Impacts of Declining Mississippi River Sediment Load on Subaqueous Delta Front Sedimentation and Geomorphology

Jillian M Maloney1,2, Samuel J Bentley2,3, Kehui Xu3,4, Ioannis Y Georgiou5 and MIke D Miner6, (1)San Diego State University, Department of Geological Sciences, San Diego, CA, United States, (2)Louisiana State University, Department of Geology and Geophysics, Baton Rouge, LA, United States, (3)Louisiana State University, Coastal Studies Institute, Baton Rouge, LA, United States, (4)Louisiana State University, Department of Oceanography and Coastal Sciences, Baton Rouge, LA, United States, (5)University of New Orleans, Department of Earth and Environmental Sciences, New Orleans, LA, United States, (6)U.S. Department of Interior, Bureau of Ocean Energy Management, New Orleans, LA, United States
Abstract:
The Mississippi River delta system is undergoing unprecedented changes due to the effects of climate change and anthropogenic alterations to the river and its delta. Since the 1950s, the suspended sediment load of the Mississippi River has decreased by approximately 50% due to the construction of >50,000 dams in the Mississippi basin. The impact of this decreased sediment load has been observed in subaerial environments, but the impact on sedimentation and geomorphology of the subaqueous delta front has yet to be examined. To identify historic trends in sedimentation patterns, we compiled bathymetric datasets, including historical charts, industry and academic surveys, and NOAA data, collected between 1764 and 2009. Sedimentation rates are variable across the delta front, but are highest near the mouth of Southwest Pass, which carries the largest percentage of Mississippi River flow and sediment into the Gulf of Mexico. The progradation rate of Southwest Pass (measured at the 10 m depth contour) has slowed from ~67 m/yr between 1764 and 1940 to ~26 m/yr between 1940 and 1979, with evidence of further deceleration from 1979-2009. Decreased rates of progradation are also observed at South Pass and Pass A Loutre, with the 10 m contour retreating at rates >20 m/yr at both passes. Advancement of the delta front also decelerated in deeper water (15-90 m) offshore from Southwest Pass. In this area, from 1940-1979, depth contours advanced seaward ~30 m/yr, but rates declined from 1979-2005. Furthermore, over the same area, the sediment accumulation rate decreased by ~81% for the same period. The Mississippi River delta front appears to be entering a phase of decline, which will likely be accelerated by future upstream management practices. This decline has implications for offshore ecosystems, biogeochemical cycling, pollutant dispersal, mudflow hazard, and the continued use of the delta as an economic and population center.
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