One Mississippi, Two Mississippi
Do you remember those old margarine ads--"It's not nice to fool Mother Nature?" It's not nice to fool Old Man River, either. In the long run, it's not possible. That's what might be concluded from the presentations in some sessions at the New Orleans meeting of the American Association for the Advancement of Science this year. The scientists were considering how lower Louisiana has been affected as people have manipulated the Mississippi River.
Some of that manipulation was unintentional (whenever a parking lot is paved in Minnesota or Ohio, runoff changes in the Mississippi system), but much was deliberate, in attempts to control the river's damaging floods and eroding banks or to improve its navigability.
Each action was reasonable in light of economics and human safety. The inhabitants of riverside towns and cities did not want their works inundated or swept away when the river flooded. Neither did the farmers who cultivated the fertile floodplain soils. Of course, while banks, levees, dikes, and revetments to reinforce collapsing shores protected some areas, they also diverted water into other places. That led to more control structures. The Mississippi and its tributaries became more artificial, and--superficially, at least--more tamed.
Especially for its lower reaches, this stability is unusual in the river's geologic history. Over millennia, the Mississippi River has established a pattern of shifting in its delta. Sometimes its main outlet to the Gulf of Mexico has been far east, sometimes far west of the mouth it has used since Europeans first described it. With every shift, the river deposited sediments in new places and new patterns. The sediment built barrier islands, renewed land eroded away by storms and currents, generated the rich marsh and swamp lands that became southern Louisiana.
Human effort halted this shifting, and that halted the natural rebuilding processes elsewhere in the delta. It didn't stop the gentle sinking of the whole area, which is subsiding under the weight of centuries of sediment deposition. And slowly but inevitably, southern Louisiana began to vanish. It's being reclaimed by the sea, at the rate of 100 square kilometers a year. The area of the protective barrier islands is down by some 40 percent over the past century, leaving the shore more exposed to storms and further erosion.
That of course damages the marshes. The threatened wetlands harbor an array of resident wildlife, but they're also wintering grounds for about a quarter of North America's entire waterfowl population.
And, like marshes everywhere, they provide enormous amounts of nutrients to the adjacent sea. Fishermen from Mexico to Florida notice in their catches that the Mississippi Delta is in decline.
Of course, management agencies are hearing complaints from many constituencies. As one speaker noted, "These phenomena are of great concern to the communities and cultures that make up Louisiana's alluvial wetlands...The population at risk is nearly half the state's total population."
That includes residents of New Orleans, a city now standing below sea level. (People attending the meeting sessions could walk outside and observe ships going by on the river beyond the levees--higher than the streets.) Yet the city's leaders aren't campaigning to let the river seek a new mouth; the present one sustains New Orleans' economic life, and that of bustling Baton Rouge upstream. If the river moved to a new channel, those cities might find themselves squatting beside a muddy trickle.
Human effort can't still the river's restlessness forever. The hydrologic workings of the system have picked the new mouth, via the Atchafalaya River to the west. The Corps of Engineers has emplaced the mighty Old River Control Complex to hold back the change, but--so to speak--they realize they're only whistling Dixie. Another earthquake on the New Madrid fault, an upstream dam failure, even another massive flood could overpower the control complex. Someday the river will have its way.
To us in distant Alaska, the continuing struggle to save everything in Louisiana--the economy and the ecology both--may seem like spectator sport, one that at worst we'll feel in our taxes (as another speaker commented, "Preliminary estimates of the cost to substantially reduce the wetland loss rate exceed one billion dollars"--and that's merely to mitigate the rate at which only one part of the problem is growing).
Or perhaps we're only a century away from similar trouble. Whenever I drive from Fairbanks to Valdez, the first part of the Richardson Highway takes me over the Chena Flood Control Project and along the dikes nudging the Tanana River away from homes and streets. These are valuable protections for human interests. Building them was obviously the right decision--just like each of the decisions to keep the Mississippi in line.
