Yet another critical problem is sediment reduction. Louisiana marshes need a source of sediment to survive. Historically, the Mississippi River provided the sediment. Now, however, levees confine the sediment to the river; thus bypassing the marshes, ultimately depositing it on the continental shelf in the Gulf. Our coastal marshes constantly undergo a natural process called “subsidence” which results in the land slowly sinking. In the past, the rate of sediment building equaled or surpassed the rate of sinking and the level of the marsh remained above the level of the sea.
Today, the river carries up to 80 percent less sediment than it did a century ago. Dams, reduction in land clearing and tilling, and implementation of conservation measures that reduce erosion upriver are the major causes of the reduction. Thus, even if all of the levees along the Mississippi River were removed today, the marshes would still receive significantly less sediment than they did in the 1800s.
Still, some sediment does move into coastal marshes during hurricanes and winter cold fronts when winds driven waves stir mud on the bottom of shallow bays. The volume of this sediment, however, is usually inadequate to counter the effects of subsidence. The existence of levees, canal banks, roadbeds, railroad embankments and changes upriver all contribute to the problem of inadequate sediment distribution in our coastal marshes.
- Navigation and oil/gas extraction canals
- Diking and leveeing of wetlands
- Spoil banks from dredging activities
- Upstream diversions of the Mississippi River into other basins resulting in less water and sediments available for the estuarine complex
- Locks and dams on the Missouri, Ohio, and upper Mississippi rivers
- Sedimentation rate becomes less than the rate of apparent water level rise (subsidence and sea level rise)
- Submergence and mortality of wetland vegetation
- Internal fragmentation of wetlands
- Lowered productivity of wetland vegetation