Just when you think things are all settled.
Oxfordshire of One Hundred Years Ago (One Hundred Years Ago series) Hardcover – 28 Oct 1993
Bottom Ice in the Freshwater Thames!
One of the great scientific facts of my childhood was the understanding that whilst most liquids become denser as they get colder this only applies to water down to a point, and that point is at about 4ºc. Water is at its densest at this temperature and then as it cools further it actually gets less dense.
This strange quirk of nature is probably responsible for the survival of life on earth and certainly for life on land. Together with the fact that ice is less dense than water and therefore floats – it means that even relatively small bodies of water never freeze through to the bottom – and therefore life could survive.
There is another thing which I suspect the Conservancy, by their dredging, may have stopped, but I am not certain, as it is so long since we have had a severe winter, and that is the formation of ‘ground ice’, ice that is that forms on the bottom of the river.
When I spoke to my science master about it he talked about the maximum density of water, and told me that the thing was impossible, but I took him down to the river, and showed him, opposite the barges [ie the College Barges along the left bank at Christchurch Meadow], the bottom all covered in ice.
I think he was annoyed, but at the river for behaving so unaccountably – indecently even, he seemed to think – and not at me. He was so far right that in a lake or in a river of uniform depth the ice cannot so form, but in the Thames in those days there were deep reaches followed by banks of gravel over which the water was shallow.
In times of frost the heavier warmer [ie 4ºc] water sank and remained in the deep parts, and what flowed on was the lighter water at or close to freezing point, and when the crystals formed in this they attached themselves, as forming crystals will, to any solid they could find; in this case to the gravel at the bottom.
This ice rose from time to time in spongy masses, bringing with it some of the gravel, and floated on until it reached the lock. Here it packed, and if the frost continued, formed a thick mass of rough ice which, as more came down extended further and further up stream; and it was on this ice, far more than surface ice, that on three occasions I remember a coach and four was driven from Folly Bridge to Iffley.
Owing to the deepening of the river I doubt that this will ever again be possible, though at Binsey, the other point near Oxford where I have seen ground ice form, it may well do so.
[ I suspect there is another physical factor at work here. I guess there comes a point where almost freezing water does not freeze because it is moving. The gravel would offer many tiny ‘shelters’ between the stones in which the water would be relatively still and perhaps that is where the crystallisation started?
One would be interested to hear from a real physicist about this … ]
Dredging at Oxford, 1885, Henry W Taunt
© Oxfordshire County Council Photographic Archive; HT219
“There is another thing which I suspect the Conservancy, by their dredging, may have stopped, but I am not certain, as it is so long since we have had a severe winter, and that is the formation of ‘ground ice’, ice that is that forms on the bottom of the river.”
Now where have I heard that before!