A Discourse of Rivers
Before You Take the Plunge —
The Anatomy of a Plunge Pool
By Tamia Nelson
April 23, 2013
Geologists vie with paddlers in their love of waterfalls. Back in my student days, I spent hours studying cross sections of Horseshoe Falls — also known as the Canadian Falls, it's the largest of the three falls that make up the Niagara Falls complex — tracing the successive layers of dolostone (dolomitic limestone), shale, and sandstone. These strata vary in their ability to withstand the constant assault of rushing water, with the result that the resistant dolomite capstone juts out over the deeply eroded, softer shales below.
The riverbed, too, is shale, and a deep pool has been scoured below the falls by the turbulent current. Such pools can be found below all falls — though only a few falls are as spectacular as Niagara — and their name reflects their common origin:
Given time, falling water and its accompanying burden of sediment can scour away the hardest rock. The resulting pool — often deep and always turbulent — is a plunge pool. You can see one in this simplified cross‑sectional sketch of Horseshoe Falls:
And while the falls along your favorite rivers probably don't measure up to Horseshoe, you'll find that their cross sections bear a striking resemblance to the picture above. (This is yet another example of "self‑similarity over changes in scale.") Not every falls cuts down through such tidily arrayed horizontal strata, to be sure, but each one will have a plunge pool at its feet.
The architecture of a typical pool bears closer study. While plunge pools vary greatly in their depth and extent relative to the parent falls, most will contain larger chunks of displaced rock in the upstream portion. (In the case of the American Falls — another part of the Niagara Falls complex — huge slabs of parent rock almost fill the plunge pool.) A little farther downstream, a fan of smaller rocks and other stony detritus carpets the riverbed, while the lightest sediments are swept away. This distribution follows from the simple fact that rivers drop their heaviest loads first. In that respect they're not unlike canoeists at the end of a long portage.
Of course, it's not easy to get a close look at the bottom of most plunge pools. But paddlers seeking a better understanding of the mechanisms behind their creation can learn a lot by studying river potholes, since many of the same forces are involved. I touched on potholes and their formation in an earlier article:[A]s bedrock is roughened by the impact of transported sediments (a process known as corrasion), the resulting turbulent flow generates subaqueous vortices called kolks. These spin round like small tornadoes, plucking material from the riverbed. And that's not all. The "white horses" that make life lively for canoeists and kayakers are also reproduced on a smaller scale beneath the surface. When stream velocities are high, river‑bottom turbulence generates bubble trains that subsequently burst with explosive force. Over time, these tiny explosions will pockmark even the hardest rock, in the same way that cavitation pits the surface of high‑speed propellers.So it's not a simple matter of waterborne sediment grinding away at riverbed rock. The process of erosion in more complicated than that. But the end result, while seldom as spectacular as Niagara, has important implications for unwary boaters, who may find themselves spending more time than they'd like in the resulting "keeper," trapped by the implacable reversal created when plunging water strikes the bottom of the pool and rebounds.
Cross Section of a Reversal
This combination of powerful upstream flow, turbulence, and riverbed debris can make any sojourn in a keeper very interesting, indeed, though what some boaters see as a trap to be avoided, others will regard as a challenging playground to be enjoyed.
Of rather more … well … academic interest is what waterfalls, large and small, teach us about the mutability of rivers. As the Greek philosopher Heraclitus reputedly observed, you can never step in the same river twice. He was right, too. Moving water is constantly reordering the landscape through which it flows, in ways both large and small — something that thousands of waterfront property owners discover every spring. Nor are waterfalls exempt. The same forces that excavate the pool in the rocky riverbed are also at work on a falls' cap rock and face, eroding existing cracks and opening new ones. And sooner or later, the weakened rock inevitably gives way, adding to the debris in the plunge pool at the same time that the lip of the falls takes a step back upriver. This process of erosion and retreat is called sapping. Here's a schematic snapshot:
What did I tell you? It's as easy as ABC, at least in my somewhat idealized sketch. But the result is readily apparent in the real world, too. Unless you're Methuselah's child, though, you'll need to read the book of the rocks to see it. A for‑instance: Niagara Falls has retreated upstream some seven miles in the last 12,000 years. No doubt local chambers of commerce are already making contingency plans for the millennia to come.
Now let's examine some more accessible real‑world examples, beginning with this five‑foot drop on The River, one of a succession of steps in a long falls:
Look closely and you'll see the aerated jet rising behind the curtain of the falls. It won't be so pronounced in all drops — this drop is already undercut — but it's tangible evidence of the forces constantly working to pit and scour the falls' rocky face, and to deepen and extend the plunge pool.
The next photo looks down on a small falls. The drop in this instance is only a foot or so, but it exhibits a complexity that belies its diminutive size. Note the plunging water of the falls proper, the upwelling and outwelling currents, and the multiple examples of turbulent flow in the plunge pool itself. You can also make out an intermediate basin immediately below the falls' semi‑circular lip, the result of bedrock fracturing and water erosion.
