USGS Research Links Weather Extremes to Coastal Sediment Supply in California
As climate change intensifies, parts of the western U.S. are experiencing a growing swing between extreme drought and intense rainfall, including during several recent extremely wet winter seasons. A new USGS study examines how these hydrologic extremes can dramatically reshape sediment transport in a coastal California river—and how those changes ripple downstream to impact coastlines.
Focusing on the San Lorenzo River, which flows through the Santa Cruz Mountains to the California coast, the study tracked river and coastal sediment movement over more than a decade. The researchers found that extremely wet years (such as 2017 or 2023), characterized by multiple floods and landslides, play an outsized role in delivering sand-sized material to the coast—critical for rebuilding eroded beaches and nearshore sediment deposits.
Sediment Transport in a Changing Climate
During dry or average rainfall years, rivers typically carry mostly fine, mud-sized sediment to coasts, which doesn't do much for beach building. But in extreme wet years, it’s a different story.
“When the storms hit hard and landslides occur, they dump huge amounts of fresh sediment—including coarse material like rocks and sand—into rivers and tributaries,” said lead author and USGS Research Geologist Amy East. “Our data show that the San Lorenzo River responded to these wet years by transporting a much greater load of sand downstream.” Floods late in a wet season are carrying much coarser sediment—more sand—compared to similarly sized floods earlier in the season.
This sand tends to reach the coast late in the wet season, after repeated storms have loosened hillslopes in the watershed. Once there, it settles in nearby beaches and underwater sandbars, where it can remain for 4 to 5 years, offering a temporary but valuable buffer against erosion and sea-level rise.
Implications for Coastal Communities
Beaches serve as natural barriers, protecting coastal infrastructure and habitats from rising seas and storm surges. But many California beaches are at risk of shrinking due to rising sea levels, raising concerns about long-term resilience.
“This study suggests that extreme wet years, like California had in 2017 and 2023, are delivering a major portion of the sand budget that helps sustain these coastal areas,” said East. “In a warming world, where sea levels are rising and droughts are more likely to be followed by intense rain, understanding this sediment delivery cycle is essential for future coastal management.”
Science for Better Resource Management
The study also highlights the need to consider both short-term and long-term sediment dynamics in river and coastal planning. Infrastructure like dams, flood control channels, or development in landslide-prone areas can disrupt this natural sediment supply chain, especially during the very wet years when it matters most.
“Our findings emphasize that coastal resilience depends not just on managing erosion at the shoreline, but also on maintaining sediment pathways from source to sea,” said East.
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