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Our Readers Write

Thinking Like a River

By Farwell Forrest
farwell@paddling.net

January 29, 2002

The snow's gone from the slope outside my window, but the 'Flow's still locked in ice. Spring's on its way to the northern hemisphere, though. And if spring's on the way, can whitewater season be far behind? Certainly not!

I'm not the only paddler with water on the brain. Ric, who's been reading this column just about as long as we've been writing it, wrote to me recently, summing up an extended correspondence we've been having about the dynamics of moving water. In effect, he challenged me to join him in thinking like a river, and since the rivers will be running soon, I thought I'd try to rise to the challenge and adapt our exchange for this month's "Our Readers Write." In the interest of clarity, and in order to present what follows in question-and-answer format, I've played fast and loose with both Ric's original words and my replies, but I think I've kept the sense. I hope so, at any rate. Tamia's also been good enough to work up some illustrations. They should shed light on anything that words can't. OK. Here goes, but first…

A WARNING to the reader: Even when spring comes early and winter snowpacks are light, rivers run high, fast, and very cold at the start of the season. Spring torrents are no place for novice paddlers to learn the elements of river craft. Even experts should paddle in company, and rivers in flood are best avoided by everyone, expert and beginner alike. Floodwater can make even the most placid stream a killer. 'Nuff said, I hope.

Now, let's join Ric and see if we can learn to think like a river:

If I see a big wave rising up in mid-stream, am I right in thinking there's a rock hiding somewhere underneath it?

Most likely you are, Ric. You've just described a "pillow": a wave formed when flowing water strikes the upstream face of a rock or other underwater obstruction. These waves surge and subside as the flow of the river swells and diminishes, but they don't move downriver. They are, therefore, "standing" waves. The river rushes along until it meets an obstacle. Then it piles up and spills over, or around, the obstruction. In general, river waves stay put as the water flows through them. Ocean (and lake) waves are just the opposite: the waves are pushed along by the wind, but the water stays put. (It does, however, circulate within the waves, and ocean water can really move out in a tidal current!)

Pillows don't often stand alone. A companion wave also forms downriver of most rocks, at the downstream margin of the eddy. Though these are also standing waves, they're sometimes called "lifts" or "reaction waves" in order to distinguish them from pillows. They're analogous to the stern wave created when a boat moves through the water, in fact. (In the case of a boat, of course, it's the "obstacle" that's moving.)

A Pillow Stuffed with Rock

If I see a pillow–reaction wave pair, does this mean that there's a souse hole between the waves?

Possibly, Ric—though this isn't always true. When fast water rushes over a barely-submerged rock, the downstream eddy often takes the form of a "hole" in the river—the water level drops noticeably. When the drop is dramatic, water flows into the hole from all sides. The result is a "souse hole", and the reflux flow at the downstream margin of the eddy will take the form of a "stopper"—or, even more evocatively, a "keeper"—a large, backward-curling standing wave that breaks UPSTREAM. This combination of hole and stopper can trap an unlucky swimmer for several anxious seconds (or even longer) before he's flushed out. Skilled boaters, of course, see holes as opportunities to play the river—they often surf the upstream faces of the stoppers. This is a trick that's best practiced in company, of course, and both boater and boat should be prepared for a wild ride.

Hole in the River

I've seen washing-machine-like turbulence in rivers below low-head dams. If you dumped in the wash below such a dam, it looks like you could go round and round just about forever. Am I right?

You sure are, Ric! ALL low-head dams and weirs are dangerous, but some are even more dangerous than others. The risk to life is greatest when the lip of the dam is smooth and the volume of water flowing over the drop is high—as it probably will be in springtime. (The drop needn't be very high, though. Six-inch-high weirs have drowned people.) A dangerous dam will have a distinctive "reversal"—a sort of trough in the river—extending four or more feet downriver from the lip, with a big stopper just below that. As the name suggests, the surface flow in a reversal is UPSTREAM, and the trough is often full of debris. Such reversals are, in effect, river-wide souse holes without exits, and they can be deadly.

