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 would likely worsen these trends.

Sedimentary Processes and Instability on the Mississippi River Delta Front near the Shipwreck of the SS Virginia

“…This study area is affected by multiple sediment dispersal processes, including hypopycnal river plumes [11][12][13], storm wave/current-induced resuspension [14], sediment slides driven by gravity acting on a slope [2], and possibly wave/current-enhanced sediment gravity flows [15], although this type of flow has not yet been observed directly on the MRDF. The subaqueous delta is also now showing the effects of both recent and long-term declines in sediment supply [3,16]. This change is manifested as the first observed transgression of the subaqueous delta, indicated by the landward migration of 10 m isobaths near the three main historical river outlets, and upstream migration of substantial river discharge to new and expanding outlets upstream of the three major historic outlets (Figure 1) [16].…”

“…The subaqueous delta is also now showing the effects of both recent and long-term declines in sediment supply [3,16]. This change is manifested as the first observed transgression of the subaqueous delta, indicated by the landward migration of 10 m isobaths near the three main historical river outlets, and upstream migration of substantial river discharge to new and expanding outlets upstream of the three major historic outlets (Figure 1) [16]. Interaction of these processes control the morphology and morphodynamics of the study area.…”

“…Interaction of these processes control the morphology and morphodynamics of the study area. In addition to historical and recent changes in sediment supply from the river [16], the downslope advection of the Virginia lobe should impact sediment accumulation rates on the lobe from the mid-20th century onwards, by changing the local sediment supply and depositional setting of the Virginia sediment lobe, causing sediment accumulation rates (SARs) to decline over time due to both changing flux from the river and changing depositional setting (due to lobe motion).…”

Short-Term Sediment Dispersal on a Large Retreating Coastal River Delta via 234Th and 7Be Sediment Geochronology: The Mississippi River Delta Front

“…Suspended sediment loads leaving SW Pass, S Pass, and PAL were estimated as 20.8, 4.7 and 4.8 Mt/yr during 2008-2010, respectively [34]. Since then, S Pass and PAL discharges may have been partially blocked due to in-channel dredge spoil disposal and associated in-channel sedimentation [36], with smaller fractions of total upstream load likely now exiting these major river outlets [37].…”