Mississippi River Basin Dynamics
The Mississippi River Delta Basin is defined as the land and estuarine area between the two northernmost passes of the Mississippi River and the Gulf of Mexico (figure 14). The Mississippi River alluvial valley drains over two-thirds of the continental United States, and, as a result, carries an enormous amount of water and sediment captured by the river during storm events and high water periods. The average water discharge for the river is 470,000 cfs, and the average suspended sediment load transported by the river is 436,000 tons/day. For the last 1,200 years, this sediment has been deposited primarily at the mouth of the Mississippi River's current Plaquemines-Balize Delta. In recent decades, the delta front has been building laterally into the Gulf of Mexico at a rate of 300 to 400 feet/year. Currently, the delta front is located at the edge of the Gulf of Mexico's continental shelf. As a result, large volumes of sediment are now being lost to the continental slope or ocean floor, where water depths are up to 1,000 feet and, therefore, not conducive for land-building.
Much of the Mississippi River Delta Basin has experienced rapid coastal land loss (figure 14). Recent land loss estimates for the basin average between 819 (Barras et al. 1994) and 1,337 acres/year (Dunbar et al. 1992). Since 1932, the basin has lost approximately 70% of its total land area (LCWCRTF 1993). At current loss rates, up to 26,740 acres will be lost during the next 20 years (53,600 acres over 50 years) if no wetland restoration efforts are implemented.
This land is being lost due to many factors, including compaction causing a subsidence rate of 5 feet per century, loss of sediment, possible fault zones, tidal and boat wake erosion, sea level rise, hurricanes, and human activities such as maintenance of navigation channels and construction of canals for mineral exploration. The basin comprises approximately 521,000 acres, of which 83% is open water. The remaining 101,000 acres consists of freshwater to brackish marshes that are highly valuable as fish and wildlife habitat.