Drowning of the Mississippi Delta due to insufficient sediment supply and global sea-level rise

Over the past few centuries, 25% of the deltaic wetlands associated with the Mississippi Delta have been lost to the ocean . Plans to protect and restore the coast call for diversions of the Mississippi River, and its associated sediment, to sustain and build new land . However, the sediment load of the Mississippi River has been reduced by 50% through dam construction in the Mississippi Basin, which could affect the effectiveness of diversion plans . Here we calculate the amount of sediment stored on the delta plain for the past 12,000 years, and find that mean storage rates necessary to construct the flood plain and delta over this period exceed modern Mississippi River sediment loads. We estimate that, in the absence of sediment input, an additional 10,000-13,500 km will be submerged by the year 2100 owing to subsidence and sea-level rise. Sustaining existing delta surface area would require 18-24 billion tons of sediment, which is significantly more than can be drawn from the Mississippi River in its current state. We conclude that significant drowning is inevitable, even if sediment loads are restored, because sea level is now rising at least three times faster than during delta-plain construction. © 2009 Macmillan Publishers Limited. All rights reserved. 1 2,3 4-6

The mineral sediment loading of the modern Mississippi River Delta: what is the restoration baseline?

A restoration baseline for river deltas establishes a framework for achieving goals that can be thwarted by choosing an improper historical background. The problem addressed here is identify the size of the modern Mississippi River delta that restoration should use as that baseline. The sediment loading to the Mississippi River main stem delta fluctuated over the last 160 years with a consequential dependent plasticity in delta size. A visual time series of the delta size is presented, and the area: sediment loading ratio is calculated. This ratio ranged from 1.8 to 3.9 km(2) per Mmt sediment y(-1) during the pre-European colonization of the watershed in the 1800s, a maximum size in the 1930s, and then lower after soil conservation and dam construction decades later. This land building rate is similar to the 1.3 to 3.7 km(2) per Mmt sediment y(-1) for the Wax Lake and Atchafalaya sub-deltas located to the west, which receives some of the Mississippi River sediment and water from the main channel below St. Francisville, LA. The significance to restoration of delta land lost since the 1930s is that the baseline for the 1930s was conditioned on previous sediment loading that has since declined. Most sediment is trapped in the delta, and so the existing situation is close to a zero-sum land balance. The restoration potential should be based on the delta land area that could be built from the current sediment loading, not from those of the era during peak agricultural expansion and soil erosion in the watershed. Sediment diversions upstream will, therefore, deplete sediment supply downstream where delta land will be lost. The choice of which baseline is used can be seen as a choice between unrealistic perceptions that leads to unachievable goals and agency failures, or, the realism of a delta size limited by current sediment loading