Wetlands in the Mississippi River deltaic plain are deteriorating 1 in part because levees and control structures starve them of sediment [2][3][4] . In spring 2011 a record-breaking flood brought discharge on the lower Mississippi River to dangerous levels, forcing managers to divert up to 3,500 m 3 s −1 of water to the Atchafalaya River Basin 5 . Here we use fieldcalibrated satellite data to quantify differences in inundation and sediment-plume patterns between the Mississippi and Atchafalaya River. We assess the impact of these extreme outflows on wetland sedimentation, and use in situ data collected during the historic flood to characterize the Mississippi plume's hydrodynamics and suspended sediment. We show that a focused, high-momentum jet emerged from the leveed Mississippi, and delivered sediment far offshore. In contrast, the plume from the Atchafalaya was more diffuse; diverted water inundated a large area, and sediment was trapped within the coastal current. The largest sedimentation, of up to several centimetres, occurred in the Atchafalaya Basin despite the larger sediment load carried by the Mississippi. Sediment accumulation was lowest along the shoreline between the two river sources. We conclude that river-mouth hydrodynamics and wetland sedimentation patterns are mechanistically linked, providing results that are relevant for plans to restore deltaic wetlands using artificial diversions 2-4,6-8 .Protecting and expanding coastal wetlands is vital for ecosystem services of the Mississippi River Delta 9-12 , and harnessing natural processes of wetland building using the Mississippi River and its sediments is an essential component of restoration plans [2][3][4] . The only portion of the delta experiencing significant expansion of coastal wetland at present is at the mouth of the Atchafalaya River (Fig. 1a), where a higher mineral (that is, non-organic) sediment concentration 3 and hydrodynamic factors 8 allow sufficient sediment deposition 6 to outpace subsidence and sea-level rise 13 . The recently released 2012 Coastal Master Plan 14 proposes river diversions and channel realignment to divert sediment and fresh water from the Mississippi River and Atchafalaya River into adjacent basins, to reconnect the river to delta wetlands. Successful design and implementation of such measures require an understanding of diverted sediment movement and deposition, especially during high-water events when the potential sediment load is greatest.The Mississippi River flood of spring 2011 was one of the largest on record 5,15 . Both the Mississippi River and Atchafalaya River

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TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe easternmost gas hydrate site in Mississippi Canyon (MC) was discovered using the Johnson Sea-Link research submersible in 2002. Free gas (C 1 -C 5 hydrocarbons and minor CO 2 ) vents from the seafloor to the water column at ~890 m water depth where temperature is ~5.7° Celsius. Vent gas rapidly crystallizes as massive white fracture-fillings of gas hydrate in mud at the seafloor. The Structure II gas hydrate is relatively deficient in methane (70.0%), relatively rich in ethane (7.5%) and propane (15.9%). The site is characterized by crater-like depressions and mounds of authigenic carbonate rock over an area of ~1 km 2 . The authigenic carbonate rock is the result of episodic microbial hydrocarbon oxidation. Chemosynthetic communities include bacterial mats (Beggiatoa) with tube worms, mussels, and bivalves. The MC 118 site is a unique seafloor laboratory to address important questions concerning the processes that result in crystallization of gas hydrate in fractures, microbially-driven precipitation of enormous volumes of authigenic carbonate rock, and the development of complex chemosynthetic communities in an extreme environment for life.

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