A regime shift in sediment export from a coastal watershed during a record wet winter, California: Implications for landscape response to hydroclimatic extremes

Abstract

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 21st 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 2016–2017, the San Lorenzo River, coastal California, had nine flow peaks representing 2–10‐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, these flows exported more than half as much sediment as had a 100‐year flood 35 years earlier, substantially enlarging the nearshore delta. 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 several years. 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 USA, 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. Published 2018. This article is a U.S. Government work and is in the public domain in the USA.

Commentary: Variability in Shelf Sedimentation in Response to Fluvial Sediment Supply and Coastal Erosion Over the Past 1,000 Years in Monterey Bay, CA, United States

“…The former was attributed to "anthropogenic modification to sediment dispersal systems" that increased river sand discharge, and the latter was attributed to the effects of dams on decreasing river sediment discharge thereby accelerating coastal erosion. These suggestions are inconsistent with studies of the Monterey region's rivers that find coherence in the discharge relationships of sand fractions (Gray et al, 2014;East et al, 2018).…”

“…Evidence for changes in sediment supply rates and physical conditions of Monterey Bay abounds. For example, the 1982-1983 winters had record river sediment fluxes in the region (Hicks and Inman, 1987) that fundamentally increased landscape sand production (East et al, 2018). Internal waves are dependent on ocean water densities, and temperature-based variability of the region's ocean water density, including significant warming after 1977, is well-documented (Field et al, 2006a,b).…”

River response to large‐dam removal in a Mediterranean hydroclimatic setting: Carmel River, California, USA

“…This indicates that despite large sediment exports from the river system and especially the former reservoir, the river remains in a state of sediment‐supply limitation. In contrast, other turbidity records from the same floods on the central California coast on a watershed 65 km to the north show counterclockwise hysteresis (Conaway et al ., ; East et al ., ), reflecting arrival of substantial sediment from the upper watershed late in the flood hydrograph. Our turbidity data from the Carmel River do not show this (Figure ).…”

Widespread Initiation, Reactivation, and Acceleration of Landslides in the Northern California Coast Ranges due to Extreme Rainfall

“…Recent climate shifts in California have already had severe consequences on water supply, agriculture, infrastructure, wildfires, ground subsidence, sediment flux, and landslides (Bekaert et al, ; Bennett, Roering, et al, ; Chaussard et al, ; Diffenbaugh et al, ; East et al, ; Handwerger et al, ; Murray & Lohman, ; Swain et al, ). Between the 2012 and 2018 WYs, California experienced one of its most extreme droughts and the second wettest year in recorded history (Diffenbaugh et al, ; Griffin & Anchukaitis, ; Swain et al, ).…”

“…Recent and ongoing climate shifts in California have already had an impact on the behavior and activity of landslides (Bennett, Roering, et al, ; East et al, ; Handwerger et al, ; Nereson & Finnegan, ). Over the past decade, California has experienced both a historic drought (2012–2016) and the second wettest year on record (2017; Griffin & Anchukaitis, ; Robeson, ; Swain et al, , ).…”