On Thin Ice

Stepping Out —
Keeping the Odds on Your Side

By Farwell Forrest
farwell@paddling.net

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 count wind-driven 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 as well.

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 end.

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 reserved.