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Between a Rock and a Hard Place

The Ins and Outs of Potholes

By Tamia Nelson
tamia@paddling.net

March 13, 2007

All kids look forward to summer, and I was no exception. Summer was when I went to camp — not the sort of camp you see advertised in the back pages of the New York Times Magazine, however. Not someplace with dormitories and counselors and group sing-a-longs around the fire (and breathtaking fees). No, camp was my Grandad's cabin in the Adirondack foothills. It wasn't much to look at, perhaps, but it had everything I needed — beaver ponds, dense woods, swamps, and mountains. There was even a ghost town, reached by roads so overgrown that only a few locals remembered where they were. But best of all was the river. It ran through the heart of Grandad's stomping ground. Bounded by steep cliffs and knife-edged outcrops, the river poured from pool to pool, cascading over a series of waterfalls. It wasn't very big, as rivers go, but it had broad shoulders and the sinews to match. Upstream of the first waterfall, boulders the size of Volkswagen Beetles littered the wide, cobbled banks. Still further upriver, countless small, cold creeks emptied into the main channel. I waded the shallows of every one, surprising fat bullfrog tadpoles as they hunted caddis fly larvae, while I scattered the small trout that were dining on both.

As I grew older and bolder, though, I abandoned the creeks for the cliffs above the river, scrambling from ledge to ledge like a young mountain goat, doing battle against gravity with only my PF Flyers™ and fingernails, exploring as I went. That's when I discovered The Mystery. The cliffs were pockmarked with holes, each one carved into solid rock. Some were only a few inches deep, others went down farther than I could reach. Occasionally, two or more adjacent holes seemed to have grown together, forming a three-dimensional labyrinth with a cross section like the Venn diagrams in my math book. And every hole contained a hidden treasure. The smallest usually yielded no more than a single polished pebble or a spoonful of sand, ground almost as fine and soft as the face powder on my mother's dresser, but the largest often contained several softball-sized cobbles.

And that wasn't all. Some of the deeper holes held water, and in one of these natural basins I found a solitary trout, swimming round and round, searching frantically for an outlet that didn't exist. Moved by his apparent plight, I scooped him up from his prison cell, only to confront the problem of carrying a slippery fish when I needed both hands to cling to the cliff face. The trout may have sensed my hesitation. In any case, he acted. Wriggling out of my grasp with a single sinuous twist, he plunged straight down to the river below. Free at last!

So I was left alone to ponder how a trout got into a basin no bigger than a washtub, some 30 feet above the river. And as I wrestled with this conundrum, the mystery deepened. What had created the basin in the first place? What force could carve a hole in solid rock? And how did the hole come to be filled with water in the middle of a hot, dry summer? I decided to put these questions to my Grandad, whose authority on all matters concerning the natural world I'd never had reason to doubt. But this time I was doomed to disappointment. When I found him in his garden and asked him, his only reply was a string of curses, after which he had a question of his own for me: Why hadn't I brought the trout home for dinner?

I had no answer to this, either.

Several years passed, and I gave no more thought to The Mystery. Then I returned to Grandad's camp — but this time it was April, not July, and the river wasn't at all as I'd remembered it. It no longer gurgled. It roared. The falls were torrents, and the water in the pools was either ominously slick and fast or roiled by whirling eddies. And there was something else, too. The river was now as high as it was fast. The cliffs I'd scrambled over in summertime were lost beneath the heaving surface of the rushing water. I could even feel the ground tremble ever so slightly as Volkswagen-sized boulders clashed on the river bottom. In a flash, I understood. The Mystery was a mystery no more. Here was the secret of the hanging pools and the captive trout. Both were a legacy of the spring floods. Henry David Thoreau was right, I decided. "Some circumstantial evidence is very strong," he wrote in his journal, "as when you find a trout in the milk." Or in a basin carved in rock on a cliff face, I added silently.

 

Fast-forward now. It's September, years after I first saw my Grandad's river in flood. I'm taking a college geology class. And the subject of today's lecture is…

Potholes

It's a déjà vu moment for me. I scribble away furiously so as not to miss anything, and here's what I learn: Natural potholes — that's the proper name for holes like the one I found the trout in — are hollows carved into the bedrock exposed along river courses. They're smooth inside and generally cylindrical in shape, and most contain "milling materials," ranging in size from silt to large cobbles and, in rare instances, even small boulders. The potholes themselves also vary greatly in size. The smallest are smaller than a Sierra Club cup. Others are big enough to bathe an elephant. (Getting the elephant up the cliff face must be a job, I start to say, but the lecturer has already moved on. It's just as well. He's a rather humorless gent.) Large or small, however, potholes are a scale-independent phenomenon, meaning that the big ones look exactly like the little ones — except, of course, that the big ones are, well, bigger. And the biggest of all are the potholes found in the Channeled Scablands of Washington state. (That must have been a hell of a flood, I think, and I want to ask about it…. But the lecturer has once again moved on.)

My question will keep, I decide. Meanwhile, I make a sketch for my notes.

