The Last Naturally Active Delta Complexes of the Mississippi River (LNDM): Discovery and Implications

Abstract

The most ambitious ecological restoration project yet attempted is just getting started to renaturalize the Mississippi River Deltaic Plain. All the channeling, leveeing, lumbering, damming, dredging, and polluting of this system over the past 300 + years make it difficult to envision today how a more natural ecosystem might look and function. Our hope is that an awareness of the protohistoric deltaic plain may help guide the modern restoration program. To accomplish this, we explore the historic record for a description of the last naturally active delta complexes of the Mississippi River (LNDM) as the most appropriate restoration model for Louisiana’s coast. The LNDM is our reconstruction of this system as it was encountered by the first Europeans to navigate it. To accomplish this, we focus on Alonso de Chaves’ ca. 1537 manuscript. We find Chaves’ latitude estimates accurate (R² = 0.99), his league to equal 6.3 km, and his location of the LNDM consistent with the most authoritative first-hand accounts of the protohistoric and colonial period (Barroto, Iberville, Evía, and Dumain) . We find the LNDM was a vast seaward-advancing arc that occupied, through four distributaries, all of the five most recent delta complexes of the Mississippi River and extended across all of coastal Louisiana east of the Chenier Plain. It was characterized by plumes of freshwater that extended for more than 10 km into the Gulf of Mexico (GoM) during the spring flood of the Mississippi River and by a vast offshore oyster reef covering > 2,000 km², impeding navigation, and functioning as an offshore harbor near the reef’s western end. Our findings support “reconnecting the river to the deltaic plain via … the reopening of old distributaries” (Day et al., Science 315:1679–1684, 2007) and the desirability of “a fully revised delta-lobe-scale chronostratigraphy” (Kulp et al., Soc Sediment Geol Special Publ, 83:279–293, 2005). Implications of our findings are discussed in light of what we view as fundamental errors in Louisiana’s coastal restoration plan and the “Berms to Barriers”/post Deepwater Horizon oil spill efforts. Here we find that many of Louisiana’s coastal restoration benchmarks —diversions restricted to the lower regions of coastal Louisiana (i.e., the Birdsfoot and the Atchafalaya delta complex); oyster reefs confined to estuarine environments; brackish-water dominated estuaries in the spring; deepwater shipping channel inlets; and artificial levees—are incompatible with a sustainable coast and that recent data are consist with a constant rate of land loss in coastal Louisiana of 69.1 km²/yr (1.47 football fields/hr) for 1932 through 2010 . We also find that the “Berms to Barriers” concept is necessarily going to fail unless the natural flows of the Mississippi through and across the LNDM are sufficiently restored so as support Louisiana’s barrier islands and coastline against the forces of the GoM. Our findings support Lamb’s (Separata du Revista da Universidale de Coimbra 24:9, 1969) argument that Chaves (ca. 1537) provides our earliest comprehensive view of the coasts of the Americas and Ovieda’s (1851) argument that De Soto’s men sailed out the mouth of Río del Espíritu Santo (River of the Holy Spirit)—which we conclude flowed through the Atchafalaya/Vermilion Bay complex and not the Birdsfoot.