A Discourse of Rivers
By Tamia Nelson
April 14, 2009
Hurrah! The sun is returning to Canoe Country, and many northern rivers are already running free. But that doesn't mean it's time to haul your boat to the nearest put-in. Ice still sheathes the margins of plenty of streams, while others are now in flood, choked with toppled trees, sodden lawn furniture, and (sometimes) the bloated bodies of unlucky livestock. Teams of hard-charging canoeists and kayakers won't be dissuaded by such piffling considerations, of course. They'll head out on the water as soon as a channel opens. But cautious paddlers will hold back, waiting for the last of the ice to melt and for their favorite stream to retreat within its banks. Even if your risk-taking days are long past, however, it's not that easy to stay on shore when the water is singing. Luckily, there's a sort of halfway house for impatient boaters. We may not want to challenge a river in flood or court hypothermia—let alone chance a desperate swim under an ice shelf—but we can always walk the riverbank, can't we? Watching moving water at work is unfailingly captivating. It's also a useful dress rehearsal for the Real Thing: scouting drops later in the season. So why not join me as I check out The River at my doorstep? It's a great way to catch up on…
I guess you'd have to call me a reformed hard charger. Or maybe I've just gotten timid. (You've heard the one about old pilots and bold pilots, haven't you? It's worth mulling over.) In any case, I no longer jump into my boat as soon as cracks appear in the ice. I take my own advice, instead—I go for a walk. And I'm seldom disappointed. Sometimes the adventure begins even before I catch sight of The River. On one of my first pre-season scouting trips this year, I found the precipitous town road leading to the trailhead awash in meltwater. As this impromptu stream coursed through the deep drifts of sand left behind by the winter road crews, it reproduced The River in miniature. The little rill was a textbook example of "self-similarity over scale changes," an intriguing phenomenon that I've written about before. And like any good textbook illustration, it taught me a thing or two. Look at this photo to see what I mean:
We're facing downhill. The water is running under our feet, toward the top of the shot. At first glance, you might think you're looking at an aerial photo of a braided stream complex, perhaps one of the many that dissect the outwash plains of Alaskan glaciers. But you're not. The photo spans an eight-foot lane on a town road. Now look closer. Not all the flowing water twists and turns in glorious confusion. In fact, the yellow arrows bracket the tail end of a wide channel that offers a classic example of laminar flow. (It's actually a tire track.) The current lines here are parallel and distinct. They don't cross. They are, as it were, well behaved. Drop a feather onto one current line and, barring an untimely gust of wind, it will flow straight and true. Or at least it will till the channel sweeps to the right, when microeddies—not visible in the photo—complicate the picture. A drifting boat on a similar stretch of a real river would behave the same way, as many paddlers have discovered while navigating long "straights." That's a Very Good Thing to know—and to recognize.
But nothing stays the same for very long on moving water. A little further downstream on my river-in-miniature, I came across a striking counterexample:
Here the flow is from left to right, and this time my boot provides the scale. A sandbar has formed in the middle of a wide channel. Standing waves mark its location. Moreover, the bar breaks up the orderly, linear pattern of the streamlines. While some water flows over the obstacle, the rest is diverted to either side. The streamlines divide, jostle along in the two narrow channels, and then join up again below the bar. The result is a confusion of cross- and countercurrents, the hallmark of turbulent flow. Scale this up, and you've just encountered another common phenomenon on many rivers. It's not a place for lazy drifting!
But there's more to be learned here. Once again, look closer. The two channels, right (that's "river right," of course—the side on your right when you face downstream) and left, are not equivalent. The right channel seems pretty tranquil, if rather shallow, whereas the left is deeper and more lively. On a river, you'll often find that the "quieter" channel around an island is bony, almost too shallow to float your boat. Sometimes—and this happened to Farwell only last year—it peters out altogether, leaving you high and dry. He wished he'd opted for the gutsier ride around the other side of the island, instead. And what about the "overland" route, the route that takes you right across the submerged bar? Well, it could be an exhilarating roller-coaster ride. Or it could leave you stranded, with no choice but to wade your boat out to deeper water. The moral of the story? It's best to avoid all such surprises. That, in a nutshell, is the case for scouting.
Enough of snowmelt on roads. Let's go and see…
What's Happening on The River
As I scrambled along the bank, picking my way carefully over slick rocks, I soon found life-sized counterparts to the phenomena I'd already seen in the miniature river on my doorstep. Here, for example, is laminar flow, though this time it's not in a place you'd want to drift into (or over):
The River drops about four feet at this location, and the water's running high and fast. But the flow over the ledge is still laminar. To borrow a couple of terms from the lexicon of close-order drill, the streamlines maintain their dress and interval. You can actually see the parallel lines of current.
