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Blum and Roberts, along with LSU graduate students, educating other geoscientists about the process of delta-building in the Wax Lake-Atchafalaya Deltas.

LSU Study Finds Tough Choices Ahead for Louisiana’s Deltaic Coast

Former LSU Harrison Professor Mike Blum and Boyd Professor Harry Roberts find sediment loss and global sea-rise threaten to submerge delta by 2100

Former Harrison Professor of Geology and Geophysics Mike Blum and LSU Boyd Professor of Oceanography and Coastal Sciences Harry Roberts recently published the first study ever to quantify sediment available to build the Mississippi Delta and fill the southern part of the alluvial valley over the past 12,000 years. The results, titled “Drowning of the Mississippi Delta due to insufficient sediment supply and global sea-level rise,” were published in Nature Geoscience.

“Many in the geological community have understood the findings of this paper to be basically true in a qualitative sense for years,” said Roberts. “But no one ever quantified the boundary conditions for delta plain development and now large-scale land loss. We simply quantified the sediment available for delta-building in the past as compared to the present and predicted the fate of the coastal plain under different observed and predicted rates of sea level rise.”

According to the study, the Mississippi River Delta was never fully sustainable; there were always deteriorating parts, while other areas receiving sediments thrived. This shifting of sites of sediment deposition was part of the natural cycle because the river has historically changed its course every 1,000-1,500 years. Now, man-made construction no longer lets the river change course, or even transport sediment to the delta plain because of the continuous artificial levee system. Flow down the Atchafalaya represents the Mississippi River’s most recent attempt to switch its course.

“Currently, only 30 percent of the Mississippi goes down the Atchafalaya River,” said Roberts. “If nature had its way, most of the Mississippi’s discharge would have shifted to the Atchafalaya channel by now.”

Subsidence of the delta plain and subsequent land loss has traditionally been measured primarily as disappearance of our marshlands. Early on, researchers focused attention on marsh management to finding a solution for the growing threat of disappearing deltaic wetlands. But this study has a different view. The authors believe that large-scale and meaningful restoration efforts must involve sediment from the Mississippi River. However, they point out that this sediment should be placed so as to enhance the growth of existing marshlands and maximize the building of new land.

“Due to more than 40,000 dams on the Mississippi and its tributaries, the river now transports less than half of the sediment load it had available when the delta plain was built, starting about 7,000 years ago,” said Roberts. “Results of our study paint a pretty bleak picture at first reading of the paper. We’re witnessing the effects of two major problems, an increase in the rate of sea level rise and man’s intervention into the natural allocation of sediments. Structures like dams and levees have retained sediments and do not allow the natural processes of sediment distribution to take place ‘overbank’ and through channel networks.”

The final analysis of their data provided a somber statistic: according to their study, if things remain unchanged, the Delta region could be completely submerged by 2100.

Blum and Roberts greeted the results of their unique study with something less than enthusiasm. “Although we are glad to have this published, I think it also fair to say we both feel sad about the conclusions,” said Blum. “Speaking for myself, if there was ever anything I hoped to be completely wrong about, this would be it. But we hope the results will spur action to address the problems we highlight.”

Although the researchers acknowledge that their findings don’t paint an optimistic future for the Delta region, they point out a positive: having such quantifiable data allows decision-makers to have a clear, numerical picture of the scale of the problem. But those solutions won’t be easy or painless.

“This study really will challenge both state and federal governments to prioritize where to focus large-scale restoration efforts, and the priorities will be painful because they deal with humans and human interests,” said Roberts. “But choices need to be made soon because the longer we wait, the harder it becomes to fill the space being created by subsidence and sea level rise. The good news is that our results may encourage the coastal restoration community to think differently about how to use one of our most precious Louisiana natural resources: the Mississippi mud in our waterways.”

Ashley Berthelot | Editor | Office of Communications & University Relations
July 2009