All dams should be scouted before running, of course. If the reversal extends more than a few feet downstream, or if you see debarked tree limbs in it, tumbling round endlessly and stripped of all their branches, DON'T EVEN THINK ABOUT RUNNING THE DAM!

On the other hand, floodwaters can submerge a dam so deep that you don't even know it's there. But you'll have a lot of other things to watch out for in floods!

Not by a Dam Site!

OK. Let's go back to mid-river rocks for a minute. There'll always be an eddy below a rock, won't there?

Right, Ric. There's an "island" of relatively quiet water, or eddy, below every obstruction, large or small. In gentle rivers, an eddy may be nothing more than an all but invisible "slick," with a barely-perceptible circulation. In steep, powerful rivers, however, the water in eddies often flows forcefully upstream. These eddies aren't "quiet" at all. You can even find secondary eddies upriver of rocks located within them.

Eddy-lines are sometimes hard to spot in slow-moving rivers, but in big water, eddies often become holes, and eddy-lines are marked by dramatic "steps" in the river. You can't avoid noticing them then!

Big Eddy

When scouting a route through a rapid, you should always look for the Vs, shouldn't you?

Yes—but be sure to pick the right kind of V! An upstream-pointing V marks the location of a rock or other obstruction. The obstruction is just below the point of the V. Sometimes there'll be a pillow. Sometimes there won't. In either case, you're better off avoiding it.

A downstream-pointing V, on the other hand, indicates a "chute," or passage. Even when you find such a chute, however, you shouldn't assume that you can ride it out to the end. Often there are rocks hiding in the turbulence. Look for them! And you're likely to find a big standing wave at the bottom (near the point of the V), too. It forms when the jet of fast-moving water in the chute slams into slower water just downstream. This wave is usually just a gentle roller. In high water, though, these rollers can become towering curlers, breaking upstream. That's when a chute can be a passage into danger.

Safe Passage?

Is the current always strongest in mid-channel? Do rivers flow fastest in mid-stream?

They do, Ric, but only if you're on a straight reach and if the river isn't too wide. In big rivers, the main current doesn't follow a straight line, and the thalweg (the river's "fast lane") meanders from one side of the river to the other— even when the river itself runs straight.

OK. What about places where the river doesn't run straight, then: the water in a bend is always faster on the outside, isn't it?

True. That's why cut-banks are usually found on the outside of bends, while sandbars form on the inside, where the slower water drops some of its "burden" of sediment. The outside of a river bend also collects trees which have been uprooted by the collapse of undercut banks. These "sweepers" and "strainers" can be deadly, especially in high water. That's why prudent paddlers stay close to the inside of bends in spring.

Round the Bend

Let's get out of the fast lane, shall we? In places where the current slows down and the water isn't too deep— less than three feet, say—it seems like I have to work harder to go anywhere. Is this some kind of drag?

It is. It's called shallow-water drag, and you'll notice it whenever you've got less than three feet of water under your keel. The magnitude of the drag depends on your speed: the harder you paddle, the more noticeable the drag. (The drag gets worse as the water gets shallower, too. It really mounts up at depths of one foot or less.)

I keep a notebook to write down the things I learn about rivers and water. Do you do this?

Sure do! Both Tamia and I keep paddling notebooks, and we have for years. We've also found that it helps to watch how water behaves in tiny run-off streams and rivulets, even if they're no wider than a few inches. Eddies, holes, pillows, stoppers, bends, bars—you'll find them all (and more) if you look closely. Drop a leaf or a twig in a tiny stream and watch it run the "rapids." You'll be surprised at how much you can learn about the dynamics of moving water.

All Write!

That's it. An exercise in thinking like a river, courtesy of an observant paddler and long-time reader. (Thanks, Ric!) Look for "Our Readers Write" again at the end of April. Next week, however, we'll be rejoining Ed and Brenna on their Trip of a Lifetime.

Have a safe spring. Keep reading, keep writing, and keep telling us what's on your mind!

Copyright 2002 by Verloren Hoop Productions. All rights reserved.










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