Impact of Water Loss on Sustainability of the Mississippi River Channel
in its Deltaic Reach
Abstract
The Mississippi River channel from New Orleans to the Gulf of Mexico
(GOM) is a key deep draft navigation channel and an active deltaic lobe.
Natural and engineered lateral exits from this reach into adjacent
receiving basins historically has provided mineral sediment for wetland
accretion in the face of rising relative sea level and supported
estuarine-coastal food webs. However, our analysis indicates water
losses from the channel have increased by 25% since 2004 due to (1)
bank failures during large floods since 2012 that have created several
large exit channels downriver of the flood protection levee, and (2) the
opening of an engineered diversion at West Bay in 2004. This has
resulted in a 60-80% loss in stream power in the lowermost navigation
channel that is accompanied by net shoaling between 2012 and 2022 and an
increased dredging need. Our 2022 survey in the GOM exit passes
indicates that only 20% of the freshwater, 5% of the total suspended
sediment (2% of the sand) at New Orleans now reaches the GOM: this
supports previous research indicating the delta front is retreating
after centuries of progradation. Together these results indicate that
(1) river containment and the sustainability of the navigation channel
is threatened, (2) sediment load reaching the seaward end of the delta
may be insufficient to avoid major degradation, and (3) the increased
freshwater flux into adjacent shallow coastal water bodies has unknown
implications for coastal hypoxia and food webs, including commercial
species (e.g., oysters) and marine mammals. Future acceleration in sea
level rise rates and tropical storm frequency/intensity likely will
worsen these trends.