Editor's note: Paul Kemp is the Audubon Society's vice president of the Louisiana Coastal Initiative and was associate research professor in Louisiana State University's School of the Coast and Environment. He was a National Oceanic and Atmospheric Administration fellow on the staff of U.S. Sen. Edward Kennedy. John Day is a distinguished professor in Louisiana State University's Coastal Ecology Institute.
(CNN) -- The U.S. Army Corps of Engineers on Monday opened the Bonnet Carré Spillway for the 10th time since 1932. This 7,000-foot structure of gates on the east bank of the Mississippi River, 30 miles above New Orleans, relieves pressure on levees protecting the city by shunting river water into nearby Lake Pontchartrain.
As the crest of the historic 2011 flood rolls downriver from Memphis toward an arrival in Louisiana in two weeks, carrying up to 2 million cubic feet of water per second, we who wait at the bottom of the Mississippi's vast watershed are painfully aware of our dependence on an 80-year-old flood protection system that's functioning on borrowed time.
Consider the plight of Morgan City, Louisiana, on the Atchafalaya River near the Gulf of Mexico. If, as now seems likely, the Bonnet Carré Spillway cannot divert enough water to save New Orleans, the Corps will open another outlet, the Morganza Floodway, perhaps as early as the end of this week.
This structure on the west bank of the Mississippi upstream of Baton Rouge would transfer water from the main river to the Atchafalaya, increasing discharge to more than twice the normal spring flow and perhaps raising water levels beyond the record elevations reached at Morgan City in 1973, the only time Morganza was opened before. Morgan City's 13,000 residents are desperately raising levees and sinking barges to try to keep the water out but are also grimly preparing to evacuate if the order is given.
Extreme floods have always plagued the Mississippi, and the annals of America are filled with heroic efforts to stem the rising tide. Winter storms and snow melt-saturated soils in the Ohio River Valley typically combine to produce most of the water that enters the Mississippi. But the recent intense weather systems that generated so many tornadoes and record rainfall throughout the Midwest and Eastern United States have concentrated runoff in streams to raise this Mississippi crest.
Although this event is historic, it's part of an emerging pattern with clear implications for America's relationship to her greatest river. Intense precipitation has increased in most temperate regions around the world and is widely understood to be a consequence of global climate change.
Higher spring sea-surface temperatures in the Gulf of Mexico put more water in the atmosphere and cause intense thunderstorms, spawned when warm and cold air masses collide over the interior of the continent. Some projections indicate that the mean discharge of the Mississippi could increase by as much as 40 percent in this century. But it is also possible that the frequency of major spring floods triggered by more extreme rainfall over a shorter rainy season will increase without changing total annual discharge.
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Consider this. The Bonnet Carré Spillway has been operated according to the same plan for 80 years whenever Mississippi flow past New Orleans is projected to exceed 1.25 million cubic feet of water per second. The record 1973 flood led to the fourth opening of the spillway 40 years after construction. So, the structure was opened three times before 1973 -- 1937, 1945, 1950 -- and six times in the second 40 years --1975, 1979, 1983, 1997, 2008, 2011. The spillway is being opened more frequently over time, a statistic consistent with global change projections.
And in that context, consider this: Like the Bonnet Carré Spillway, the rest of the Mississippi River flood control system dates from the early 20th century, and so do the attitudes and assumptions about its operation. It's not the early 20th century anymore, and change is sorely needed.
A final consequence is that, except for the sand and mud that enters the Atchafalaya River, the massive sediment load carried by this once-in-a-generation flood will be lost to the deep waters of the Gulf of Mexico, and much of it will have to be dredged at great expense out of navigation channels. This, at a time when sediment is desperately needed to prevent the further ecological collapse and loss of the Mississippi River Delta's swamps and marshes we depend upon for seafood and protection from hurricane surges.
As the citizens of Morgan City meet an unprecedented wave of water, there are no plans to open additional river diversion structures upstream and downstream of New Orleans, which have been built in recent decades to start the process of reconnecting the levee-bounded river with its delta. Although they are small, these structures could take another 20,000 cubic feet of water per second off the Atchafalaya River levees and provide a healthy infusion of sediment for the disappearing marshes.
It's time to strike a new deal with the Mississippi River to meet the challenges of a new century, to keep people and businesses safe in the future, and to sustain the ecosystems that support us all.
The opinions in this commentary are solely those of the writers.
