CONTROVERSIES AND CONNECTIONS IN THE RESTORATION OF RIVER-DOMINATED DELTAS (Invited Presentation)

River deltas are inherently complex systems, and these complexity lead to confusion in both the scientific and management communities regarding the best way to restore delta. In the Mississippi River Delta, there is considerable debate regarding "river diversions-” areas where sediments and water are diverted from the Mississippi River into degrading wetlands with the hope of reinitiating deltaic land building processes. Some have argued that diversions lead to reduced shear strengths of wetland soils that make them more vulnerable to storm driven erosion, while others argued that sediments from river diversions will develop stable land.

This controversy was addressed in the Cubits Gap Subdelta, an analogue for a large (>1,420 m³ s^-1) river diversion. This study tested the hypothesis that areas of land gain, and/or resilience to erosion occurred in areas that actively received river sediments, thus having mineral rich soils with high shear strength. A new Normalized Difference Water Index was developed for Landsat-7 Enhanced Thematic Mapper (ETM+) and Landsat-8 Operational Land Imager (OLI) images, which yielded land/water boundary maps with 30 m resolution. Land gain occurred predominantly in the riverside section of this subdelta where sediments were imported from Mississippi River crevasses and/or dredging. Land loss typically occurred in the distal regions of the subdelta, which had lower levels of sediment supply and greater wave exposure. Geotechnical analyses revealed land loss pixels generally correlated sediments with to high organic contents, water contents, and salinity, and with low shear strengths and bulk density, whereas land gain pixels generally correlated with low organic content, water content, and high shear strength and bulk density. Results indicate that diverting the flow of the Mississippi River to build land will be most successful if such systems carry high loads of sediment; concerns about the integrity of fresh marshes may be unwarranted if those marshes are rich in mineral sediments.