Ice Thickness Safety

by Laura Emerson

Every year, especially when cabin fever sets in and we want to be out and about IMMEDIATELY, it is prudent to review ice thickness safety before zooming toward the horizon. Let’s face it: those annual photos of the butt end of cars and snowmachines protruding above thin ice almost always belong to beleaguered locals, not tourists.

The general rules of thumb for safe activities on ice are:

3-4”: single person cross country skiing or walking

4-5”: group of people cross country skiing (or spread out walking single file)

5-6”: a solo snowmachine or ATV (or several spread out single file)

7-8”: a group of people gathered in a small space

8-12”: a car (groups of cars should spread out, about 50 feet between vehicles)

How are these numbers determined? By what is called “the Gold Formula.”

Its calculation, P = A * h^2, multiplies ice strength (A) times thickness (h) plus a safety factor (2) to determine load capacity (P).


Ice fishermen/women can measure thickness with the bulky gas or hand augers that dig their fishing holes. However I must confess that a few winters ago, the weight of my husband, the augur and me in close proximity cracked through thin ice that we should have assessed visually first (see below)! Thank goodness that my motor was still a warm spot for soaked socks to dry. A more convenient tool for anyone, a battery powered drill with a long WOOD augur can drill through 8” of ice in 30 seconds. What great peace of mind for a pretty outing over any surface you question.


New ice is stronger than old ice because the bonds between ice crystals degrade over time, even in cold weather.

Blue and black ice surfaces are stronger than white ice.

The former indicates fast frozen water. Light colored ice, on the other hand, incorporates snow, which contains air molecules, rendering these surfaces weaker. A hard ice layer may be easy to assess but a challenge to cross. We pull studs over our boots often during the winter to traverse slick paths between our cabin, outhouse, and food shed. We also added studs to our snowmachine treads to claw up a steep ridge that always gets icy in March, as well as scratchers to generate flakes for engine cooling.

Icy surfaces are even more troublesome for ski planes than for cars because the former lacks brakes! After careening across lakes waiting for belated friction to do its thing, we finally installed external brakes last winter. Phew! Worse than slick ice are surfaces marred by the rutted tracks of snowmachiners who have illegally traveled along an active runway. Long and deep indentations capture wheels and skis of planes, dangerously limiting their mobility. Last winter, we had to jump out of the plane on an active runway and manually haul the plane out of the rut so we could turn and taxi out of the way of incoming planes.

Still ponds and lakes are generally safer than rivers, and they freeze earlier and thaw later than moving waterways. For example, our lake tends to freeze in mid-October and thaw in mid-May.

The faster the river, the later it freezes and the sooner it thaws. To cross the two or three rivers between us and the road system to haul in fuel and other supplies, we wait until January and hope for 8-10 good weeks of safe surfaces. Even when frozen though, river ice is considered less safe than ponds because of underlying currents as well as branches and rocks that poke holes in the surface. In fact, engineers quantify this danger with a 15% decrease in strength/safety even for the SAME THICKNESS as lake ice. Inside river curves (slower moving water) are generally sturdier outside curves (faster water).

Snow covering confers several important benefits, such as traction for any mode of transportation, and a medium for cooling snowmachine engines. Even my chickens and ducks prefer walking on snow to slipping and sliding on ice (which I must admit is hilarious to watch).

A disadvantage is that it obscures important clues about stability, like color, holes, and cracks. Speeding through fresh powder is undeniably gorgeous, but it may not be prudent to be the first person out, ESPECIALLY on a route you do not know well. The exception is probably on well marked, well used trails that are likely the hardest packed surface in the vicinity. On our lake, for example, two spider holes linger all winter long. From the vantage point of our plane, they look big, but snowmachiners can’t see them. Those who follow the trails of locals will safely avoid them, but each year, some joy-riders take a “short cut” straight across the lake. Last year, two women got wet, and their companions had to haul out the machines with a come along. Fresh snow also obscures the width of a trail. Having to dig out a snowmachine that has plunked off the hard surface, and then build up a ramp to drive it back onto the trail is a a long and sweaty kill-joy to a winter outing.

Overflow is always an issue where ice weight presses on underlying water and forces it upward through cracks. Recent winters of warm/cold, rain/snow conditions also produce “pancake and syrup” like layers of ice/water/ice/water. Your body weight might crack through one layer but your snow machine or group of friends could crush through several. If you aren’t familiar with underlying springs and water flow, at least take note of low lying areas where rainfall can pool.

Ice fisherpeople are polite and prudent enough to position their holes away from marked trails, and often to mark them with crossed sticks. However, it is easy to see the tracks of snowmachine hotrods who go off trail, running circles around well populated ice fishing lakes. Duh: fishing holes and the surrounding turrets of ice thrown up by augers can cause a quick stop to a snowmachine and a long walk for the inattentive rider.

March is one of my favorite months of the year, because of the dazzling sunlight reflected off snow, but this combination (of sun and snow) generates challenging conditions for all sorts of pretty outings. One late winter day, at my ill-advised “cabin fever” suggestion, we pointed our snowmachines in the direction of feeble tracks we had not followed before. The route was lovely, with long views toward mountains before plunging into the woods. My husband was some 70 feet ahead of me on a rise when the trail disappeared in some dense trees. When he tried to turn around, we discovered he was on top of a springy alder thicket. The more he tried to turn around, the more intent the mass seemed to be to swallow his machine and him. He sank waist deep in air pockets because we had forgotten to bring snowshoes). It took us more than two hours, with a come along, a shovel and a saw to extract ourselves from this predicament. Not a great day for marital harmony.

Some years, spring arrives so fast that we suddenly realize “today is our last load.” (This has delayed some construction projects for 18 months). One late March morning, Bryan powered his snowmachine through a low spot in the Big Susitna River that had formed a pond on top of the ice. When he retraced his path six hours later with a sled full of heavy supplies, that pond had grown to a river! Was the source an open lead? How safe was his route? He stopped his machine at a prudent distance to wade in and assess the water’s depth (about 6 inches) and the strength of the underlying ice. On the far side, several snowmachiners watched him. Fortunately, they pointed out the route they had successfully traced through the water, so he did the same. His hauling season had just ended.

I absolutely believe that the best way to weather our long winters is to get outside for fun and exercise. On the one hand, I don’t believe than any of us can beat “Old Man Winter.” As seasonal news repeatedly proves, some goofballs may not be destined to contribute to the gene pool or to retain that brand new truck. On the other, thousands of Alaskans enjoy all sorts of methods for crisscrossing our beautiful state throughout frigid months, as this magazine beautifully attests. Those who are prepared, can joyfully do so again and again. Have fun. Be safe.



One thought on “Ice Thickness Safety

  1. thanks Laura I was thinking where the first casualty of the year was going to be this year, there is always some one who thinks they KNOW it is safe and has to be first!!!!!



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