Louisiana Takes Precautions for Mississippi River Flooding

03/15/2011 04:43 pm ET | Updated May 25, 2011

The last time the Bonnet Carre Spillway, situated on the Mississippi River 33 miles above downtown New Orleans, was opened was spring 2008, and that event was a must-see for weekend picnickers and school field trips. As the river rises from the Midwest snow melt and rain, the U.S. Army Corps of Engineers plans to hold a practice drill in case the spillway -- which was built after the great flood of 1927 to protect communities -- needs to be opened this spring.

Meanwhile, businesses along the lower river are preparing for high water. And the oyster industry, deflated by river diversions during the BP spill, is bracing for more complications if the spillway is opened. Too much fresh water kills oysters, and Bonnet Carre discharges excess water into Lake Pontchartrain and then into the Gulf of Mexico.

Mike Stack, chief of the Army Corps' Emergency Management Office in New Orleans, said the high-water season for the lower Mississippi River is March into early June. "On March 9, the National Weather Service issued a forecast that the current river rise will crest at 14.5 feet on the Carrollton gauge in New Orleans on March 26," he said last week.

Stack said the Corps considers opening the Bonnet Carre when the river flows at 1.25 million cubic feet per second through the city of New Orleans. Water levels are monitored daily, and the flow is increasing now because of rain in the Mississippi and Ohio River Valleys this month, he said.

"Today, the flow passing through New Orleans is 800,000 cubic feet per second, so we have a ways to go before we would consider opening the spillway," Stack said. "But we'll schedule a test opening of the spillway soon to make sure that everyone knows what to do in the event of an actual opening."

Structures above Baton Rouge, including the Old River Control Complex and the Morganza Spillway, which diverts flows into the Atchafalaya Basin, are also used to help regulate the river, Stack said.

If the Bonnet Carre is opened, river material will be deposited in Lakes Pontchartrain and Borgne, and movement of fresh water into the brackish and saline lakes will affect fisheries, including oysters.

A century ago, oyster growers welcomed spring's high water, but circumstances have changed. Al Sunseri, president and co-owner of P&J Oyster Co. in New Orleans, said, "Before the Army Corps built the levees and navigation channels and the oil companies built thousands of miles of pipeline canals, spring flooding of the river rejuvenated the wetlands, as well as marine life -- including oysters. However, that's all been altered by the re-engineering of waterways in Southeast Louisiana."

Richard Boe, supervisory environmental-resources specialist at the Army Corps in New Orleans, said the impact on oysters from opening the Bonnet Carre depends on the timing, duration and volume of water discharged, along with weather. "Oysters survive low salinity better when the water temperature is cool to cold, so spillway operations earlier in the year are less likely to have an adverse impact on oysters," he said. "The longer the spillway is operated, however, and the larger the total volume of water discharged, the more likely it is for very low-salinity water to adversely affect productive oyster areas."

Sunseri agreed, saying, "Oysters can live for two to three weeks sealed up, in zero salinity, when the water's cold. But if the water gets too fresh, when estuary water temperatures are warm, oysters open up and die from too little salt water."

Boe points to the short- and long-term effects of opening the spillway. He said, "During past openings, the combination of low, ambient salinity levels in oyster-producing areas and water discharged through the spillway combined to produce significant mortality of oysters. However, it's been documented in numerous studies that oysters that survive spillway openings grow very rapidly, reproduce well, and are of good quality because of the growth of plankton populations on which oysters feed."

Opening the spillway in the spring fills Lake Pontchartrain with nutrients, and as a result fishing in the lake is usually good later in the same year.

Meanwhile, the local oyster industry is trying to recover from last summer's fresh water diversions. Because of damage to beds, "we're like a cork bobbing in the water," Sunseri said about P&J, which was founded in 1876. "We've cut costs and reduced expenses back to 1980 levels and are getting by with a staff of just me and my brother and a few people working 15 to 20 hours a week, until more oysters become available."

Sunseri said, "It should take two and a half to three years for oysters to become market size, that is 3.5 to 4 inches, if everything goes well and we don't have another freshwater kill-off." He noted that in St. Bernard Parish, "We only saw oyster spat set in the north end of the parish on the east side of the Mississippi River because of fresh water diversions to keep oil outside of the marsh after the Deepwater Horizon rig disaster." Spat are baby oysters. Fresh water was diverted starting last May from the Mississippi River into nearby salt marshes in St. Bernard, Plaquemines and St. Charles Parishes.

St. Bernard President Craig Taffaro said last week that his parish's diversion gates don't address issues with a rising river, and added, "We have no flood gates here." Upriver spillway structures like the Bonnet Carre relieve pressure from a rising river, he noted.

Stack at the Corps said, "Fresh water diversions from the river are used for purposes other than flow regulation. They don't make much of a difference in easing the flow of the river. They're done for environmental purposes -- for example, to build marsh."

As the river rises, Stack said, "We're coordinating flood-fighting preparations with local levee districts, the Governor's Office of Homeland Security and Emergency Preparedness, and state and local agencies. Companies should adjust their tie-down procedures, and businesses along the river should take precautions to protect their facilities."

This article was published in the Louisiana Weekly in the March 14, 2011 edition.