Atmospheric rivers — also known as "rivers in the sky" — are long, narrow bands of concentrated water vapor that can produce high winds and massive amounts of snow and rain when they make landfall. The storms they produce have been double-edged phenomena in California this winter, creating devastating flooding and mudslides, but also replenishing freshwater reservoirs in the drought-stricken state.

New NCCOS-funded research shows that atmospheric river storms may have another benefit: marsh building.

The study covers a four-year period of monitoring at five marshes in northern San Francisco Bay and includes atmospheric river storms in 2017 that coincided with increased sediment accumulation at sites that were flooded.

Marshes more closely associated with a riverine setting experienced the greatest sediment accumulations following the storms, with an average surface elevation gain of 46 millimeters, while a tidal saline marsh had the smallest increase, with an average gain of 4 millimeters. The gains are significant, as anthropogenic activities throughout the region — such as riverbank armoring, levee construction, and land development — have decreased the availability of suspended sediment to San Francisco Bay by roughly 50 percent between 1957 and 2001.

The findings suggest that atmospheric river storms may help create positive elevation trajectories to offset sea-level rise at some locations. Incorporating sediment accretion data from these storms in models that estimate marsh responses to sea-level rise could improve projections that guide coastal management.

The project received funding from NCCOS's Effects of Sea Level Rise Program. For more information, visit the project webpage.

The science behind atmospheric rivers. Credit: NOAA.