Consequences of Sea Level Rise: Implications from the Mississippi Delta

Abstract

Sea level rise is expected to increase worldwide over the coming decades, and its impacts are beginning to be felt in many areas. Two major direct impacts of sea level rise are submergence and salinity increase. Historically, the Mississippi River Delta has experienced a relative sea level rise (RSLR) and thus serves as an analogy or model for what can be expected elsewhere. Despite long term RSLR primarily due to subsidence, the Mississippi has grown in size over the past several thousand years since eustatic sea level stabilized. Within this century, the net positive growth rate has been reversed and net wetland loss rates as great as 100 km² per year have occurred. Much of the wetland loss is associated with human activities that have resulted in a reduction of sediment input to wetlands. Because of this reduction, vertical accretion of the wetland surface is less than RSLR and plants are disappearing due to waterlogging and salinity increase. The resulting loss of wetland plant vigor complicates the problem because the production of plant roots is an important component of soil formation and vertical accretion of the wetland surface.

Two important points to consider in addressing the problem of sea level rise is that there is often a lag time of decades before the response of the natural system to sea level rise becomes evident and that changes in the natural system may be slow at first and then accelerate. The institutional response in Louisiana is complicated, but many of the actions taken may be detrimental in the long run. A common response to rising water levels will be flood control. But, in the Mississippi Delta, dikes along the river have greatly restricted sediment input to wetlands. Additionally, semi-impoundments with water control structures are being considered to protect wetlands from increasing water levels and salinity increases. Many of these depend on gravity drainage, but in a microtidal area such as the Gulf coast, gravity drainage has a finite life span due to rising water levels. Land ownership patterns also complicate a comprehensive approach to the problem due to units selected for management and conflict between short-term and long-term benefits. We conclude that coastal wetlands can be managed to survive rising sea level but that only comprehensive, integrated, long-term planning can effectively deal with the problem of sea level rise. The principle of ecological engineering, where the energies of nature are used as much as possible, should play an integral part of any management plan. Because deltas are probably one of the most threatened of coastal landscapes, an early warning monitoring system is recommended for selected deltas of the world.

This article has also been published in Coastal Management 17: 241–257 © 1989 Taylor & Francis, reprinted by permission.