Hydrologic Modification

Hydrologic ModificationHydrologic modification is considered the “linchpin” problem of the basins, indicating that all other problems revolve around, and are often affected by it. When we build levees, dredge canals, or cut through natural ridges, the natural flow of water is changed. In some cases, such changes accelerate erosion. In other cases, it can result in changed salinity of water bodies. As a result, fresh marsh can be changed to a more “salt tolerant” type. In more extreme cases, marsh can be converted to open water.

Because of flood protection measures instituted by Congress following the 1927 flood, 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.

Aerial views of the estuary also reveal another type of hydrologic modification: canals for navigation and oil and gas exploration and production. 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 either 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.

Probable Causes

  • 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 

 Probable Impacts

  • Reduced sediment flows
  • Habitat loss/modification
  • Changes in living resources
  • Eutrophication
  • Pathogen contamination
  • Toxic substances
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