Feb 17 2009

Of Time and The River: A Study in Potholes

Potholes are one sign of spring that no cyclist welcomes. They loom large around this time of year, too, as early thaws reveal the true extent of winter’s damage to streets and highways. But not all potholes require immediate evasive action. In fact, some are part of the scenery—though paddlers are more likely to see them than cyclists. I’m thinking of the potholes scoured in river rock, of course. These hold a special place in my memory, since several of my earliest independent forays took me to the cliffs along the river on which my grandfather’s camp stood. And those cliffs were studded with potholes. I can still remember my puzzlement at finding water and pebbles—not to mention the occasional trout—in sculpted hollows many feet above the river’s surface. The mystery was only resolved when I realized that summer storms had raised the river to flood stage only days before. It was a eureka moment of sorts, and it helped awaken my interest in the science of geology.

Anyway, many years later, my childhood memories gave rise to an article for Paddling.net (“Between a Rock and a Hard Place: The Ins and Outs of Potholes“). That article, in turn, elicited a pleasant note from a Texas teacher who’d used it when introducing the subject to her sixth grade class. As delighted as I was that my scribblings had found such happy employment, however, I wasn’t blind to the article’s shortcomings, the most glaring of which was the lack of good illustrative photos. The explanation for this lapse is simple: I wrote the column before I’d acquired a digital camera.

Now I’m going to put matters right. And I hope it won’t come too late for the students at Galena Park Independent School District in Houston. The photos below illustrate three stages in the process of pothole formation, beginning with these nascent potholes, little more than shallow depressions in The River’s bedrock:

Potholes in the Making

Nascent potholes they may be, but they’re already filling with rock fragments, and those fragments will become millstones when The River next rises, working in concert with swirling sediments and the explosive force of tiny bursting air bubbles to enlarge and deepen the shallow depressions. And how big are they now? The pine needles give the scale. These embryo potholes are no larger than saucers.

They won’t be this size forever, though. Let’s sneak a peek at what the future holds in store for them:

More Potholes in the Making

This pothole is about as big as a punchbowl. The attendant millstones are larger, too. And if you look closely, you’ll also see several satellite potholes below the first, each about the size of a baseball. Some are dry; others are filled brimful with water. (Often you can find tiny frogs in these small potholes, each one lord of all he or she surveys.) Note the polished surface of the rock. It’s further evidence of the power of moving water.

So far, so good. But we haven’t finished our tour. Take a look at these beauties:

BIG Potholes

Here we have mature potholes in all their glory. The larger ones could contain a washing machine with room to spare. (The hemlocks and birches along The River’s bank hint at the scale, as do the hikers who are just visible above the falls.) Do you see the slight concavities on the cliff face? Once flowing water has done its work over many tens of thousands of years, these too will grow to rival their companions. The story of time and The River is a tale without end.

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