Document Type

Article

Publication Date

2026

School

Ocean Science and Engineering

Abstract

This project examines ecological effects of the current operational strategy of the Bonnet Carré Spillway (BCS), a Mississippi River flood-control structure located about 21 miles northwest of New Orleans, Louisiana, and managed by the U.S. Army Corps of Engineers. The BCS is part of the larger Mississippi River and Tributaries Project, a network of levees and control structures designed to minimize flooding from the American plains to southern Louisiana. The spillway is opened when river discharge at New Orleans is forecasted to exceed 1,250,000 cubic feet per second, diverting significant volumes of Mississippi River water into the Mississippi Sound. The Mississippi Sound normally maintains salinities of 5–18 parts per thousand, fluctuating with seasonal wet and dry periods. While natural salinity deviations can occur due to local factors, these are usually brief. Extended BCS openings, however, have reduced salinity to zero for over 39 consecutive days across much of the estuary. Past BCS openings have been linked to elevated mortality in marine species, including oysters. Over the past 14 years, the BCS has been opened seven times, which is more frequent than during the previous six decades. After the two BCS openings in 2019, it became clear that spillway operations can have severe impacts on ecosystems and fisheries, as harvestable oyster reefs in the Mississippi Sound experienced up to 100% mortality based on MDMR field surveys during the summer of that year. Mississippi’s on-bottom oyster fishery was closed for five years after this event. With increasing rainfall trends at both local and national scales, there is an urgent need to explore alternative flood-control strategies and management approaches that minimize ecological harm and protect Mississippi’s marine resources. This is the report on the ecological component of the project “Development of an operational alternative to the Bonnet Carré Spillway accounting for ecological tipping points in the Mississippi Sound”. The objective of this component is to identify where and when environmental stressors (e.g., low salinity) lead to the ecological tipping points whereby (or salinity thresholds when) oyster mortality is significantly increased due to BCS releases, and at what discharge these occur.

Download

Share

COinS