When sea level rise threatens coastal wetlands, don’t turn to rivers for help, scientists say

Amid climate change, large dam removal projects have gained attention as a solution to the loss of coastal wetlands that reduce flooding, filter water and provide habitat for wildlife. But in an article published in Sciencethe researchers conclude that this strategy will not work in most American rivers.

The reason, they say, is the lack of sediment. Of the nearly 5,000 rivers analyzed, nearly three out of four rivers could not provide enough sediment to offset sea level rise in their connected coastal areas. Nearly half were at least 10 times less than the amount of sediment needed.

This is the first national study examining the amount of watershed sediment likely to be deposited by rivers in coastal areas. Research so far has focused on a few very large rivers like the Mississippi and steep rivers like the Elwha in Washington state, which are not representative of most other rivers in the contiguous United States. .

Most U.S. watersheds are small and not the primary source of sediment accumulation in wetlands, researchers say. It is on these small rivers that most of the dams are located.

Researcher Scott Ensign, Ph.D., of the Stroud Water Research Center, a nonprofit organization that studies freshwater streams and rivers around the world, led the study. He said: “The Elwha is a textbook example of a dam removal project restoring coastal sediment, and for good reason: it released a huge amount of sediment and sand. »

“However, rivers along the East and Gulf coasts are less steep than on the West Coast, and they contain less sediment that can reach wetlands – larger wetlands, requiring more sediment to keep them at “above the rising seas. Basically, the numbers don’t matter.”

Christopher Craft, Ph.D., a professor at Indiana University who focuses on wetland restoration and climate change, said: “The in-depth and comprehensive spatial analysis conducted by the authors strongly suggests that the ‘Sediment supply to most coastal watersheds is insufficient to sustain’

Ensign and her co-authors, Joanne Halls of the University of North Carolina and Erin Peck of the University of Massachusetts, used publicly available data from the U.S. Geological Survey and the National Oceanic and Atmospheric Administration to model sediment supply from watersheds to coastal wetlands using Esri’s ArcGIS Pro technology. They then compared their predictions for intertidal wetlands in the United States with previously reported rates of change.

“Overall, the sediment that saves most wetlands from drowning does not come from the river upstream. In many places on the East Coast, removing dams won’t help. We need to look elsewhere.” , explained Ensign.

James Pizzuto, Ph.D., a professor of geological sciences specializing in river science at the University of Delaware, said the researchers cleverly approached a complex problem.

“These results, along with local variations documented by mapping the entire U.S. coast, provide critical guidance to managers and scientists, documenting areas where future efforts should focus on other processes at -beyond watershed sediments,” he said.

Such efforts could include finding ways to preserve more mineral sediment, plant matter and organic carbon in wetland soils, said Donald F. Boesch, professor emeritus in the Center for Environmental Sciences at the University of California. University of Maryland. He said: “This is true both where they lack sediment and where sediment is diverted to build and maintain wetlands experiencing high rates of relative sea level rise, such as in the Delta. Mississippi.”

Future studies are needed to measure the amount of sediment trapped behind specific dams and to accurately predict its effect on downstream tidal wetlands.

Ensign said: “Overall, the most important action to save tidal wetlands is to allow them to migrate upslope. In some areas this will require restoring natural hydrology and preserving low-lying lands. Direct application of sediment and other technical approaches may also be useful at very local scales.