Wicked River_ The Mississippi When It Last Ran Wild - Lee Sandlin [19]
Meander loops tend to form in even equidistant patterns, all other things being equal, but at any given moment their pattern on a particular river usually appears to be wildly irregular. This is because the loops are continually being reshaped by a process of fluid dynamics called helicoidal flow. Helicoidal flow is a secondary type of turbulence that forms around the main current moving in a river channel. Two things happen because of helicoidal flow: the water along the outer curve of a meander loop speeds up and eats into the riverbank, and at the same time the water on the inner curve slows down and deposits the silt that it’s already carrying downriver. The result is that the outer bank is worn away while the inner bank is built up, and so the loop becomes inexorably larger and more pronounced within the same area of land. Sooner or later, the growing curves of adjoining loops touch, the current breaks through the banks, and a new connection is formed. On the Mississippi, these connections were known as cutoffs. When the main volume of the current flows through a cutoff, the silt being carried downriver begins to be deposited around it, building up new banks on either side, and the now-landlocked curve of the loop outside the new banks either dries up or else becomes a bayou or an oxbow lake.
On the Mississippi, where the land was flat, the current was vast and strong, and the helicoidal flow was perpetually at work, the meander loops and cutoffs were constantly unfolding into strange new contortions. In the lower valley, where the obstacles were the fewest, the Mississippi bent, doubled back on itself, executed hairpin turns, and twisted around to flow in new directions. A complete map of its meander belt, as the term is, would show that over the centuries the river had writhed around its current route like a nest of anacondas.
At ground level, this shifting tangle was experienced as an unending challenge. Whenever the river people gathered, all they’d ever talk about was how the river was changing. They’d rattle on in a whole specialized technical vocabulary of homegrown hydrology that described the river’s peculiar behavior: chutes and points, bends and reaches, false points and sycamore snags. They’d debate about how the river was doing that season, where it had shifted unexpectedly, whether it was rising or falling, what easy stretches were now suddenly dangerous, and which of its most celebrated dangers were wearing away and were now just child’s play for a real river man.
The talk was a way of bonding; it enabled total strangers to chatter on together like childhood friends. But it was also an immediate practical necessity. The waywardness of the Mississippi was a constant threat. Every day, somewhere along the river, huge bluffs were collapsing; overgrown banks were falling in on themselves; ancient stands of trees were sliding down into the tide. Sandbars were growing into islets. Islets were accumulating rocks, rotted logs, and mud and sprouting with countless scattered seeds; they were bristling with new trees and underbrush; they were melting away in the current again and turning back into sandbars. On every voyage, the familiar landmarks were disorientingly reshaped or abruptly erased, while new hazards had popped up out of nowhere.
This was another big reason why there were no trustworthy maps of the river. It changed too quickly. Every pilot had to have his own mental map, which was added to, corrected, erased, and redrawn in his head on every run; and no two pilots’ maps, if they could have been compared, would have been identical. The river remade itself every day. People who lived on or around the river learned to think of it as untrustworthy, violent, deceptive, and unknowable. While the voyageurs called it the wicked river, the plantation slaves called it Old Devil River, because of its habit of playing bizarre and malicious tricks. A man would go to bed on one side of the river and wake to find that it had changed course overnight and his property was now on the opposite bank. That was