NORCO, La. — The Army Corps of Engineers began opening part of a 1930s flood-control structure northwest of New Orleans on Thursday to divert water from a rising Mississippi River into nearby Lake Pontchartrain.
The move is intended to ease pressure on the city’s levees but raises environmental concerns about the lake and the Gulf of Mexico.
Rain in the upper Mississippi Valley has caused the river to rise and increase in velocity. It was projected to reach 17 feet at a key New Orleans gauge, with a speed of 1.25 million cubic feet per second. Those forecasts triggered a decision to open the Bonnet Carre Spillway for the 12th time since it was completed in 1931.
Opening the roughly mile-and-a-half-long structure of concrete bays and wooden timbers diverts some of the river’s flow before it reaches New Orleans. The water will continue to flow into the lake nearly 6 miles away and then into the Gulf of Mexico.
Environmental concerns include the effects the fresh river water will have on the salinity of the brackish lake and the coastal Gulf waters. Changes in salinity can affect fisheries, causing finfish to move out of the lake, while potentially killing oysters.