- In Every Issue
By John Lienhard
Here’s a sight we’ve all seen on flights out of Houston. This is one of hundreds of meandering bayous. So, how does meandering work?
Water would simply flow straight ahead in a perfectly formed channel. But real riverbeds are never perfect. The mechanics of fluid flow dictate that any bend creates a downward flow on the outer side of the bend. This causes the water to move downstream with just enough helical motion to scour the outside of the bend. That slow swirl carries silt across the bottom — then dumps some of it on the inner side of the bend.
That’s how the bend grows, and grows, until it forms a circle. It finally closes in on itself and the flow is once again straight. Then the process repeats. We look again at the photo: Sure enough, we see so-called “ox bow lakes” that were nipped off and left behind. They litter the area along the bayou.
We can ask two kinds of question about anything that happens. We can ask “How?” and get a mechanistic answer. Or we can ask “Why?” and get what we call a teleological answer. Ask, “Why does Nia study math?” The answer, “Because it was assigned,” tells us only the mechanics of getting her to work on this task. It takes a teleological answer to tell its purpose — to tell us "Why." “Nia studies math so she can become a rocket engineer.”
We engineers never want to give a teleological explanation for a natural phenomenon. We need to understand the mechanics of how things work. Yet look at what happens during meandering. That helical motion keeps eroded silt moving forward. Otherwise, it would simply accumulate on the bottom. It would eventually clog the channel. The helical motion keeps sweeping excess silt along until it builds deltas out on our coastlines.
A teleological explanation of meandering might say that its “purpose” is to keep silt from blocking the flow. We engineers would never say that meandering occurs for that purpose. We would say only that it has that effect. And yet ... and yet ... nature has so many mechanisms for regulating itself. We are constantly tempted to see nature as acting purposefully.
Let’s take this a step further. Real estate becomes more and more precious as Earth’s population approaches a staggering eight billion people. We can’t let our waterways wander here and there. So we keep sheathing them in concrete. Now they can neither scour nor meander. But that means trouble.
Meandering inevitably leads rivers to lurch into new watercourses. The lower Mississippi had done just that every 800 years until the 19th century. Then we began building levees to hold the river in place. The Mississippi was due to leave its present course, and burst into the Atchafalaya Basin, a century and a half ago.
Instead, New Orleans now has a tiger by the tail — a river that is increasingly restive in its chains. We know that we should never speak of an inanimate object “wanting” to do this or that. But the mighty and restive Mississippi really does seem to yearn for that next large step in its centuries of meandering.