TRACKING WETLAND SEDIMENTATION FROM THE HISTORIC 2011 MISSISSIPPI RIVER FLOOD

The marshes of coastal Louisiana are disappearing at a rapid rate due to natural and anthropogenic processes. Maintenance of soil elevations relative to water levels is key to marsh sustainability, but leveeing of the Mississippi River prevents overbank flooding and direct delivery of sediment to counterbalance rapid rates of subsidence. Enhanced sediment deposition may occur during hurricanes or extreme flood events, contributing to marsh accretion, but their relative importance to marsh maintenance is unclear. A better understanding of routing and deposition of sediments and their role in marsh-building dynamics would help clarify these issues and aid restoration planning.

The events of the 2011 Mississippi River flood offered a unique opportunity to understand how sediment is delivered to Louisiana marshes during a geologically significant flood event and enabled the first and only detailed description of the physical, chemical, and biological properties of sediment deposited during the flood.

A helicopter survey of 45 sites was conducted along 350 km of coastline to measure sediment accumulation and determine its provenance. Flood sediments were identified by an absence of ingrown plant roots and a distinct color and consistency; however, the lithological (bulk density, organic matter content and grain size) and chemical (stable carbon isotopes of bulk organic matter) properties of flood sediment were nearly identical to pre-flood constituents. Biological characteristics distinguished the two sediment types and provided information on the mode of sediment deposition and source of flood material. The abundance of centric (planktonic) diatoms and assemblages indicative of direct riverine deposition increased significantly in the flood sediments. Flood sediment accumulation was significant in comparison to annual accretion and hurricane sedimentation estimates and was substantially larger near rivers than in basins further from the channels. Our findings not only provide insight into how large-scale river floods influence wetland sedimentation, they lay the groundwork for identifying previous flood events in the stratigraphic record.