The U.S. 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 eased pressure on the city's levees but raised 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 (5 metres) at a key New Orleans gauge, with a speed of 1.25 million cubic feet (about 35,400 cubic meters) 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.

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 finish to move out of the lake, while potentially killing oysters developing off the Louisiana and Mississippi coasts.

The river water also carries nutrients that can contribute to algae blooms, said John Lopez, a scientist with the Lake Pontchartrain Basin Foundation. He said winds can push mats of algae into harbors, where fish kills are a possibility.