There is a feeling of disquiet along the Mississippi River this morning. The water, usually the color of wet cardboard, glows an alien, electric blue. Clouds slide in from the Gulf of Mexico looking like they've been dragged through pools of used motor oil, and far on the southern horizon but headed this way are thunderheads bleeding black tendrils of rain, an army of marauding man-o-wars. I hunker down on my old maroon mountain bike and crank with a panicked vigor. It's September in southern Louisiana, hurricane season, and only a fool could stare into that storm-darkened sky without visions of Atlantis.

Well, I'm on high ground, at least—outside Baton Rouge, pedaling southeast along the gravel path that traces the spine of the Mississippi River levee, which snakes for 120 miles or so between here and New Orleans, a town I once called home. This part of the levee—a sloping, grassy ridge I'd guess is about 40 feet above the river—offers the only thing close to a vista in this terminally flat terrain, but this is not a trip I'd recommend to connoisseurs of pristine views. This leg of the river slinks through one of the country's greatest concentrations of oil refineries, toxic-waste incinerators, and fire-belching petrochemical plants. They line the river like dragons at the trough. The local atmosphere is allegedly so hospitable to rare tumors and exotic carcinomas that this route is known to locals as Cancer Alley, a moniker that does not appear on the brochures of bed-and-breakfasts in the region.

I'm taking two days to ride the levee not because I have a taste for toxic scenery but for weirder reasons that have to do with my particular love for New Orleans and my complicated feelings about this keloidal welt of earth, which for years has perverted the Mississippi and ostensibly guarded New Orleans while ensuring that, someday, it will all likely be rinsed into the Gulf.

When I lived in New Orleans, from the summer of 2002 to the spring of '04, I visited the levee daily to escape for an hour or two a city that, contrary to its best-known epithet, has never been a particularly easy place to live. I remember first meeting my backyard neighbor, who greeted me with a photograph of a gun-shot corpse she'd stumbled across nearby. "See how fresh he was?" she said, pointing out with pride the blood dripping from the dead man's mouth. (I lived in a "good" neighborhood, by the way.) My first hurricane season, I was the only person on my block anxiously boarding up windows, not having yet learned that heavy weather was properly confronted with a half-gallon jug of brown liquor and a cooler full of ice. I moved away in time to dodge Katrina, too much a fretful East Coast type to put down roots in soil perpetually sagging beneath my feet. It was not lost on me, however, that the sense of looming cataclysm—the feeling that each day might be your, and the city's, last—was part of what made New Orleans the most intoxicatingly vital, intriguing place I've ever been. I was then, and am now, in love with the city. But whenever the darker aspects got me down, I found the best panacea was pedaling along the levee path, a place I hold much dearer than Jackson Square or the oak cathedrals of St. Charles Avenue. On the levee, the sunsets were wild outrages enacted in rainbow-sherbet hues—an effect, perhaps, of the noxious effluent from the industryscape across the way. On the levee, riverborne breezes, smelling faintly of diesel, cut the city's airless fetor, which on summer days could make you feel as if you were drawing breath through a rotten sponge. On the levee, I could actually see the river, otherwise hidden from view, and be sure it was flowing at nonlethal levels. On the levee, most folks showed up in spandex, which was comforting: You could see nobody was packing heat.

After an hour or so on my bike, I'd go home restored in spirit. But returning now, a year after Katrina, with roughly 60 percent of New Orleans still in ruins, a longer voyage seemed required. So I set out from Baton Rouge full of desperate, nostalgic superstition—and the inarticulable hope that drives one to revisit a journey taken with a friend, now dead. On the path you once traversed together, you hope in some dim compartment of your heart that, somewhere along the route, she'll magically appear beside you, alive and well.