A new report published in Nature acknowledges that while hurricanes contribute a significant amount of sediment to marshes as waves lap onshore, it’s the Mississippi River that contributes most for wetland development.
“(The report) suggests that river discharge is and could be very important for wetland development,” said Sam Bentley, director of the Coastal Studies Institute at LSU and one of the authors of the report. “The river is a vital resource.”
Over the years, there have been two main camps for what makes the most significant contribution to wetland development.
One camp argues that coastal Louisiana wetlands are primarily built over time through the accumulation of plant material in organic soil with hurricanes bringing in the needed sediment, or inorganic material. The other camp says that sediment can be most effectively delivered by river.
Previous reports have attributed most of the new sediment into the marshes as coming from tropical storms and hurricanes, but those have been done just after a storm passes.
“Measuring something over the short term doesn’t give you long-term results,” Bentley said.
However, that doesn’t mean that the previous studies are wrong, but that different ways of looking at the process seems to come up with different answers.
“That’s how I do my science. Very rarely do you find a right answer, there’s just a range of possibilities,” Bentley said. “All of this material ultimately comes from the river anyway.”
Lead author James Smith IV, who graduated with his bachelor’s in geology from LSU in May, said there was a big difference in the hurricane studies and this recent one. The hurricane studies looked at sediment accumulation soon after the hurricanes passed, while this most recent study showed the lasting impact by looking at 10 years or even 60 years after a storm had passed.
The report is the result of analyzing 27 cores, or plugs of soil taken from the marsh, collected in 2008 and 2013 by the U.S. Geological Survey. The study looked at about 70 years worth of sediment accumulation in these cores from 1946 through 2005.
Smith said every two centimeters of the cores were analyzed to look for mineral sediment, which were then dated and compared to any Category 3 or stronger hurricane that had passed within 62 miles.
The analysis showed that only about 11 percent of the sediment could be directly attributed to a Category 3 or stronger hurricane, 3 percent coming from other events, such as winter storms, and the rest coming from other coastal processes including influence from the river.
“Hurricanes obviously bring some sediment into the system,” Smith said. “Hurricanes also cause a lot of land loss.”
During hurricanes Katrina and Rita, the USGS reported that 217 square miles of land were lost in southeast Louisiana due to the storms.