Frozen lakes and rivers look solid, and that is exactly what makes them dangerous. Ice thickness varies across a single lake because of currents, springs, depth, snow cover and sun, and the only way to know what is under your feet is to measure it as you go. State agencies publish guidelines, and the Minnesota Department of Natural Resources recommends at least four inches of clear, new ice for walking, five to seven inches for a snowmobile or small ATV, eight to twelve inches for a car or small pickup and twelve to fifteen for a medium truck. Those numbers apply to solid, clear ice, and they are minimums. They do not mean it is safe everywhere on a lake. The signs below help you recognize when ice is weaker than it looks. Several of them come from state natural resources departments and extension offices, and the thicknesses cited are for new, clear ice. Always check conditions with local bait shops and authorities, carry safety gear and never go out alone. When in doubt, stay off. A day on the ice is never worth the risk of falling through, and most deaths involve people who thought the ice was fine.
The first and simplest sign is thickness. Four inches of clear ice is the common minimum for walking, and anything less is not safe for a person. Many people drill a test hole at the shore and conclude that the whole lake has the same thickness, which is a mistake. Use an ice auger, spud bar or chisel to check thickness every few steps, especially as you move away from shore, and measure at the thinnest point. The Minnesota DNR and other agencies stress that ice is not the same everywhere. Early in the season, freezing starts at the edges and moves inward, so the middle of a lake may be much thinner. Late in the season, ice can look thick but weaken from the bottom up. Measure only the clear ice, not the layers of slush or snow on top. A tape measure with a hook or a purpose-made ice gauge is a good tool. Remember that the numbers refer to new, clear ice. Old, cloudy or honeycombed ice requires more thickness, and conditions can change from one day to the next.
Clear ice is strong, and white ice is not. White or opaque ice, often called snow ice, forms when slush freezes on top of existing ice, and it contains air bubbles that make it much weaker. One guideline is that white ice is about half as strong as clear ice, so you need roughly twice the thickness for the same load. Four inches of white ice is not equal to four inches of clear ice. The color is usually easy to see on the edge of an auger hole or crack, where layers of clear and white ice appear in stripes. A thick layer of white ice over clear ice may still be safe, but the clear portion is the part to count. After a heavy snowfall, water can seep through cracks and flood the surface, creating slush that freezes into snow ice. These layers stack up over the season. Treat any opaque ice with extra suspicion, and measure the thickness of each layer. If most of the ice you drill through is white, go back to shore, or set a more conservative limit.
Gray or dark patches on the ice suggest that water is present and that the ice is thin or rotting. Dark ice may indicate that water is soaking through, and it is a classic sign of weak ice. As spring approaches, ice becomes honeycombed, a state in which the crystals separate into vertical columns, sometimes called candle ice. Honeycombed ice can look thick, but it has lost most of its strength, and it can collapse without warning. Rain, warm air and sun accelerate the process. Check the ice often with your auger or spud bar, and if it is soft, mushy or crumbles in your hole, get off. Color alone does not guarantee safety, but dark or gray areas are a reason to stay away. In the late season, avoid shoreline areas where meltwater collects, and avoid areas where ice has been exposed to sun. Take care near the edges of the lake in spring, where the ice begins to separate. Conditions can deteriorate fast, so don't assume that the ice that was fine last week is fine now.
Standing water on the ice has several possible meanings, and none is good. It may be meltwater from a warm spell, or water pushed up through cracks by the weight of snow, or a sign that the ice has sagged below the water level. Slush can hide the true ice surface, so you cannot see the color or thickness, and it often covers weak spots. Walking on slush is slippery and can make it hard to feel cracks. When ice floods, the water and snow combine to form a layer of snow ice, which is weaker, and the overall thickness of good ice may be less than expected. Do not measure slush or snow as part of the ice thickness, as the Minnesota DNR advises. Avoid driving vehicles on slushy ice, because the added weight can push it down and increase flooding. If you see water around your boots, or if water seeps into an auger hole, retreat to shore and wait for colder weather. The sign is subtle but important, especially after a big snowstorm or a rapid thaw.
Moving water erodes ice from below, and the areas where it flows are typically thin. Check where streams enter or leave a lake, around narrows where currents speed up, near springs and under bridges. Rivers are especially treacherous because the current changes the thickness and the ice can differ over short distances. Even in a very cold winter, ice near inlets and outlets may be only an inch or two thick, and it can look the same as the ice around it, especially under snow. Look at a map or a depth chart before you go, and ask local anglers where the currents are. Stay well away from these areas, usually at least 50 feet or more, and don't cross them on a snowmobile. Fish movement and water currents can also create thin spots in the middle of a lake. Be wary of ice over water with a lot of wind and wave action, which prevents even freezing early in the season. If you must cross a river, consider alternatives. Because the thin areas can be invisible, treat flowing water as a red flag.
Snow looks like a solid surface, and it is an insulator. A thick layer slows the freezing of the water below, so ice under a deep snowpack can be much thinner than ice on a windswept patch of the same lake. Snow also hides cracks, pressure ridges and open water, and it makes color impossible to judge. The weight of heavy snow can push the ice down, causing water to flood the surface and create slush. Early-season snowfall is a particular danger because it can stop ice from thickening just when it is most needed. Anglers and snowmobilers should be aware that a layer of snow can disguise the difference between thick and thin ice. Check thickness often, and when you drill, look at the layers for slush and white ice. Use a tool to clear snow and look at the ice surface. After a storm, wait for colder weather, and be especially careful in the days after the first snow. A snowy lake looks inviting, but it requires more caution, not less.
