Hydrology

Current Understanding

New Orleans and indeed the entire Gulf Coast lie between two water systems: the sea and the rivers emptying into it. Before European settlement, the Mississippi carried a stream of sediment-laden waters through an intricate network of channels, natural levees, marshes, lagoons, and barrier islands to the Gulf. Today, the coastal zone of Louisiana contains about a quarter of the nation’s wetlands and almost half of the coastal wetlands. The interior areas of the river’s delta were sheltered from storm effects, and intrusion of salt water was limited. Regular flooding over banks and through channels provided sediment flows that replenished the marshlands and expanded the delta. Meanwhile, subsidence due to lithosphere depression, compression of sediments and peat, and most recently extraction of oil and gas have worked to submerge the delta. Three components of water are present in this complex and dynamic landscape: the fluvial, steadily flowing streams, the tidal stretches with reversing flows, and the permanently subaqueous plain. This watery network has changed constantly over time as some channels have become clogged from sediment deposition and others have gained active flows.

Early settlements such as New Orleans were established on naturally elevated levees and were protected from hurricanes by the natural landscape. Flooding from the Mississippi was a greater threat than storms from the Gulf. Human activities in recent decades both in the region and in the upper Mississippi basin have caused changes in the tidal wetlands that in some areas have produced rapid deterioration. Tributary channels have been closed. Artificial levees have been constructed, and people have settled in low-lying areas under their protection. Land has been extensively developed for human uses. Canals have been dredged for navigation, oil and gas production, and transportation. Coastal hydrology has been greatly modified. Wetland loss has accelerated markedly. Finally, storm surges and wave action of great storms such as Katrina may produce large temporary or long-term losses of wetlands.

Changes in the Mississippi itself have been significant. A salt wedge now penetrates the river’s deep, main channel, extending with low river flow even above New Orleans. Currently, the river system has two major active channels: the Mississippi and the Atchafalaya. River flow and sediment discharge have changed dramatically over time. In the nineteenth century there was a large sediment transport because of upstream land changes. In the latter half of the twentieth century, sediments greatly diminished because of upstream dams and locks. Jetties in the navigational channels of the delta that have been made by man facilitate the movement of sediment away from the delta. The amount of material now available to restore wetlands is limited. Since the 1930s levees have been constructed along most of the Mississippi that have prevented sediments from replenishing the bordering wetlands. Instead, those sediments are deposited at the very edge of the continental shelf in the Gulf of Mexico. The river has been prevented from switching to a new course. The Atchafalaya’s flow has been limited by control barriers; otherwise it would be the new course for the Mississippi. The mechanisms that had built and preserved the delta in the face of subsidence and erosion have been largely stifled.

Near-Term Needs

One of the simplest and thus conceptually attractive methods to provide some level of protection for New Orleans and the larger Gulf Coast area is to restore through natural processes a coastline barrier that would serve as a buffer to any incoming storm, reduce the rate of land loss, and minimize inland flooding. Given current rates of sea level rise and other difficulties in reestablishing natural processes, it is unclear at present whether such an approach is actually feasible.

The use of freshwater and sediment resources of the Mississippi and other rivers must be based on detailed predictions of discharge and sediment availability that encompass basin hydrology including land use and land cover changes.

Longer-Term Directions

In the long term, flood protection can only be secured with a combination of levees and a sustainable coastal landscape. Rebuilding the region in an environmentally and economically sustainable fashion will require integrated planning, investment, and management that recognize the forces of nature, the need to protect communities, the value of natural resources and ecosystem services, and financial constraints.