On Thin Ice
Keeping the Odds on Your Side
By Farwell Forrest
March 8, 2005
Ice. It's like I said last time.
Unless you never paddle anywhere but the
tropics, chances are that you'll find yourself on thin ice someday. What
happens then? Well, if you've made an unaak
and practiced with it, you'll at least know whether it's safe to step out.
But be warned: My idea of "safe" may not be the same as yours. Safety is
always relative. Safe compared to what, exactly? I'll speak plainly. You
risk a cold
swim each time you venture out on ice. In other words, you bet your life
with every step you take. If this worries you and it probably should,
at least a little bit you'll want to place your bets where the odds
are in your favor. How? It's easier than you might think.
First things first. River ice and lake ice are natural products. They're
not extruded from a die, and they don't come with quality-assurance seals.
The ice can be safe where you're standing and still be dangerously thin only
six feet away. So when some local sage tells you not to worry, that he
tested the ice "over there" just an hour ago and it was plenty thick, don't
automatically assume you're good to go. Each step on ice carries you into
unknown territory. It helps to know why. Let's begin by taking a closer look
at some of the things that build ice up and make it stronger.
Cold temperatures, for one. (That's no surprise, is it?) Time, for
another. The longer a cold snap lasts and the chillier it gets, the thicker
the ice on rivers and lakes becomes, all other things being equal.
Wait a minute! "All other things being equal" what's that mean? Just
what it says. It's an important qualification, and it applies without
exception to anything and everything I'll say here. I won't repeat it with
each sentence, though. You'll have to supply it for yourself.
Does anything else make ice stronger? Yes. Pressure. But we're not
talking glaciers here, are we? We're talking everyday ice, the sort of ice
that early-season paddlers can meet up with on almost any lake or stream. If
we neglect pressure, the recipe for strong ice is really pretty simple.
Water plus subfreezing temps plus time equals ice. Lower temps and more time
equals thicker, stronger ice. That's true whether the water is fresh or
salt, though salt water freezes at a slightly lower temperature than fresh.
This recipe is only half the story, however. Whenever you're on thin ice,
you'll need a lot more than a thermometer and a calendar to know if it's
safe to step out. Here's where you have to start paying close attention to
that "all other things being equal" business. Water's funny stuff. All
liquids get heavier as they get colder. (Tamia would be happier if I wrote
"denser" rather than "heavier" here, of course, and she'd be right. Still,
"heavier" works for me.) Water's no exception to this rule, but only up to a
point. When you chill pure, fresh water down to 39.2 degrees Fahrenheit (4
degrees Celsius) it's already as heavy as it's ever going to get. In a pond
or lake, it sinks to the bottom. This goes on till the
entire body of water is the same temperature at every depth: 39.2
degrees Fahrenheit. Only then does the water at the surface cool further. It
doesn't get any heavier, though. It lightens up instead, and sooner or
later, it starts to freeze (getting even lighter in the process).
In other words, water freezes from the top down. And water's the only
liquid that does. Think about what this means. If water were like every
other known liquid, all but the deepest lakes would freeze solid every
winter, as would many of the world's seacoasts. This wouldn't do the fish
much good, obviously, but the damage wouldn't end there. It would also put
an end to
wait for it
life as we know it. In fact, just about
everything we see around us owes its existence to the quirky behavior of
cold water. We do, too.
No problem. As long as water doesn't change its ways and the sun doesn't
suffer a bout of indigestion and we don't poison the well we all drink from,
we'll survive. Let's get back on the ice. Since water freezes from the top
down, the surface needs to be in contact with cold air for ice to grow. But
suppose it snows just after the first skin of ice forms. If you've ever dug
a snow cave, you'll know just how efficient an insulator snow can be.
The obvious result? When, as often happens in winter, a blast of arctic air is
ushered in by a snowstorm, the ice under the newly fallen snow won't get
much thicker, at least not right away, no matter how cold it gets. This
isn't good news for anyone thinking about taking a walk. Most times when
I've broken through and it happened more often than I care to
remember during my youthful apprenticeship it was snow-covered
ice that did me in.
The moral of the story? Don't trust any ice under a blanket of
snow. Use your unaak to probe ahead before every step. Or else.
OK. What's next? You've probably wondered why I spoke only of ponds and
lakes earlier, and not of rivers. There was a reason for this. If you don't
waves and density-driven vertical currents (that is, what happens when
surface water is chilled and sinks to the bottom), the water in lakes and
ponds is still. There are no horizontal currents to disturb things. But
rivers are another scene altogether. Rivers are
water that's going places. And moving water doesn't hang around waiting
to be cooled down. The upshot? Rivers freeze, but they don't freeze as
quickly or as predictably as lakes and ponds. In places where the water is
moving relatively slowly along the bank
and on the insides of bends, say, and in large
eddies ice grows faster and thicker than it does over the
thalweg, or main channel. The practical implications are easy enough to see.
