anchor ice

I. Frazil

In fast-flowing water during a cold snap, the surface stays liquid — the turbulence breaks up any sheet that tries to form — and instead the freezing happens in suspension. Tiny disc-shaped crystals, a few millimeters across, nucleate throughout the volume. The river becomes a slurry. The slurry has a name. Frazil.

Frazil is sticky. The crystals are too small to lock into a structure on their own, but they will adhere to anything cold enough to hold them. Submerged rocks. Fishing nets. The intake screens of hydroelectric dams. A whole river of microscopic adhesive snow, looking for somewhere to stop.

II. Anchor ice

Where it finds purchase, it accumulates. On the riverbed in turbulent winter water, ice grows upward from below, the way coral does — sometimes feet thick, taking the shape of whatever it grew on. Anchor ice. The river above stays open and dark; the rocks at the bottom go pale and furred.

When the sun rises and the water warms a fraction of a degree, the adhesion fails. The ice releases all at once. Because it is less dense than water, it floats. Whatever it was holding comes up with it — gravel, mussels, sometimes boulders, sometimes the bodies of fish that were sheltering in the wrong cleft when the ice closed over them. The mass drifts downstream until it melts, and then everything it was carrying drops back to the riverbed, often miles from where it started.

Geomorphologists use this to explain how stones the size of a curled cat end up on top of a sandbar with no current strong enough to have rolled them there. The river didn’t roll them. The river lifted them.

III. Candle ice

A lake-ice sheet that has been weakened by sun rots along its grain boundaries. The horizontal bonds melt first. What remains is a layer of vertical hexagonal columns standing shoulder to shoulder — packed candles, in cross-section. From above the surface still looks like ice. Step on it and it disintegrates into the columns and you go through.

Scoop a handful out and pour it back into the lake. It rings. Each column strikes the others on the way down, hollow and high, like a wind chime made of glass straws. Candle ice. A solid that has remembered it was made of pieces.

IV. The top

Water is densest at 4°C. As a lake cools toward freezing, the 4°C water sinks; the colder, lighter water rises and freezes at the surface. The ice insulates whatever is below. Most lakes do not freeze solid — only the top few feet — because of this one anomaly in the density curve of one of the simplest molecules.

If water behaved like almost any other liquid, freezing from the bottom up, every lake on Earth would solidify every winter. There would be no overwintering fish, no benthic life. The whole arrangement — the rivers with their anchor ice, the lakes with their candle ice — runs on a quirk in how H₂O packs itself near zero.