Because of flood protection measures demanded by the public and then instituted by Congress following the Great Flood of 1927, in conjunction with those of private landowners and the State of Louisiana prior to the flood, artificial levees now line much of the Mississippi River.
The levees coincidentally prevent sediment and water from being dispersed into the surrounding wetlands through periodic flooding and levee breaks. Concrete mattresses placed along the channel bank have prevented the natural tendency of the river to change course. In fact, the length of the river has been shortened by approximately 150 miles by cutoffs in the central portion of the lower Mississippi River. Both shortening of the river and placement of concrete mats on the banks have reduced the river area exposed to erosion. In the past, soil from the river’s edges was the primary source of sediment that fed the marshes.
Canals for navigation and oil and gas exploration and production are another type of hydrologic modification. When canals are constructed, the excavated material is placed alongside the canal, creating spoil banks. The impact of this type of activity can be threefold. First, the canal itself creates paths of ingress for waters of higher salinity, forcing animals to either adapt or relocate. Native plants have little choice but to adapt to their new environment or die. Second, erosion can occur along the canal banks with the passing of each vessel, converting more land to open water. Third, the dredged material alters the natural flow of water across the estuary landscape, sometimes creating lakes and in other cases, depriving large areas of water, nutrients and sediments.
Impacts of canals are not, however, all necessarily negative. Canal banks do provide some diversity of habitat, especially in coastal areas. Canals provide significant recreational opportunities and aquatic production potential as well.
- Diking and leveeing of wetlands
- Maintenance dredging; spoil banks
- Excavation of channels and canals for navigation and/or oil, gas, and mineral exploration; particularly those excavations deeper than surrounding waters
- Diversions of freshwater flows and sediment loads for navigation, flood control, or water supply purposes
- Reduced sediment flows
- Habitat loss/modification
- Changes in living resources
- Pathogen contamination
- Toxic substances