Those who know lakes never completely trust the ice, ever. They always are on the lookout for thin places. The thin places are sometimes in predictable places but not always. If you know a particular lake, you know where to expect open water in the middle of the coldest months but every season is different. 

One phenomenon that often is associated with thin ice is a “pressure ridge”. Pressure ridges form as the ice warms and cools. After ice freezes, it behaves as a normal solid. When it warms, it expands and when it cools it contracts. This is why in the evening on a frozen pond or lake, you hear great noises as the ice cracks. It has frozen onto the rocks along the shore and when it cools and contracts, it can’t pull the rocks away from the shore so it cracks. The cracks then fill water and generally the water freezes quickly but then the next day, the sun warms the ice and all the ice expands. It has to expand somewhere and often one section pushes up over another section to produce a ridge of ice. This ridge, though, is associated with cracks and thinner ice than normal so if there are any currents in the lake (which do occur even in the winter on a lake strangely enough), these thin areas along the ridges can be very thin ice or open water.

Pressure ridges often occur near points or between islands because as the ice tries to expand, it gets stopped by the island and the pressures build up in other parts of the ice. Sometimes these pressure ridges form large piles of ice that can be quite spectacular. If you happen to be traveling to the North Pole instead of just around Lake Wentworth, getting through the pressure ridges can be a major problem as sometimes the piles of ice are many feet tall.

The photo shows a pressure ridge and open water that was visible in early February. This area between Stamp Act and Cate Island always seems to have thin ice and ice boaters have gone through the ice at various times.

Thin ice can also occur at cracks as ice is forced apart from another section of ice by the wind. Even though there is not a lot of friction between the wind and the frozen surface, with a big enough lake the force can be large. This will cause water to up-well between cracks and can often lead to large sections of open water. 

Water can also percolate into the lake along glacial deposits of gravel such as an esker. The water coming from shore will travel in this gravel bed and then come up as a spring. Because the water comes from below the frost level, it is above freezing and will melt the ice around the spring. One year between Cate Island and Stamp Act Island, I saw one of these springs that was literally bubbling water upward so fast that it raised the water level an inch or so above lake level. It was literally making the sound of a water fountain bubbler that you drink water from. Needless to say, a lot of ice was melted around the spring.

This year on Sebago Lake in Maine, an ice fishing derby had to be cancelled because people, cars and ATVs were falling through the ice. The problem was associated with warm weather but most of the incidents were along pressure ridges. No one suffered permanent harm but a number of vehicles went to the bottom and lots of people got wet. Remember, just because other people are on the ice doesn’t mean that is safe everywhere.

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