The photo above shows a natural drop, but dams often provide even better illustrations of laminar flow. To reiterate: While you can drift and dream down some straights in big rivers, laminar flow is not synonymous with "safe water." It goes without saying that no sane paddler drifts idly over a dam, right? Right. And by way of driving that point home, here's a for-instance from a dam site:
Perfect dress and interval. Perfect laminar flow. On the face of the dam, anyway. But it won't last long, will it? When The River once again runs wild and encounters obstacles, or when anything—even a bend—interrupts the smooth progression of the streamlines, the result is inevitable: turbulence. If you look closely, you can even see it in the two photos above. The froth and spume at the foot of both ledge and dam tell you that the streamlines have broken formation. Want a more dramatic shot? How about this?
The ice-sheathed boulder in the background (it's just behind the left arrow) is about the size of an old-style VW Beetle, and the river is flowing fast and furiously over this staircase falls. It's a pretty chaotic scene, and examples of turbulent flow are everywhere. Well, maybe not quite everywhere. Even in the midst of this tumult, there are places where discipline and order reassert themselves. Two such spots are marked by the yellow arrows. But such oases of order are few and far between. This is no place for drifters!
It occurs to me that I've been oversimplifying things a bit. It's not either-or, laminar flow or turbulent flow, with no middle ground. Moving water can exhibit another face: converging flow. Look for it around tongues and chutes.
The River's running from right to left in the photo above. See the dark tongue of water in mid-shot? It's hard to miss; it marks a place where the river squeezes through a narrow cleft in a rock ledge. Streamlines converge here, and converging streamlines add up to converging flow. Now check out the next photo, which shows the chute from a different perspective. (The yellow arrows mark the same spot in both photos. I put them in to help folks stay oriented as I walked downriver. They have no other significance. Think of them as benchmarks.)
This second shot was taken from a point on shore downstream of the first. You can see how the streamlines squeeze together to negotiate the gap, then flare apart before descending into a chaos of whitewater. Finally, here's yet another view of the tongue, taken from even further downriver. An icy rock outcrop frames the scene, while the yellow arrow still marks the same spot as before.
Are you having trouble following the streamlines? Then click here to open a new window with an enlargement of the last shot. I've sketched in a few lines to guide the eye.
The River has many voices, and it speaks with many tongues. In scouting from shore, perspective is all. So before we pack up and head home, let's look at two more examples, beginning with a modest chute that exhibits a classic downstream-pointing V:
We're facing downriver. Boulders to the left and right squeeze the streamlines together—the ones on the right are out of the shot—forming a modest chute. If the discharge were higher, or the gap narrower, there'd probably be a pronounced reaction wave at the point of the V, just where the streamlines converge.
Of course, tongues don't always conform to this classic scenario. Take a look at this:
Yep, you've seen it before. But did you see the tongue? If not, the following annotated photo may help:
The streamlines converge in the tongue, only to plummet over the ledge in relatively good order (near-laminar flow) and then dissolve into chaos and confusion, churning round in one of those lethal maelstroms that paddlers like to label "reversals." Reversal, indeed! The jutting rock in the background is the size of a small car, and the reversal next to it packs a hefty punch. It could certainly trigger a reverse in any inattentive paddler's fortunes. Sometimes, notwithstanding Falstaff's inglorious showing on the field of battle, the better part of valor really is discretion. And that's why I recommend a variation on…
The Not-So-Ancient Sport of Poohsticks…
To any and all Canoe Country paddlers who are impatient for the ice to leave the rivers and the floodwaters to recede. Don't get me wrong. The works of A. A. Milne leave me cold, and my reaction to The House on Pooh Corner pretty much paralleled Dorothy Parker's. But canoeists and kayakers owe Pooh's creator a debt of gratitude, nonetheless. He might even save your life someday. Instead of committing body and boat to the uncertain mercies of a chilly spring torrent, toss a stick in your chosen stream, instead. Then follow its progress with your eyes, while you remain safely on the riverbank, watching attentively as your woody emissary goes with the flow. Better yet—and success here is what distinguishes Poohstick masters from mere mortals—try to predict its path as it sweeps down though the tumult. Short of getting your paddle wet, there's no better technique for learning the ways of moving water.
We can't all be heroes, right? And who says we have to be? The calendar may tell us that it's spring, but what does a calendar know about hypothermia? Maybe you're not ready to do battle with rivers still swollen by floods of snowmelt. That doesn't mean you can't do a little scouting. This is a great time of year to catch up on current affairs. So, what are you waiting for? Lace up your boots, head on over to a nearby river, and watch moving water stretch its limbs after the Long Sleep of winter. And one more thing: Be sure to take a Poohstick with you!
Copyright © 2009 by Verloren Hoop Productions. All rights reserved.