First published February 5 2020 in the York Weekly, Portsmouth Herald and Foster’s Daily and online at seacoastonline.com
I spent almost the entire first semester last year having my freshmen STEM class run experiments about ice. Every morning, when I walk my dog, we follow a path along the river and observe the ice, marvel at the huge slabs of ice that are carried downstream and stacked like pancakes in some places, crunch our way over the paper-thin ice that coated the hummocks of the floodplain after the last brief thaw, watch dangling orbs of ice form in the waterfall that spills down to feed the river. I, like so many others, am obsessed with ice.
If you think about it, ice formation on a pond or lake is fairly straightforward. As the water gets colder, it gets denser and starts to sink. Liquid water reaches its maximum density at 4 °C (39.2 °F), after that it continues to get colder and freeze solid, but as it freezes, it becomes less dense and floats to the surface and we end up with ice-covered lakes.
At first, the ice is paper-thin, fragile sheets that crumble in your hand. As winter’s cold progresses, the ice gets thicker and thicker as more ice is added from the bottom. As long as the pond or lake is deep enough, there should always be at least some liquid water underneath the ice. This is very helpful to the animals that live in the pond or lake. They can survive in the liquid water near the bottom until the spring thaw.
Now, think about a river or stream in which the water is constantly moving. All of that turbulence makes it difficult for a nice sheet of ice to form. On cold winter nights, the surface water of a river cools, crystals of ice form and start to grow. The constant water movement keeps the crystals from growing together into a solid sheet, instead you get a slushy mixture of water and ice called frazil ice. You can see this floating on the top of a river in early winter, or instead of floating on top, because the crystals are so small, they can easily be carried by the turbulence down to the bottom of the river.
As temperatures continue to drop, the frazil ice can start to join together at the surface and form round plates of ice with upturned edges (from the plates bumping together). This is called pancake ice. Or, what was more common on my river, ice will start to form along the edges of the river. Ice forms more easily along the edges because there is typically less water movement and the temperature of the shallow water on the edge cools faster. This is called border or shore ice. Border ice will generally enlarge toward the middle of the river until the ice from both sides meet.
Another interesting way a river can freeze is when the frazil ice that is transported to the bottom of the river attaches to the streambed and builds from the bottom up. This is called anchor ice. Anchor ice can grow very rapidly and block the flow of a river or stream causing local flooding. If there is particularly low flow of water along the bottom, an enormous amount of anchor ice can build up. This can be extremely harmful to aquatic life because the anchor ice can physically lift up parts of the streambed and move it downstream, movement akin to the action of a bulldozer, killing small fish and aquatic invertebrates (like dragonfly or caddisfly larvae) that live in the streambed, or freezing fish eggs that were waiting for spring to hatch.
I thought I first fell in love with ice in some dramatic location, perhaps when I crossed Lake Champlain on the ferry one cold winter’s night and watched the prow cut through the ice and listened to it grind against the hull of the boat, or when I was travelling as a National Geographic Grosvenor Teacher Fellow in the Arctic Ocean and witnessed firsthand the ethereal beauty of icebergs and the vast expanses of pack ice in the polar sea. But watching the beautiful, ever-mutable ice along my little river this winter has made me realize that I’ve loved ice since I was a kid watching icicles dangle from the gutters of my house, that no matter where you find it, ice is magic.