 

River Potholes

Pothole Cluster in Bedrock
(The largest is about 18 inches in diameter)

 

So much for my trip down memory lane. Geology, like every other science, has a private language. It serves a purpose, to be sure, but it's also one way of separating the in-crowd from the general public. There are other problems, too. Geologists are an independent bunch, and the same word often conveys several meanings, depending on the context and who's doing the talking. In Death Valley, for example, a pothole is a brine-filled hole lined with salt crystals. To a glaciologist, on the other hand, potholes are synonymous with moulins (pronounced moo-LANH, the second syllable snorted through your nose like the vin in vin rouge), naturally occurring cylindrical shafts in glacial ice. Notwithstanding these exceptions, however, when geologists speak of potholes, they're most likely referring to the holes carved in river rock. That's also the meaning that makes the most sense to paddlers. If you spend much time near moving water, you've seen 'em. And maybe you've been wondering about the same thing that had me scratching my head back when I found a trout in the rock:

What Makes a Pothole?

The list of ingredients is pretty short. Bedrock. Turbulent water. And the abrasive mixture that geologists call suspended sediment. Start with bedrock. Without it, potholes won't form. Why? Anything less rock-solid will simply be washed away. "Washed away"? That's the key to understanding Ingredient #2. Potholes are carved from rock, and moving water provides the energy. This leaves only the suspended sediment. It provides the cutting tools for the job. Three ingredients: rock, water, sediment. Three roles: medium, muscle, tool. Nature as sculptor, in other words. So far, so good.

Let's spend a little more time on water. (No hardship for a paddler, eh?) If you've ever capsized in a big rapid or heavy surf, you've felt the power of moving water. The faster water moves, the more power it has — and the more power it has, the more suspended sediment it can carry along with it. But fast-moving water doesn't flow quietly for long. As soon it meets an obstacle, it twists and turns, bending back on itself and forming eddies, souse holes, whirlpools, and boils. These macro-phenomena have micro counterparts: as 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.

This is all it takes. Each new irregularity generates more turbulence, and every increase in turbulence breeds new irregularities. The milling materials — sediments ranging in size from silt to cobbles — then do their work, tirelessly grinding away. Soon a cavity starts to form in what was once smooth rock. A pothole is born.

 

A Pothole is Born

Great Potholes From Little Fissures Grow
(A cross-sectional view, showing sand and pebbles in yellow)

 

Of course, what goes up must come down, right? Rivers in flood are no exception. And when the high water recedes, hidden potholes are exposed to view. This is especially true on steep, flashy mountain streams — like the river that flowed past Grandad's cabin, for instance. In fact, it's Rule Number One in…

The Pothole Hunter's Handbook

Don't waste time looking for potholes along braided rivers weaving their way uncertainly across broad glacial outwash plains. You're not likely to find any. Nor will you have any better luck prospecting for potholes on languid streams meandering through sodden water meadows. Go with the flow, instead! If you're paddling a steep whitewater stretch with lots of exposed bedrock, you're in the right place. Remember to look up, too. You'll often find potholes carved right into cliff faces. Don't limit your search to big rivers. It's energy that counts here, not size. You can sometimes find potholes large enough to float a creek boat alongside flashy mountain streams no bigger than a thin blue line on your quad. And whenever you get the chance to get up close to a pothole, take a look at what lies inside. The captive millstones can be astonishingly beautiful, with a color and polish that even a jeweler would envy. So be ready to take a photo. Or make a painting. But leave the stones in place for the next paddler to enjoy, please.

Sometimes you find a treasure that you can take home without a single qualm of conscience, however. I found a watch in one pothole, and a pair of sunglasses in another. The watch was waterlogged, but the sunglasses didn't have a single scratch. This was treasure indeed.

Plus, you never can tell when you might find a trout.

Ready for the big leagues? Then head out to the escarpment above the Niagara Gorge, where ancient floodwaters carved potholes that could swallow the largest SUV without a trace. And then there are the Channeled Scablands. I first heard about them in my physical geology class. It turns out they were formed when an ice dam holding back glacial Lake Missoula suddenly collapsed, sending the equivalent of Lakes Erie and Ontario on a mad dash to the Pacific at speeds approaching seventy miles an hour. Maybe you can guess what sort of potholes this created. I couldn't, at least at first. My imagination simply didn't stretch that far. But now that I've seen the results for myself, I have to admit they live up to their billing.

Since the long-ago time when I scrambled across the cliffs overlooking my Grandad's river, I've found potholes in many places, on many other rivers. In fact, there's a fine crop to be seen along The River, less than 30 minutes' walk from my door. Yet while I know a lot more than I once did about the forces generated when water cascades over rock, potholes still leave me feeling as if I'm standing on the threshold of a great mystery. I'm sure of one thing, though. If I ever find another trout swimming round and round between a rock and hard place, I'll set that one free, too. I only hope my Grandad will forgive me.

Copyright 2007 by Verloren Hoop Productions. All rights reserved.









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