Mark Schleifstein / 10 April 2020

Several blooms of greenish algae have been spotted along both the north and south shores of Lake Pontchartrain, just a week after the Bonnet Carre Spillway was opened by the Army Corps of Engineers to reduce the threat to the New Orleans area from high water in the river.

Geologist Chris McLindon took photographs of algae-tainted water hugging the shoreline in Mandeville on Thursday, and Rachel Strassel, a spokeswoman for the Lake Pontchartrain Basin Foundation, said she spotted algae along the southernmost six miles of the Lake Pontchartrain Causeway bridge.

Corps officials said they have not yet determined that the spillway opening is the cause of the blooms, however.

“Algal blooms typically occur this time of year as water temperatures increase,” said Corps spokesman Rene Poche. “We need to continue to collect data and samples from across the lake to see how the water from the spillway operation is moving in the lake and compare that to pre-operation conditions.”

Corps workers had opened 70 of the 350 bays in the spillway structure as of Thursday evening, allowing 63,000 cubic feet per second of river water to enter the lake. That water — representing roughly 5% of the river’s flow — is high in nutrient content, the result of fertilizer running off farmland in the Midwest and other areas north of Louisiana and into the river.

In announcing the spillway opening on April 2, Col. Stephen Murphy, commander of the Corps’ New Orleans District , said he hopes workers can begin closing the spillway again by the end of April.

On Thursday, the river was held to a high of 16.8 feet at the Carrollton Gauge in New Orleans, thanks to the gates already opened. The official flood stage in New Orleans is 17 feet, though earthen levees and floodwalls protect the city from water heights of at least 22 feet. The river is forecast to rise to 17 feet by Monday and stay at that level through next Friday. The river is then expected to begin a slow fall, dropping to 13.5 feet by May 7, the end of the National Weather Service’s 28-day forecast.

This marks the fifth time the spillway has been opened in the last five years. Before that, the spillway was opened on average once every 10 years. The spillway was opened twice last year for a total of 143 days.

The lengthy opening resulted in blooms of toxic blue-green algae along the lake’s north shore, and also along the Mississippi Gulf Coast, causing beaches there to be closed to tourists. The river’s fresh water also destroyed oyster beds in the Mississippi Sound, Lake Borgne and in other Louisiana locations on the east side of the river, as well as disrupted shrimp development and commercial finfish catches.

The U.S. Department of Commerce issued an unusual fisheries disaster declaration for Louisiana and other Gulf Coast locations.

Several Mississippi coastal counties and cities, and environmental and commercial fisheries groups filed suit against the Corps, contending the federal agency had not adequately studied the environmental effects of opening the spillway or compared it to using the Morganza Floodway farther north to direct water into the Atchafalaya Basin.