Overflow and slush are a complex topic. We will be placing a photo essay on this topic in our Articles section soon, so look for that resource. Slush forms on top of lake and river ice from overflowing water. As soon as it snows on top of ice, that creates a force pushing down on the floating ice. All ice, including perfectly safe thick ice naturally cracks day and night, expanding and contracting with changing air temperatures. When the ice cracks water can rush up through the crack on top of the ice but under the insulating snow, and form slush pockets. These slush pockets can become very broad, sometimes covering entire lakes under the snow, and they are a hazard to travelers. You can get bogged down in it as the water seeps up through the snow, flash freezing on your snowshoes, skis and sleds. A slushed sled will not slide with big globs of slush frozen hard on it, so you need to do your best to avoid slush pockets, or move quickly past them sometimes breaking your own fresh trail (float) and not following the leader’s float.
If you bog down in slush, you risk soaking your boots and feet, and this is very serious in deep cold. Snowshoes will keep you floating for a while, and quickly you haul your sled off the float that is seeping in with water, and up onto unpacked snow, can buy you time on the upacked dry snow to tip the sled over and start scraping the frozen slush off on top of this unpacked snow. You need a scraper handy when traveling over potentially slushy lakes. If the slush pocket that your sled is bogging down in is very wet and oozing water up very fast, you probably missed the surface snow signs and entered a pocket you should not have. Recognition of these areas comes with skill and experience. Get the heck out of there, dragging that sled, globs and all onto drier ice and snow to scrape.
On otherwise safe thick ice, when you feel the snow underneath your skis or snowshoes turning to jello as you enter an undetected slush pocket, its time to either turn around really fast, or keep going really fast onto drier snow. Just don’t go really fast into deeper slush! Your buddies behind you will see the float start darkening with the seeping water, and they have to react: get off the float and make their own, or turn around and find another route around the slush pocket.
The slush is the worst when the snow is deepest, driest and fluffy and airy, which is typical of early winter snow. You can be in the middle of an extended deep freeze of -40 at night, but if the snow is insulating, then you can experience slush under the snow. Getting water for camp is easy – just step off your snowshoes and punch a boot down in the snow and cold filtered water seeps up for the taking.
Slush is more of a hazard for getting wet feet, rather than falling through thin ice, except for the phenomenon of “spider holes”, sometimes called air holes. Spider holes are aptly named, as several channels of melting snow form the legs of the spider. Spider holes form along a crack that closes, except for a small hole, which begins to melt out quickly into a bigger hole due to the flowing water. Most spider holes are less than a foot (30 cm) in diameter. But I have seen re-frozen holes in spring ice after the snow is gone, of about half a meter or 2 feet across. I don’t have science on the morphology of these holes, but I suspect there is also a tapered ice thickness, thinner closer to them, and so I do not recommend approaching an open spider hole to see how big it is. It will be a wading pool around it anyways discouraging anyone without waterproof rubber boots.
Spider holes and pockets of slush can often be seen on snow long before you get there. Look for dimples and depressed swales of snow. Wind will sculpt snow in many shapes and sometimes the dry wind swale can fool you. Recognition comes with experience
As the snow settles down in the season and reforms into less insulating structures, the slush will often re-freeze into white ice. Sometimes you get a slush sandwich: blue ice on the lake, then a liquid water slush layer on it, then a white ice layer on top of the slush. It will eventually freeze solid depending on the winter you get, or worst case scenario stay there all winter under fluffy snow. So sometimes a meltdown and even a rain storm after deep snows can be a blessing in disguise if it can settle down that deep insulating snow on the ice.
Wind on lakes also helps out on getting rid of slush, by blowing the snow off the open ice, thus reducing the insulation layers, allowing thicker ice to form, and allowing the overflow events to refreeze quicker. However the wind will pile the snow up along the shore often causing slush pockets to form and be sustained there all winter. Big windswept lakes can often be free of slush whereas small lakes without that wind action can be completely underlain by slush.