The distance from the Mississippi's flow or mouth is irrelevant compared to the sediment that flowed out of it before levees forced it to exit the way it does today.

Erosion from that decision was already an issue 50 years ago; weather events accelerated the pace, but so have upriver dams that cut off sediment supply, and also shipping canals that have carved gaps into it that push surges further inland than they would otherwise, and also the chéniers that diverted floodwaters regardless of cause into more fragile marshes, and also farmland that was in active use as far south as Pecan Island up until Rita salted it, and also 50 years of oil-industry support infrastructure, and also the Deepwater rig explosion that leaked oil that killed off marshes (the settlement of which is how the state ended up capable of awarding this land), and also rampant invasive nutria that negated efforts to use agricultural fresh-water discharge to reverse previous damage.

https://svs.gsfc.nasa.gov/11042/

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/201...

https://repository.library.noaa.gov/view/noaa/44432/noaa_444...

https://repository.library.noaa.gov/view/noaa/45895/noaa_458...

https://gulfspillrestoration.noaa.gov/media/document/2018-07...

https://www.usgs.gov/publications/accelerating-elevation-gai...