrophic flood, the flows during this extremely wet year exported more than half as much sediment as had a 100-year flood 35 years earlier. Annual sediment load in 2017 was an order of magnitude greater than during an average-rainfall year, and 500-fold greater than in a recent drought. These anomalous sediment inputs are critical to the coastal littoral system, delivering enough sediment, sometimes over only a few days, to maintain beaches for years; the massive sediment export of 2017 substantially enlarged the nearshore delta with new deposition that remained through spring 2018.

Future projections of megadroughts punctuated by major atmospheric-river storm activity suggest that interannual sediment-yield variations will become more extreme than today in the western U.S., with potential consequences for coastal management, ecosystems, and water-storage capacity. The occurrence of two years with major sediment export over the past 35 years that were not associated with extremes of the El Niño Southern Oscillation or Pacific Decadal Oscillation suggests caution in interpreting climatic signals from marine sedimentary deposits derived from small, steep, coastal watersheds, to avoid misinterpreting the frequencies of those cycles.