If you have to cross a frozen river, you need to be extra careful. This is a
good time to put your scouting
skills to use. A frozen-over river is still a river, after all.
Sometimes, though, a river can ambush you, turning up where you least
expect it, right in the
middle of a lake, or something that looks a lot like a lake. Most
reservoirs have thalwegs, for instance; they are, after all, just dammed
rivers. So do some lakes. Don't be fooled by the ice on top. The old river
is still alive down there somewhere. I was reminded of this when we lived on the
Flow. Early one spring morning Tamia and I were awakened by an eerie,
ululating song. It went on and on, with several competing themes spiraling
up and down the scale in some sort of crazy counterpoint. We followed our
ears outside, not knowing what we'd find in the half-light under the big
pines a ghostly 'shee riding the wind, maybe. Or a convocation
of coyotes with mischief in their hearts. But it wasn't a banshee's wail
we heard, and it wasn't a coyote chorus. It was the ice, thinned by the
warmth of the spring sun and the rasp of moving water and then set vibrating
by the snowmelt-fed surge of the buried river. The music that called us from
our bed was only the ice singing. Only? Like the Sistine Chapel is
only a church, right?
A magical moment it was, too. Yet recollect what the Sirens' song did to
heedless sailors. It lured them to their deaths. Anywhere there's moving
water, there's a good chance you'll find thin ice. On a mountain creek. Over
the buried channel of a drowned river. Wherever a stream empties into a lake
or pond. If time and cold temperatures help ice grow strong, snow stifles it
at birth and moving water grinds it down or at least stops it from
getting thicker. But are these the only enemies that ice has? No. Anything
which traps or holds the sun's heat can weaken ice. Cattails on the fringes
of a pond. Large rocks close to the surface of a frozen lake. The
oil-blackened track left by passing snowmobilers. Even a windfall pine
bough. The shallow margins of a pond or lake are normally the first places
to freeze in the fall. Because the sun can so easily warm the bottom in
these shallows, however, this ice is usually the quickest to rot in spring
Reservoirs and rivers also experience overflows. Whenever the
volume of water flowing through the main channel slackens, the ice sags.
Then, when the flow picks up again when the spring sun melts the
snowpack in the mountains, or when a cold snap increases the demand for
power and the engineers open the intake gates at the upstream turbines
the ice is buoyed up again, often higher that it was before. Ice is
like most people. Given enough time, it can adjust to change, but it
doesn't like sudden shocks. If it's heaved high up on the back of a surge,
it buckles and cracks, and some of the floodwater spills out onto the
surface. The same thing can happen when a heavy fall of snow weighs down the
ice that covers a frozen pond or lake. Cracks form and spread, and water
bubbles out. In either case, the resulting overflow loads and weakens the
underlying ice. (Water is
very heavy.) Pools then form on the surface, and ice that was
safe becomes honeycombed and rotten, a pulpy trap for the unwary
traveler. So if you see a yellow-brown stain spreading out in front of you on the
snow-covered surface of a northern lake or river, head in another direction,
probing carefully as you go.
There's another hazard associated with fluctuating water levels.
Sometimes the ice doesn't drop when the water does. This happens
fairly often on narrow, flood-prone creeks, in fact. Why does it matter?
Ice that isn't supported by
water isn't safe. It's doubly dangerous, in fact. With nothing but air under
it, ice can be fairly thick and still be weak weak enough to break
under your weight. But it's still thick. Your unaak may not punch through to
alert you to the danger. Your only warning then is the hollow ringing sound
the probe makes with each thrust, quite unlike the comforting Thud!
of safe ice. If you ever hear this warning bell toll while you're crossing a
creek, beware. It tolls for you. You may already be walking on air, and your
next step could send you plunging into the unknown. It's no fun. I once
broke through an ice bridge on a steep local stream. My pack caught me and
held me up, but when I looked down I saw no fewer than three more ice shelves
below me, each with a hole in it larger than the one whose
crumbling edges were now the only thing holding my pack. And at the very
bottom, some twelve feet beneath my dangling boots, a torrent of frigid
water rushed pell-mell into a dark tunnel. I couldn't see any light at the
Well, accidents happen, and it's important to know what to do when things
don't go your way. I was lucky. When I went through the ice over that little
stream I wasn't alone, and though I hadn't yet discovered the unaak, both I
and my companion had sturdy knives. That was enough. Next month, I'll have
more to say about self-rescue and about the art of getting a boat over the
ice. It's both easier and harder than stepping off unencumbered.
See you then.
Copyright © 2005 by Verloren Hoop Productions. All rights