Science of the Seasons: Overflow, take care on thin ice

By Dr. David Wartinbee, for the Redoubt Reporter

Photo courtesy of Dr. David Wartinbee. Snowmachine tracks navigate through an area of overflow along Snag Lake along the Swanson River this winter. While overflow doesn’t necessarily mean lake ice has thawed to the point of being impassible, it is still unstable and dangerous.

During the recent Yukon Quest sled dog race, four-time winner Hans Gatt was forced to scratch after plunging into river overflow. The temperature was probably 50 below zero and he was completely soaked. Another of the race leaders, Sebastian Schnuelle, helped him by providing dry socks and wrapped dog blankets around his boots. But Gatt suffered serious frostbite on his hands and had to retire from the race.

On a recent weekend, while snowmachining along the Swanson River, I came upon frozen tracks through previous overflows, as well as some sections with current overflow. That got me to thinking, just what is overflow and what causes it to happen?

Overflow can occur on lakes or around streams in winter. In either place, the most common situation involves liquid water flowing or sitting on top of a frozen layer of ice. But why does it appear in the first place?

Let’s look at streams and rivers first. In virtually all streams, there is water flowing underneath the ice. When more water collects than can flow under the covering ice layer, it starts to back up. Eventually the water backs up far enough upstream, and uphill, that it finds a break in the ice where the water escapes to the surface. The liquid water flows out from under the ice cover in quantities that reflect the amount of water being backed up.

The backup may be caused by a flow restriction that is a long way from where the water is appearing. If there is a large amount of water trying to get through the blockage, there can be massive amounts of water flowing toward the surface.

If the water comes to the surface and is completely exposed to frigid air, it may freeze quickly and create an undulating ice surface that covers snow, river ice and any nearby vegetation. More likely, the water will flow into and through an overlying snow layer, which can cause serious problems for the unwary.

Snow covering overflow water will act as an effective insulator. There are air spaces in between individual snowflakes that function just like a down comforter. With a thick insulator protecting the overflow water from the cold air, the water remains liquid.

Even if the top of the overflowing water gets exposed to cold air, only the surface freezes and liquid water continues to flow underneath. Additional blockages in the newly formed channel may occur and another layer of liquid water may flow on top of the first. It is not uncommon for there to be several layers of overflow ice with liquid layers trapped underneath each one. The upper layers now insulate the lower ones, so overflow areas can remain liquid for weeks or months at a time.

The problem here is that these layers may not be thick enough for a person on a snowmachine, a musher on a dog sled, or maybe not even thick enough for a dog to safely cross. Additionally, one area with overflow may be safe to cross without problem, while an adjacent area may be deceptively dangerous.

Some years ago a friend and I were crossing a local lake when suddenly his snowmachine began to throw a watery rooster tail. We both increased our speed and turned toward a nearby shore to get away from the underlying slush. Fortunately, the slush was localized and only about 10 inches deep. However, there are all kinds of stories about snowmachines being bogged down and partially submerged in lake overflow.

When lakes freeze, the ice is only covering the surface and the water underneath is liquid. Lake overflow occurs when liquid water flows on top of the ice. Since most lakes have snow cover of varying thicknesses, the slushy snow underneath is usually hidden from view to the casual observer and well insulated by the overlying snow.

Lake water levels may increase or decrease during the winter season. Subtle changes in water depth can cause some sections of ice to collapse and cause long cracks in the ice. Those who have spent any time around frozen lakes are all too familiar with the haunting sounds of ice cracking.

Alternatively, wind can push the ice and cause the entire ice plate to shift position relative to the shore, or shift position relative to other sections of ice. Either of these situations can cause the ice to crack. These cracks then provide places where liquid water can flow to the surface.

Particularly large lakes, like Tustumena or Skilak, may have ice plates push against each other and create pressure ridges. Ice may buckle and produce visible ridges of ice or force one layer underneath the other. Either way, these are usually areas of ice weakness and should be avoided whether skiing, riding a smowmachine or landing a ski-equipped plane.

If the area where water flows through a crack is exposed to cold air, it might simply freeze on top of the previous ice layer. But as with stream overflow, when snow is present it insulates the water and the slush may remain in liquid form for weeks. Not only does liquid water have to drop in temperature to zero degrees Celsius, it also has to lose 80 calories per gram of water in order to change from liquid to ice.

Several lakes I have visited in the last month have varying amounts of overflow. When it is thin, the slush is merely an inconvenience. When it gets thicker it can cause a variety of problems. Cross-country skiers whose skis contact liquid water may suddenly find ice forming on ski bottoms. Snowmachines may have trouble pushing the sleds through deep slush and the tracks get less traction when pushing against water-covered ice.

Ski-plane pilots are advised to make a light touch-and-go landing on lakes and then fly over the ski tracks to look for overflow. If the tracks are darkening due to underlying water, another spot on the lake would be better to land.

Lake and stream overflow can simply be an oddity until you are caught in it. Wet feet or wet equipment in the winter is no fun. Just ask Hans Gatt.

David Wartinbee, Ph.D, J.D., is a biology professor at Kenai Peninsula College’s Kenai River Campus. He is writing a series of columns on the ecology of the Kenai River and Cook Inlet watershed.

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