Inside the barge's wide-open room, Danny offers other measures: "Two years ago we drove a wooden two-by-four into the mud on the edge of a canal, to stake our alligator trap. I went past it the other day; the edge has receded 18 feet from the stake. Doesn't much matter, though. The gators are gone. Water's too salty."
With the marsh disappearing, the delta's only remaining defense is some crumbling barrier islands that a century ago were part of the region's shoreline. The next morning Penland and I travel an hour down the coast to the Louisiana Universities Marine Consortium, a scientific outpost in Cocodrie, an encampment of scientists and fishermen on the coast's edge. From there we head out in one of the consortium's gray research boats.
The boat pounds across what appears to be choppy sea for 50 minutes before we reach Isles Dernieres ("last islands" in French). But the open surf is never more than seven feet deep. The vast reach of shallow water was once thick with swaying grasses, parted occasionally by narrow, serpentine waterways full of shrimp, oysters, redfish and trout. Penland beaches us in the bayside mud. We walk across a mere 80 yards of barren sand before we toe the ocean. A similarly diminutive outcrop is visible in the distance to either side. They are what remains of a once very long, staunch island lush with black mangroves. "It broke up ocean waves, cut down storm surges and held back saltwater so the marsh behind it could thrive," Penland says in mourning. Now the ocean rushes right by.
Louisiana's barrier islands are eroding faster than any around the country. Millions of tons of sediment used to exit the Mississippi River's mouth every year and be dragged by longshore currents to the islands, building up what tides had worn away. But in part because levees and dredging prevent the river's last miles from meandering naturally, the mouth has telescoped out to the continental shelf. The sediment just drops over the edge of the underwater cliff into the deep ocean.
Back in New Orleans the next day it becomes apparent that other human activities have made matters worse. Cliff Mugnier, an L.S.U. geodesist who also works part-time for the Corps of Engineers, explains why from the third floor of the rectangular, cement Corps headquarters, which squats atop the Mississippi River levee the Corps has built and rebuilt for 122 years.
Mugnier says that the earth beneath the delta consists of layers of muck--a wet peat several hundred feet deep--formed by centuries of flooding. As the Corps leveed the river, the city and industry drained large marshes, which in decades past were considered wasteland. Stopping the floods and draining surface water lowered the water table, allowing the top mucks to dry, consolidate and subside, hastening the city's drop below sea level--a process already under way as the underlying mucks consolidated naturally.
That's not all. As the bowl became deeper, it would flood during routine rainstorms. So the Corps, in cooperation with the city's Sewerage and Water Board, began digging a maze of canals to collect rainwater. The only place to send it was Lake Pontchartrain. But because the lake's mean elevation is one foot, the partners had to build pumping stations at the canal heads to push the collected runoff uphill into the lake.
The pumps serve another critical function. Because the canals are basically ditches, groundwater seeps into them from the wet soils. But if they are full, they can't take on water during a storm. So the city runs the pumps regularly to expel seepage from the canals, which draws even more water from the ground, leading to further drying and subsidence. "We are aggravating our own problem," Mugnier says. Indeed, the Corps is building more canals and enlarging pumping stations, because the lower the city sinks, the more it floods. In the meantime, streets, driveways and backyards cave in, and houses blow up when natural-gas lines rupture. Mugnier is also worried about the parishes (counties) bordering the city, which are digging drainage canals as they become more populated. In St. Charles Parish to the west, he says, "the surface could subside by as much as 14 feet."