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A Discourse of Rivers

Going Round the Bend? Expect Cutbanks!

By Tamia Nelson
tamia@paddling.net In Harm's Way

August 26, 2014

In the short space of ten millennia, give or take a thousand years or so, the River of My Youth has carved many meanders into the hodgepodge of sediments left behind by the retreating Laurentide Ice Sheet. Homo sapiens had little incentive to settle down in this corner of Canoe Country until the ice melted, of course, though our immediate ancestors had already been making themselves at home elsewhere, in more hospitable climes, for at least 100,000 years. But a chilly mantle of ice insured that "modern" humans wouldn't set foot in northern New York till comparatively recent times. Now that we've taken up residence, however, we like to think we're in charge, and we certainly aren't alone in this. Hubris seems to be a universal human trait. Some scientists have even floated the notion of relabeling the current geologic epoch to reflect our absolute dominion over the world's lands and waters. Farewell Holocene, then. All hail the Anthropocene.

But our arrogance on this point is misplaced. While it's true that we have nearly unlimited power to foul our own nest, we really aren't calling the shots. A case in point: Every now and then the River of My Youth shows anyone with eyes to see that it's still the boss. Sometimes it floods, washing away homes and businesses, despite the best efforts of engineers and public works departments. At other times it just dumps a canoeist out of his boat and sweeps him into the deadly embrace of a partially submerged tree, a tree which was tumbled into the channel by a recent landslip at the outside of a bend. This is a small matter in the global scheme of things, to be sure, but it's a very big deal to the canoeist and his frantic companions, who must now struggle to free him before he drowns.

In fact, I played a small part in just such a life‑and‑death drama some years back. That performance had a happy ending, but it taught me an important lesson:

A River Is a Living Thing

And like most living things, it's full of surprises. Here's what I mean. A river's thalweg — sometimes called the river's "fast lane," the thalweg is the deepest part of the bed — snakes sinuously from one bank to the other as the river twists and turns. The surface (primary) flow in a bend is directed outward to some degree, and the surging water gnaws away at the fabric of the river's outer bank, eroding its substance and ultimately undercutting it. If the bank is made up of hard bedrock, this process is necessarily a slow one. But if the rock is soft, or if the bank is composed of poorly compacted sediment, erosion proceeds at speed, accelerating the pace of meander growth and movement. (Want to see meander formation in action? Then check out this short video from the Science Education Resource Center at Carleton College in Minnesota.)

In any case, whether the outside of a bend is sheathed in rock or soil, the end result of erosion is a cutbank. These often have steep, undercut slopes, exhibiting pronounced overhangs. Cutbanks can be only shoulder‑high — or they can tower far above your head. The thalweg runs close to the cutbank, too, so the water on the outside of a bend is much deeper than it is on the inside. This sketch may help you visualize what I've described:

Anatomy of Meander

Of course, my sketch simplifies the untidy reality of a living river. It will do for our purposes, however. In the drawing, the river flows from top to bottom, and the thalweg is shown by the sinuous blue line. Where the river runs straight, the deepest channel is more or less equidistant from the banks, but as soon as the river enters a bend, the thalweg swings toward the outer bank. You can see this in the red‑lined cross sections above, while blue arrows indicate the likely locations for cutbank formation. The stippled areas identify point bars, the sandy or gravelly shallows built up from sediment swept inward by the subsurface ("secondary") flow at every bend.

So much for the sketch. Let's look at some real‑life cutbanks, beginning with one carved by a rill that rushes down a steep, wooded slope:

Small World

As you can tell from the photo, this cutbank was created by an intermittent stream — the flow, although quite lively during the spring runoff, had subsided to a trickle when I took the picture — but thanks to the phenomenon of self‑similarity over scale change, it exhibits all the salient characteristics of cutbanks on larger rivers. Both undercutting and overhang are readily seen, and there's clear evidence of slope failure, otherwise known as slumping.

Now for an example on a somewhat larger scale. It's obvious that rock walls are slower to yield to the force of moving water than unconsolidated sediments, but the water always wins in the end. Look at the photo below:

Big River

The rock is limestone, its layers somewhat weakened by percolating groundwater. Time and the river did the rest. In fact, all rivers, everywhere, work tirelessly to remodel the landscape through which they flow, something that this paddler, negotiating a river constrained between banks of gneiss — an obdurate metamorphic rock — is about to discover:

Cruising for a Bruising

Obdurate the rock may be, but the surging water has still had its way, undercutting the bank and creating a shallow overhang. And the river was flexing its muscles on the day I snapped this shot. A fraction of a second after I pressed the shutter, the kayaker was slammed against the rock wall. He was lucky to escape uninjured.
 

He was lucky. You or I might not be. You can't depend on luck. Still, as a great scientist once observed, "fortune favors the prepared mind." And since we'd all like luck to be on our side, let's see what we can infer about …

Cutbanks and Their Dangers

We'll start with a summary. Cutbanks form on the outside (concave surface) of bends, where you'll also find:

  • Fast, often turbulent, currents

  • Overhangs

  • Unstable slopes

  • Slumps

  • Piles of rock rubble (these would be called talus slopes in the mountains)

  • Sweepers and snags

Those elements combine to create a uniquely hazardous environment. Consider the following:

  • The fast, turbulent water makes it hard to control your boat, increasing the likelihood of a capsize.

  • Overhangs are unstable, and if they collapse …

  • The resulting slump can bury anyone unlucky enough to be standing (or floating) below. It's not good news for any sightseers up top, either.

  • Undercut ledges can trap a boat (or a swimmer), as can …

  • The sweepers and snags that are created when trees, uprooted by a slump, topple into the water.

  • Boaters who get in over their heads on the outside of a bend, yet who somehow avoid being captured by a strainer, can nonetheless be swept over ledges or into rapids downstream. That's enough to dampen any paddler's spirits.

Despite this long list of potential hazards associated with cutbanks, however, mercifully few paddlers get trapped by undercut ledges, buried by slumps, or entangled in sweepers. Even feckless boaters have the odds on their side. But Pasteur was right: fortune does favor the prepared mind. In other words, a little caution — the sort of thing Edmund Burke had in mind when he wrote of "early and provident fear" — yields a big dividend in safety, making a small risk smaller still. I think it's worth the effort, don't you?

 

Staying Out of Trouble

 

"What's around the next bend?" It's a familiar question on river trips. And the answer is often, "A cutbank." But while cutbanks make for spectacular scenery, they can also create hazardous conditions for unlucky paddlers, both on and off the water. Still, if you've read this far, you'll be ready to meet these dangers head on, no matter what the next bend holds in store.

 


 

Related Articles From In the Same Boat

 

Plus a fascinating collection of simulations from the
Science Education Resource Center at Carleton College in Minnesota:

 

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