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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.
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.
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.