CARBON AND SEDIMENT SEQUESTRATION PATTERNS WITHIN THE ATCHAFALAYA BASIN AND MORGANZA SPILLWAY BEFORE AND AFTER THE LOWER MISSISSIPPI FLOOD OF 2011

Sediment and carbon sequestration are an important ecosystem function of forested bottomlands. Our study is located in the Atchafalaya Basin (Basin) and Morganza Spillway (Spillway), Louisiana. The Basin is a distributary of the Mississippi River and contains the largest contiguously forested riparian wetland in North America. The area experiences high sedimentation in well hydraulically connected areas and hypoxia and sediment starvation in hydraulically isolated areas. The Spillway diverts water from the Mississippi River to the Atchafalaya Basin during exceptional floods (approximately 56,000 m³/s). The Mississippi River experienced the second largest recorded flood in the spring of 2011 forcing the opening of the Spillway for the first time since 1973 resulting in record flows in the Basin.

In 2010 we established approximately 20 floodplain transects in areas identified as highly depositional (previous study: 2000 to 2003). The sites were visited twice in 2011; once before the flood and again after the floodwaters had receded. Sediment cores were taken after the flood in the Spillway at 10 locations with sedimentation measurements at another 30 sites. Deposition rate, sediment texture, bulk density, and loss on ignition (percent organic material) were determined at each site. Mean sedimentation rates ranged from slightly erosional to greater than 1 meter/yr. Organic material ranged from 1% to 28%. Sand ranged from about 5% to 90%, and density varied from 0.4 to 1.3 g/cm³. Sites with low elevation, high hydrologic connectivity, and hydraulic damming (flow stagnation) featured the highest amounts of sediment trapping in the Basin; the converse in any of these factors typically diminished sediment trapping. The forebay of the Spillway trapped the majority of the sediment diverted from the Mississippi during the opening of the floodgates. Almost all remaining sediment that went through the structure was deposited within 3 km of the floodgates. The Atchafalaya Basin plays a substantial role in lowland sediment and carbon sequestration, though a large interior area is currently losing land due to anthropogenic alteration of flow. These findings on local sedimentation patterns may aid in management of flow to control sediment deposition and reduce hypoxia.