Our Readers Write
Thinking Like a River
By Farwell Forrest
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.)
If I see a pillowreaction wave pair, does this mean that
there's a souse hole between the waves?
Possibly, Ricthough 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 riverthe 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 riverthey 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.
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 highas 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 riverextending 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!
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
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!
When scouting a route through a rapid, you should always look for
the Vs, shouldn't you?
Yesbut 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.
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
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.
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, sayit 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, barsyou'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.
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
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