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

Abstract

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 km2 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 km2 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.