Abstract

Over the past few centuries, 25% of the deltaic wetlands associated with the Mississippi Delta have been lost to the ocean¹. Plans to protect and restore the coast call for diversions of the Mississippi River, and its associated sediment, to sustain and build new land^{2, 3}. 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^{4, 5, 6}. 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 km² 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.