Why the Thames Turned to Ice

Between the sixteenth and nineteenth centuries, Londoners occasionally woke to find their river gone. In its place lay a road of ice thick enough to walk on, and within days a whole temporary town would appear on top of it. These were the frost fairs.

The fairs were not gentle affairs. Traders dragged printing presses onto the ice and sold souvenir cards stamped with a customer's name and the date, proof that the buyer had stood where boats normally floated. There were swings, puppet shows, skittle alleys and stalls roasting whole oxen. In 1684, a coach service ran along the frozen river as though it were a high street. Londoners called the largest fairs "blanket fairs", after the tents that covered the ice.

Two things made them possible, and only one of them was the weather. Europe was passing through a colder period, with harsher winters than we now expect. But the more important cause was the old London Bridge. Its many narrow arches, crowded with waterwheels and shops, acted like a dam. Water slowed almost to a standstill as it squeezed between them, and slow water freezes far more readily than fast.

That is why the fairs ended when they did. The last great frost fair was held in 1814, when an elephant was reportedly led across the ice below Blackfriars. Barely two decades later the medieval bridge was demolished and replaced with one of wide arches. The Thames, released, began to run faster — and stopped freezing.

Winters after that were sometimes just as cold. The ice, though, never returned. A city had changed its river without meaning to, and in doing so quietly cancelled its own strangest festival.