Now here is a photo of another small falls, but this time we're looking upstream:
The amber staining of the water below the drop is largely the work of tannins, but the water carries a significant burden of fine sediment, too — The River's chosen cutting tool. When Longfellow wrote that "the mills of God grind slowly, yet they grind exceeding small" he wasn't thinking about plunge pools or potholes. But he might have been.
And here's the result:
Needless to say, I'm exploiting our old friend "self‑similarity over changes of scale" in this example. The plunge pool in the photo was created by runoff from torrential rains, spilling across the shoulder of a poorly maintained road. But it wasn't raining when I snapped the shot, so the pool was high and dry (and therefore open to inspection). The blue arrows show the path taken by past runoff, and the blades of grass provide scale. As water has repeatedly cascaded across the road, the cracked asphalt surface has begun to crumble away, exposing an underlying stratum of older asphalt and cold patch above a base of coarse sand. And when I chanced upon it, this was what I saw:
It's certainly not Horseshoe Falls, but there's no denying the family resemblance — and I didn't have to wait 12,000 years to watch the story unfold. The "cap rock" in this case is asphalt, comprising (in the words of M. G. Lay, whose Ways of the World is a fascinating history of roads and road‑building) "pieces of stone, sometimes called 'aggregate,' bound together by a bituminous adhesive." It's labeled A in both the photo and the sketch. (All other identifying tags correspond, as well.) Layer B is older asphalt, now discolored by dried silt. Both asphalt layers exhibit undercutting, though it's more pronounced in layer B. The plunge pool (C) has been gouged out of the underlying sand. Large pieces of aggregate populate the upstream end (1), while a fan of coarse sand mixed with smaller fragments spreads out below (2), with a berm of finer sand marking the downstream margin of the pool (3). Further "downriver," a resistant stratum of asphalt (B again) diverts the runoff's flow.
OK. So far, so good. But you may be wondering …
Why Paddlers Should Care
It's a fair question. Of course, boaters from novice to expert benefit from learning the many ways of moving water: how a river's currents sculpt its bed, and how, in turn, the bed channels and choreographs a river's flow. That much is obvious. But there's a more important lesson to be learned here, one bearing directly on the safety and well‑being of enthusiasts who see waterfalls not as obstacles to be circumvented, but as opportunities for exciting river play. And that lesson concerns what lies in wait beneath the surface of each plunge pool.
Let me explain. Every year I watch eager creek boaters getting air time going over some of The River's spectacular falls. To my knowledge, only one unlucky paddler has died in the process, though I suspect that many others have suffered several anxious moments before they broke free from the current's grip and returned to the open air. And while I can understand the excitement of running waterfalls — I've run more than a few in my time, I admit — I can't help but wonder if the eager boaters would be quite so gung‑ho were they aware of what awaits them below The River's taller drops. As we've seen, plunge pools are a river's holding tanks. All manner of debris accumulates in them from one season to the next. There are rocks, ranging in size from cantaloupes to cars. That goes without saying. But pools can also conceal waterlogged tree trunks, some still bearing their branches. Tangles of 20‑pound‑test monofilament aren't unknown, either, many with the treble‑hook lures still firmly attached. I've even seen discarded shopping carts. And then there's that bane of paddlers everywhere: barbed wire, looped in untidy coils around tumbled heaps of submerged fence posts, stripped from the banks of some tributary creek by the spring floods, only to come to rest, sodden and comparatively immobile, deep in a plunge pool. A deadlier trap than this would be hard to imagine.
But I don't have to imagine it. I frequently explore The River at low water, when the bottoms of many of the pools are exposed to view, and I've often spotted barbed wire and subaqueous thickets of tree limbs, not to mention enough fishing lures and monofilament to outfit any angler for years. Cautious creek boaters will, I imagine, do just as I do, visiting their favorite rivers in the off‑season to conduct low‑water tours of inspection. Indeed, I'd recommend the practice to every canoeist and kayaker who paddles moving water. Who knows? The day may come when something — hubris, high spirits, or simply the love of meeting a waterfall's challenge head on, rather than skulking ignominiously around it on the portage trail — will impel you to launch yourself over a high drop. I've hinted at one possible consequence in the sketch at the head of this column. Then again, it's just a cartoon. It's not real life. Or is it? There's only one way to be sure: Look before you take the plunge.
Waterfalls fascinate everyone, it seems. And paddlers are particularly vulnerable to the siren song of falling water. Which is why it pays to give some thought to the forces that shape the deep pools beneath our favorite falls — and to what surprises we might encounter in those pools. Not every surprise is a happy one, after all. That's worth considering the next time you're tempted to take the plunge, isn't it? Sure it is!
Related Articles From In the Same Boat
- Air, Earth, and Water — Natural History for Paddlers, a topical collection of columns, including …
- "A Discourse of Rivers: It's a Matter of Scale," and …
- "A Discourse of Rivers: Current Affairs." Plus …
- "Keeper by Name, Keeper by Nature: The Whys and Hows of Avoiding Reversals."
And some articles from my own website, as well:
Copyright © 2013 by Verloren Hoop Productions. All rights reserved.