Patterns and mechanisms of wetland change in the Breton sound estuary, Mississippi River delta: A review

Document Type

Article

Publication Date

2-1-2025

Abstract

The Breton Sound Estuary, located within the Mississippi River Delta, has experienced significant wetland loss over the past century due to a combination of natural and anthropogenic factors. This study examines the patterns and mechanisms driving wetland change in the upper Breton Sound Basin and focuses on the impacts of riverine isolation, hydrological alterations, and human activities. Prior to human interventions, the basin received regular large riverine input via overbank flooding and crevasse channels. Levee construction began in the 18th century, but it wasn't until the great Mississippi River flood of 1927 that continuous levees were built that completely isolated the river from the upper Breton Sound Basin. The reduction of riverine input led to replacement of fresh and low salinity marshes by more salt-tolerant species. Sediment introduced during this single 1927 event created a 130 km2 crevasse splay still visible in marsh cores more than 10 km from the levee breach. Higher levees built afterward as part of the Mississippi River and Tributaries Project (MR&T) have almost completely isolated this basin from the river. A small river diversion was constructed in 1991 to divert river water into the basin, but its impact has been minimal compared to the historical crevasse channels. Starting in the 1970s, wetlands in the upper basin began to deteriorate and was dramatic during the passage of Hurricanes Katrina and Rita in 2005, and wetland loss was much higher in some areas. For example, some wetlands in the path of diverted water had loss rates of about 7% per year during the passage of the hurricanes compared to <1% per year for most of the rest of the upper basin. Here we describe patterns of wetland change and analyze factors responsible for the loss. There have been pervasive changes in hydrology of the upper Breton Sound Basin due to flood control levees. But dredging of an extensive canal network is mostly responsible for the majority of wetland loss in the central part of the upper basin. Marshes facing the open waters of Breton Sound had low loss rates and receive sediments and nutrients from resuspended seafloor materials and Mississippi River water discharged to open waters of Breton Sound. Nutrients played a negligible role in causing wetland loss. Sediment introduction is likely the most effective restoration strategy for mitigating further wetland loss, and specific recommendations are provided that are tailored to the unique conditions of different zones within the Basin. Understanding the complex interplay of factors influencing wetland dynamics in the Breton Sound Estuary is crucial for developing effective restoration and management strategies in the face of ongoing environmental changes.

Publication Source (Journal or Book title)

Estuarine Coastal and Shelf Science

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