When I travelled by train during the recent cold weather I was surprised to see that some smaller streams and rivers had partially frozen.   I don’t ever remember seeing this before in the UK.

Most people have heard of the Frost Fairs that were held on the River Thames in London with the last fair being held in 1814.   You can read more about these in Lucy Inglis’ excellent blog Georgian London.

I’m not sure why but I’d not heard of the other fairs that were held in the rest of the country. Fairs were held on the rivers Severn and Wye in Worecestershire and the River Tyne in Newcastle into the Victorian era.   Rivers froze over in other major cities, as shown in this wonderful photograph.

The River Thames in Oxford in the 1890s

It is unlikely that any of these rivers will freeze solid again as they have been changed significantly over the last century.   Most rivers in urban areas are far narrower than their natural width as land along the banks has been claimed for development.   Narrowing a river makes it flow faster, which in turn means it is less likely to freeze.

I wasn’t quite clear on the exact science behind this so my favourite Twitter scientist @SmallCasserole has helped me here.   Still water settles into layers, so in a pond the top layer can be significantly colder than the bed.   That is how your goldfish can survive underneath the ice in your garden pond.   With no flow there is no mixing of these layers so the top layer can freeze even if the average temperature of the water is above zero.   In a fast flowing river all the layers of water mix together (in river terminology it is “delaminated”) so the river is the same temperature throughout.   If the temperature is cold enough any river will freeze however fast it is flowing.

This wonderful Pathe clip shows that it can get cold enough for Niagra Falls to freeze (short advert at the start)