In the winter I spend an average of six days a week crossing ice. Since I do a lot of guiding in winter, my safety and the safety of my clients depends on my knowledge of ice conditions. Beginning in November, I keep a daily log of high and low temperatures, wind speed and direction. This gives me valuable base-time data, and I also chisel-check the ice in bays as it begins to form throughout November. An ideal fall freeze is when we have an extended period of cold weather (single numbers to lower teens), with little wind, until we get an ice thickness of about four inches before snow comes. To understand ice, one needs to be aware of the special properties of water. As with most liquids, as water cools it becomes denser — up to a point. Water reaches its maximum density at 4°C (39°F). In the fall when the surface water reaches 4°C, it sinks. This is called turnover, and it happens in spring and fall. Once a lake has cooled to 4°C, the surface water can continue to cool below 4°C until it freezes. If water was most dense at its lowest liquid temperature, just above 0°C, a lake would freeze from the bottom up. This new ice is black ice, so called because it has very little air in it, and it is like looking through glass into the deep black water of the lake. Ponds and Lakes Calm, cold nights in November are needed for the surface water to freeze. Lakes begin to freeze first along the shoreline, eventually freezing across bays. The middle of the lake or areas that receive more wind are the last to freeze. If you could view the progression of a lake freezing from an airplane, you would see it freezing in many large plates. All the bays or coves skim over. In the main lake the ice tends to grow out from the lee shore further than from the windward shore. Where these plates meet is where you will find pressure ridges later in the season. Pressure ridges are formed where plates of ice meet. As the ice continues to freeze, it expands, but the shoreline keeps it from expanding outward. The energy associated with expansion has to go somewhere; in this case it is subduction (downward) and abduction (upward) at pressure ridges. In my experience, pressure ridges tend to be most active earlier in the winter, in December and January, where there is not much snow on the ice to insulate it from extreme temperature swings. Thaws, especially early in the season, are also associated with pressure ridge activity. Lake ice basically occurs in two different colors, black and gray. Black ice is stronger because it has less air in it. Gray ice obtains its color from the presence of air, and the lighter the shade of gray, the more air the ice contains, and the weaker it is. Black ice is formed by top-down freeze at the surface; gray ice is usually a mix of melt freeze snow, refrozen slush, or a combination of all that formed above the black ice. I have observed over the past few years, as we have gotten more thaws in winter, that gray ice is becoming the dominant form of lake ice in the northeast. Slush is often a feature of our lakes, especially after a heavy snowfall followed by mild temperatures. Slush can either be pressure or over-flow related. Pressure slush occurs when the weight of the snow on the ice is great enough to create enough pressure for lake water to percolate up through cracks in the ice and spread over the surface. Over-flow slush most often occurs near shore and at inlets and outlets. Ground water springs near shore flow all winter under the snow pack, and anything that causes an increase in water level (e.g. human controlled dams) will cause overflow slush at inlets or outlets. Slush is not dangerous in and of itself, but it makes for difficult travel conditions if it is deep and covers an extensive area. River and stream ice begins to form first along the shoreline, because the current is slowed and because of the cold conduction of frozen ground. The water is often 32°F, and only its movement keeps it from freezing. However, even streams and rivers with strong currents can develop frazil ice — small disc shaped platelets drifting in the current. Frazil ice sometimes becomes attached to the bottom of the riverbed and grows by accretion to form anchor ice. The shore ice develops first and becomes thickest in areas of slower current, like the inside of river bends. Shore ice continues to grow outward from shore and may eventually span the entire channel. As winter progresses, water levels drop and cracks that run parallel to the shoreline begin to develop in the shore-fast ice. RIVERBED CROSS-SECTION Ice Safety and Rescue I always carry a small lightweight (5/16″ diameter x 50ft) throw-bag with me, whether I am traveling by snowmobile, dog team, or snowshoes. If I am checking the ice to find a safe route, I also use an ice chisel-to-chisel check as I go, and I have a pair of self-rescue ice picks on a lanyard around my neck. The bevel of the ice chisel should be fairly low angle, 15 to 20 degrees, and very sharp. Mine is mounted on a 6′ long spruce pole, and with a very hard thrust it will go through 2″ of ice. With a moderate hard thrust I know I have at least 2″ of ice under me; I am able to check ice thickness over some distance reasonably quickly. Two inches of ice will hold a person or a snow machine, but I recommend a minimum of four inches. Early in the season when traveling on thin ice, mark the trail with evergreen boughs and stay on this trail. Under these conditions I chisel check a trail approximately every 50′ and will do so again if the temperature goes above 24°F. On lakes the last places to freeze are also the first to open up in the spring — the inlets and outlets of rivers and streams. Narrows in big lakes often produce enough current to open and close throughout winter. Always chisel-check and use extreme caution in these areas. The size and activity of pressure ridges varies greatly from winter to winter, even on the same lake. The rule of thumb is to always cross perpendicular to the ridge itself. Slush often forms on top of ice that is thick enough to safely travel on unless it is near the edge of open water. Pressure slush is most common on lakes and overflow slush on rivers. Slush is water on top of the ice, so it seeks the low spots. You can often avoid it by hugging the shoreline where the ice is higher. If you have to cross a river, do so with extreme caution. Wide places will have shallower water, thinner ice and faster current. Narrow places will have slower current, thicker ice, but also deeper water. The bottom line is that there is no effective rescue method for someone going through river ice in current where the water is over your head. Thoroughly chisel- check every step if you must cross. Traveling by snowmobiles on ice has advantages and disadvantages. The main disadvantage is that you don’t feel the snow and ice under your feet, and you can very quickly get yourself onto thin ice or deep slush due to the speed that even a Tundra can travel. The two greatest advantages of traveling by machine are that it spreads the weight out over more surface area and that you can use it as a portable anchor on the ice. I usually do my first ice check by a combination of on foot and snowmobile. I use Ski-Doo Tundras or Skandics for bush travel because of their superior flotation. Standard equipment on all sleds includes an axe, throw-bag, snowshoes, ice chisel, two mountaineering ice screws, and a shovel. Any suspect areas are chisel- checked first on foot; then I bring my Ski-Doo up to the last chisel hole and repeat the process. I’m backed up by another person, keeping a safe distance (100 feet), who is also on a Ski-Doo and who has a throw bag. Should the first person go through, the second tosses the throw-bag. On glare ice, where the thrower has poor traction, clip the t-bag to the handle on the front ski for an anchor, and the swimmer can pull himself or herself out. A larger throw-bag (1/2″diameter x 75′ L) is preferable for snow machine travel because it can double as a towrope if one machine gets stuck in the slush. If you are alone and get stuck in slush, you can use the ice screws for an anchor. Then cut two poles, and with the rope make a Finnish winch that will pull as well as any come along to get yourself unstuck. Along with judgment and the proper safety/rescue gear, curiosity, respect, and a healthy amount of fear should be your constant companions when crossing ice. Travel well! Kevin Slater operates Mahoosuc Guide Service in Newry, Maine and offers wilderness canoe trips in Maine, Utah and Montana. MGS also offers exceptional dog sledding expeditions in Maine, and in Northern Quebec in partnership with a remote Inuit tribe.You can contact Kevin at: info@mahoosuc.com
