Ice thickness is measured with an auger and a tape, and canal ice is white because it is full of trapped air. The pound above lock 14 makes three inches overnight when the rest of the canal makes half an inch, and you can read a newspaper through it.
I am a waterway operative on a canal and I have fifteen miles and nine locks.
What you actually do about ice, and why it gets measured.
Ice damages lock gates, because a sheet of it presses on the mitre and a boat driving through lifts it against the timber. So when it comes you stop passage, and the decision to stop is mine, and it is made with an auger and a tape.
You drill a hole, you put the tape down with a little hinged foot on the end, you pull it up against the underside and you read it. Under an inch you can usually carry on. Two inches and the gates stay shut.
Ice also tells you about water. Moving water does not freeze readily, so a pound with a leak in it, or a bywash running, stays open longer, and where it freezes first tells you where your flow is not.
The pound above lock 14 is eleven hundred metres long and it makes three inches in one night.
Not in a cold spell. In one night, from open water at four in the afternoon to three inches at eight the next morning.
The pound below 14, the same width, the same depth, the same water, fed through the same lock, makes half an inch.
Three inches overnight is not a British canal.
I have looked it up properly, because I wanted to know whether I was being ignorant. There is an old rule of thumb that uses the accumulated degrees of frost, and on a night of minus five you would expect well under an inch on still water.
Half an inch is what the rest of my length does, and half an inch is right.
The ice is clear.
Somebody with a degree in this ought to be able to explain that to me, and nobody has.
Canal ice is white and slabby. It forms fast, it traps air and sediment, and it is opaque, and when you break it with a bar it goes milky along the fracture.
The ice on that pound is glass. You can lie on the towpath and look through three inches of it at the silt and a shopping trolley that has been there since I started.
Clear ice is what you get when water freezes slowly from the top down with nothing dissolved in it and no turbulence. Slowly. Over days.
Three inches in fourteen hours, clear.
What I have measured and who I have told.
Water temperature, with a probe on a line, at four points along that pound and four along the next one, on eleven mornings over three winters.
Both pounds are at nought point two, which is water about to freeze.
The air is the air. There is no shelter difference that matters: the pound above 14 runs through open fields and the one below runs through the same fields.
Depth, with a sounding pole, because shallow water freezes first and a silted pound would freeze faster. The pound above 14 is slightly deeper, which is the wrong way round.
Flow, with a float and a stopwatch over a measured hundred metres, which is the crudest method there is and the one I trust. Both pounds are near enough still in frost because the feeders are shut down.
I have told two people. My supervisor, who came out and looked at it and said he had never seen ice like it and that I should keep the gates shut, which I was going to do anyway. And a hydrologist at a university who answered an email I sent in 2021 with real generosity, asked me three good questions, and said that what I was describing did not fit any mechanism she could name and that if I could get her a sample she would look at it.
The sample.
I cut one that February with a saw, a block about nine inches square, and I drove it to her in a cool box with four freezer blocks.
Her report is two pages and I have it in the van.
It says the sample is low in dissolved gas and low in dissolved solids, consistent with slow formation from water of low conductivity, and that the measured conductivity of the melt is about a tenth of what she would expect from canal water.
A tenth. The canal is rainwater off fields and run off from a road and a hundred and ninety years of boats.
She also wrote, at the end, a sentence I have read more times than the rest of it: the isotope ratios in the sample are outside the range for local precipitation and would be more typical of water from a considerably colder climate.
What I do about it.
Gates shut at two inches, which on that pound means gates shut by the first hard frost, every year, and the boaters know it and there has never been a complaint because there is a winding hole below the lock.
I do not walk on it. We are not supposed to walk on ice anyway and I am the one who tells other people that.
What I have changed is the order of my round. The pound above 14 used to be my first job on a frost morning because it is the one that needs the decision.
I do it last now, after the sun is up, because in February 2022 I was there at twenty past seven with a torch and an auger and the ice made a noise under the bit as it went through that I have heard since in the yard when somebody is drilling a brick wall from the other side.