摘要
The Mississippi River and its Atchafalaya distributary contribute 81 percent of river-borne freshwater entering the northern Gulf of Mexico, and 95 percent of the sediment from all sources. This flux nourished North America's largest and most biologically productive delta ecosystem. “Anthropocene” management has, however, separated the river from its delta in pursuit of flood control and navigation goals conceived after the great 1927 flood. Within the delta plain, there has been pervasive hydrologic alteration due to energy production, navigation channels, and impoundments. As a result, 25 percent of Mississippi River Delta wetlands have disappeared in less than a century. The surface of most marshes that remain are sinking relative to sea level toward the bottom of the tidal frame and, ultimately, conversion to open water. Deterioration of the barrier islands and wetlands, important components of once vast deltaic estuaries, has exposed more than a million delta residents to increased risk from hurricane surge and waves, while threatening fisheries throughout the northern Gulf. Recent observations from the back-to-back record 2011 high-, and 2012 low-Mississippi discharge events show that global sea level rise, subsidence and a marked shift in sediment dynamics over the past 50 years have affected lower Mississippi River hydraulics despite efforts to freeze the capture of the Mississippi by the Atchafalaya River distributary. A retreat of the “Birdsfoot” delta from the shelf edge toward a position last occupied 900 ka ago appears to be in progress. Opening of new, or enlargement of existing lateral outlets between New Orleans and the Gulf appears favored by this trend. Large sediment diversion projects to restart delta building and maintenance are being designed to take advantage of this shift. Large sediment diversions may also lower flood flow lines in the river and reduce dredging volumes sufficiently in the present navigation outlet to make it feasible for Mississippi River ports to service larger, Post-Panamax ships with greater than 15 m draft. Sediment diversion projects anchor a new 50-year, $50 billion “MasterPlan” that also seeks to enhance fish and wildlife resources and reduce hurricane flood risk to delta communities. Other land building goals are to expand use of sediments dredged and piped from the river and offshore to rebuild marshes and barrier islands, and to repair marsh hydrology wherever possible. We address a factor that has gotten less attention, namely how the increasing cost of energy will differentially affect the feasibility of specific restoration strategies over time.