Cracks are normal in ice, because it expands and contracts with temperature, and the booms and groans you hear at night are usually just that. But some cracks are warnings. Wet cracks, where water seeps up through the surface, indicate that the ice is weak and that its load capacity is low. Large cracks that run across the lake, especially those with offset edges, show that sections of ice are moving. Pressure ridges form where plates of ice push against each other, creating raised ridges and open leads of water. They can be unstable and may open or close with changes in wind or temperature. Many experienced anglers avoid crossing ridges, or cross only at places they know are safe. Dry cracks that are narrow and white are usually harmless, but avoid straddling any crack. When you find a crack, stop, drill beside it and assess the ice. Do not jump or run on the ice, since vibrations can stress it. If the ice cracks loudly underfoot, move away quietly and spread your weight. Keep your distance and observe signs of change.
Anything that sticks out of the ice can create a thin spot around it. Reeds, rocks, logs, docks and pilings absorb sunlight and radiate heat, melting the ice around them and leaving a ring of weak ice. The same is true of areas where plants grow or where dark objects sit on the surface. Bubblers and aerators, which are used to keep docks and marinas free of ice, create open water or very thin ice around them, and these areas can extend well beyond the visible hole. Warning signs are posted, but they may be buried by snow. Stay far from them. Some power plants and industrial sites discharge warm water into lakes and rivers, producing thin ice or open water downstream. Treat any dark patch, steam rising from the surface or open lead as a sign of heat, and avoid it. Boat docks are best approached from the shore side. Fish houses and equipment left on the ice can also affect the ice by absorbing heat, so be careful around old shanties. Ask local anglers about hazards before you go.
Ice thickness is a snapshot, and weather changes it. A few days of above-freezing temperatures, a heavy rain or strong sun can weaken ice quickly, even if it was thick a week earlier. Warm air and rain reduce the strength by melting from the top and loosening the crystals, and meltwater trickles through cracks to weaken the ice from within. Ice also thins from below when spring-fed water or warm currents flow beneath. Check recent weather records, not just the thickness you measured last time. After a thaw, the ice may refreeze, but it may not regain its former strength. Be cautious in early and late season, when the ice is either forming or breaking up. In spring, the safe season ends quickly, and ice that looked solid in the morning can be dangerous by afternoon. Avoid driving vehicles on ice during warm spells. Local authorities and bait shops often post ice reports and closure notices. If the forecast shows warming, plan to be off the ice early, and consider canceling.
Your own senses can warn you before you fall in. If the ice sags under your weight, if you hear sharp popping or cracking close to you, or if water seeps up around your boots or into your tracks, retreat at once. Sagging means the ice is bending, and it is close to its limit. Cracking sounds from far away are the voice of expansion and are usually harmless, but sudden cracks close to your feet are not. Water appearing in a drilled hole that rises quickly and floods the surface suggests the ice is under pressure or thin. Move calmly: walk back the way you came, keep your feet apart, and avoid running. If a snowmobile or ATV starts to sink, lean back and accelerate gently toward the shore, if you can do so safely. If you do fall in, remember cold-water survival basics. Do not panic, control your breathing, turn toward the direction you came from, kick your legs to get your body horizontal and use ice picks or a pair of awls to pull yourself out, then roll away from the hole. Carry picks around your neck.
It is tempting to follow tracks across a lake, assuming that if a vehicle or a person went there safely, the ice is fine. Tracks are not proof. Conditions change from hour to hour, and the ice that held someone yesterday may be weaker now after a warm afternoon. A track may also have been made by a lighter vehicle or a person who crossed a thicker part of the lake. A line of tracks can lead directly over a spring or a current that is not obvious. Some people take risks, and their tracks reflect judgment that may not match yours. Always check the ice yourself, with an auger and a gauge, and make decisions based on what you measure. This applies to snowmobile trails too: groomed trails over lakes can be closed if conditions change, and a marked route does not guarantee safety. Local clubs and the DNR post updates. Never assume that ice is safe because other people are on it. Crowds can add weight and stress, and the results of a collapse can be tragic. Make your own assessment and carry rescue gear.
Thickness guidelines assume a given load, and it is easy to underestimate how much weight you put on the ice. A person alone counts for little, but a group clustered around a hole, a sled loaded with a shelter, heater, auger and fish, a snowmobile and a trailer, or a truck all add up. Weight is not distributed evenly, and moving vehicles create pressure waves in the water beneath the ice that can crack it, especially at speed or in shallow lakes. The Minnesota DNR recommends at least four inches for walking, five to seven for a snowmobile or small ATV, eight to twelve for a car or small pickup and twelve to fifteen for a medium truck, and these numbers assume good, clear ice. Many experienced anglers prefer to leave vehicles on shore and use light sleds. If you must drive, go slowly, unbuckle your seat belt, open the windows and keep the doors ajar. Keep groups spread out and keep heavy gear off the ice. Ice shelters, once placed, can hold heat and thin the ice below, so move them. Remember that each increase in load requires more ice.
