Midwinter Holes

Holes in Pressure Ridges

Pressure ridges are really cracks although they often have true holes in them after a period of warm weather. They are the most popular way to fall through the ice. On Lake Champlain I estimate that over 80% of the car-through-the- ice-accidents occur at pressure ridges.

They are a feature of all ice with dimensions larger than about a mile. They tend to run from point to point and along the shore. They are caused by buckling failure of the ice sheet as it is heated during the day. They tend to occur in about the same locations every year but can occur in unexpected places. They usually form when the ice is 2 to 4 inches thick. When they form in much thicker ice the event is similar to an earthquake. Pressure ridges will be covered in more detail in a future article on cracks.

Pressure ridges form "holes" in three ways:

1. The plates can push down to form a ice bottomed puddle that can be 15 feet deep and 50+ feet wide. This is common on at the ends of ridges that do not go to shore. Keep this in mind when you are sailing around the end of a ridge.
2. The ice in the active part of the ridge may be quite broken up with no solid sections.
3. There can be places where there the plate is missing. Usually this is because it melted during warm weather. In sunny thaw conditions, the underwater ice in a ridge melts more quickly than the ice on the surface.

These areas are often characterized by a lack of pushed up plates in this part of the ridge. In thaw conditions, a ridge can deteriorate into open water very quickly.

In general ridges are easy to see because of the pushed up ice blocks along the ridge. They can often be crossed safely if they are checked out carefully first. Some sort of a probe is helpful for this. The boat makes a good bridge to cross the wet side of the ridge. Crossing ridges always requires good judgment and experience.

On new black ice in the 3 to 4 inch range I have seen pieces of the plate break off under the leeward runner when we are trying to cross a ridge. So far we have been able to get the runner out with out getting wet. A wet suit, bear claws and a rescue throw rope are in order for crossing ridges in marginal conditions.

Sailing across pressure ridges is always a bad bet. The little time it takes to check the ridge and walk across greatly reduces the chances of getting in trouble.

In Trenton, Ontario at the 1988 North Americans, Joe Norton slowly sailed across a small ridge directly behind another boat. The first boat was lucky and went across with out a problem. Joe was lucky to, he only broke off the nose of his boat and hit his jaw hard on the tiller. It could have been worse.

Reef Holes

Reef holes are similar to warm water holes except that they occur at the same spot each year. They tend to be about the same size as the reef and often have thinner ice right at their edge. They tend to skin over in cold spells and reopen in less cold weather. The reefs that run between islands and shore are often open or covered with much thinner ice than most of the ice sheet. Keep this in mind when sailing between islands.

We have a couple of reef holes in Lake Champlain that almost never freeze. These holes are typically about 75 by 150 ft. The water under the holes is about 15 feet deep in what is otherwise the deepest part of the lake.

Characteristics of Reef holes:

1. Occur over reefs.
2. Can be as small as to feet up to almost any size.
3. Tend to be round or oval in shape.
4. Often have well developed rim of splashed out water and frozen slush.
5. Are often populated by ducks as they often are the last available open water in the area. (Ducks are a reliable sign of open water).
6. Reef holes can freeze over but will generally remain thinner than deep water ice and will then reopen in thaw conditions.

Duck Holes

Ducks, geese and other water fowl will congregate in larger holes of any sort. These holes are often the only open water left in the area. I suspect the birds are not usually the source of these holes although they help keep them open.

Characteristics of duck holes:

1. Ducks.

Current Holes

In rivers and lakes where rivers enter them there are often holes and thin spots associated with moving water. The ice gets thinner or opens with warmer weather and/or higher flow rates. Current holes are particularly dangerous because if you do go through one you can be swept down stream under the ice! When rescuing a boat in this situation be sure you have a belay rope on the rescuers. Sailing on large rivers (as the Montreal sailors do) requires special knowledge of the ice.

Spring Holes

There is a widely held belief that underwater springs are responsible for most holes. In my ten years of looking at ice holes I have yet to find one that I thought was caused by a spring. Most of the supposed spring holes are gas, warm water or reef holes. There probably are real spring holes, but they are clearly much less common than many people believe.

Man Made Holes

Bubblers are probably the second most popular way to go swimming in the winter. Air bubblers or underwater propeller stirrers are used to keep the ice away from docks or boats. They typically keep the water ice free for about 5 to 25 feet away. They often are turned off at night to save energy cost. The open water generally skims over resulting in a difficult to see area of the thin ice. In general it is a good idea to approach unknown docks cautiously.

Fishermen certainly make the most plentiful holes in the ice. In most areas the fish are too small to warrant holes larger than about 8 inches in diameter. Unless these are eroded into larger drain holes they are too small to get a runner into. In some areas there are big fish under the ice and larger holes are cut, some with dimensions more than large enough to fit a runner. There is often a block of ice nearby. Bob Cummins reports that sturgeon fishermen make holes that are refrigerator sized! And they push their blocks under the ice.

Fishermen are well equipped to make holes of just about any size. This is one more reason to avoid conflicts with them.

Divers, ice cutters and others also cut holes in the ice. Local knowledge is probably the best protection from these types of holes.

If you have thoughts on this subject you would like to share with the class, write me a letter or give me a call.